Vaccination of Foals and Influence of Maternal Antibodies on Vaccine Responses

Maternally derived antibodies (MDAs) and perhaps other immune effectors such as lymphocytes that are concentrated in colostrum and are passively transferred to the foal play a crucial role in defense against pathogens encountered during the first few months of life while endogenous immune function continues to mature. Passive transfer of MDAs should therefore be exploited in immunization programs for foals by consistently administering booster doses of selected vaccines to mares 4 to 8 weeks before foaling and by ensuring that foals ingest adequate amounts of high-quality colostrum within 24 hours of birth. In addition to passively protecting the foal, MDAs may also exert a profound inhibitory effect on the active immune response of the foal to antigens, including those contained in vaccines. This phenomenon is known as maternal antibody interference.

Several studies reported during the 1990s brought this issue into focus by demonstrating that foals less than 6 months of age consistently failed to mount serologic responses to inactivated influenza vaccines.^23-29 Of potentially greater concern was the finding that a high proportion of foals vaccinated under the cover of MDAs not only failed to seroconvert in response to the recommended primary series of two or three doses of influenza vaccine, but many also failed to respond to multiple additional doses administered during the next year, suggesting induction of a potentially detrimental “immunotolerance-like” phenomenon.^26,27,30 Our studies confirmed an apparent lack of response of foals to multiple doses of inactivated influenza vaccines when the hemagglutination inhibition (HI) test was used to detect serologic responses, but responses were detected when the same samples were assayed using sensitive isotype-specific enzyme-linked immunosorbent assay (ELISA). Rather than representing true tolerance, it appears that MDAs may cause misdirection of the immune response away from the more important virus-neutralizing (VN) IgGa and IgGb subisotypes in favor of the less effective IgG(T) subisotype of IgG.²³ Subsequent studies in which titers of total rather than antigen-specific IgG subisotypes were determined documented that the age-related increase in concentrations of IgGb lagged significantly behind increases in concentrations of other isotypes and remained below adult levels beyond 6 months of age.³¹

Maternal antibody interference has now been documented to be a significant issue for many other antigens, including tetanus, eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), and equine herpesvirus types 1 and 4 (EHV-1 and EHV-4), contained in vaccines administered to foals.^23,32-36 Even low levels of antibody, below those detectable by many routine serologic tests and below those thought to be protective, can completely block the serologic response to some vaccines, resulting in a potentially prolonged period of susceptibility before the foal is capable of responding appropriately to vaccines.³⁵ These findings also indicate that it is not typically feasible to test samples from foals serologically to predict whether they will respond to particular vaccines. We now recommend that primary immunization with most vaccines containing inactivated antigens should be delayed until foals are 6 months of age or older and that, with the exception of rabies vaccine, three doses of vaccine should be included in the primary series rather than the two doses routinely recommended by vaccine manufacturers. Typically, the third dose stimulates a serologic response of greater magnitude and durability than two doses and may also contribute to a higher “set point” for the response to subsequent booster doses.^23,35,37,38 In contrast to the results cited previously, maternal antibodies do not appear to exert a marked inhibitory effect on the response of foals to either the inactivated or recombinant live West Nile virus (WNV) vaccines (West Nile-Innovator, Fort Dodge; Recombitek, Merial), thereby permitting antibody-positive foals as young as 3 months of age to be immunized successfully.³⁷

Study results should be interpreted with caution because only humoral responses are typically assessed and infectious challenge is not performed to confirm that lack of serologic response equates to lack of protection. Lack of a serologic response may correlate well with lack of protection for some diseases and some vaccines, whereas for others this may not be the case. In contrast, the presence of a serologic response may not correlate well with protection, as is frequently the case for respiratory tract pathogens. With the exception of the intranasally (IN) administered strangles and influenza vaccines (Pinnacle IN, Fort Dodge; Flu Avert IN, Intervet), the modified live virus EHV-1 vaccine (Rhinomune, Pfizer), the modified live virus EVA vaccine (Arvac, Fort Dodge), and the canarypox-vectored WNV and influenza vaccines (Recombitek, Merial), most commercially available vaccines are inactivated, adjuvanted, and administered by intramuscular injection (Table 48-1). Because inactivated vaccines administered by injection have limited potential to stimulate cellular and mucosal responses, serologic responses to these vaccines likely correlate well with their potential to induce protection. In turn, MDA interference with serologic responses to inactivated vaccines likely equates to failure to induce protection. In contrast, failure to detect a serologic response to a modified live, vectored, DNA, or mucosally administered vaccine may not equate to lack of protection because vaccines of these types induce a broader array of systemic and local responses that may not be affected by MDAs.

Table 48-1 Types of Equine Vaccines Commercially Available

If maternal antibody interference were not an issue, the approach to vaccination of foals would be greatly simplified because primary vaccination against all important diseases could be completed before MDAs had declined to nonprotective levels. In effect, the “window of susceptibility” would be eliminated. In reality, an attainable goal is to maximize the beneficial effects of MDAs while minimizing their negative impact on primary immunization. In order to best meet this goal, one or both of the following should be determined to be the primary focus:

Assessing risk takes into account both the likelihood that the foal will become infected as well as the risk of serious sequelae or death if the horse does become infected and develop disease. If the disease affects the foal early in life, such as is the case with rotavirus (RV) infection, there is usually insufficient time to induce a protective immune response by actively immunizing the foal. Under these circumstances, the approach should be to maximize the degree of protection passively transferred from the dam via colostrum. Other diseases, such as rabies, affect horses of all ages, but the risk of acquiring infection is generally low.

Diseases of Moderate to High Risk to Young Foals but Low Risk to Adults

Diseases of moderate to high risk to young foals but low risk to adults include RV infection (on certain breeding farms in certain years) and, in geographic areas such as -Kentucky and some other eastern states, type B botulism. For these diseases, the following approach is appropriate:

Booster-vaccinate the dam before foaling to maximize uniformity of passive transfer.

Ensure good passive transfer of maternal antibodies.

Introduce management practices to reduce exposure to the infectious agent.

Vaccinate the foal if risk continues beyond the first few months of life.

Diseases of Moderate to High Risk for Weanlings and Older Horses but Lower Risk to Young Foals Born to Vaccinated Mares

Diseases of moderate to high risk for weanlings and older horses but lower risk to young foals born to vaccinated mares include EHV-4, EHV-1, strangles, influenza, tetanus, EEE, and WNV infection. For these diseases, the following approach is appropriate:

Vaccinate the dam before foaling to maximize uniformity of passive transfer.

Ensure good passive transfer of maternal antibodies.

Start foal vaccination after the risk of maternal antibody interference is no longer present in most foals. When several vaccine types are available for a particular disease, the vaccine that is least subject to MDA interference should be used. Introduce management practices to reduce exposure to the infectious agent while primary vaccination is being completed.

If a two-dose primary series is recommended for adult horses, use three or more doses of vaccine in the primary series to improve the chances that foals that do not respond to earlier doses will respond to additional doses administered later.

Diseases of Low Risk to Foals

Diseases of low risk to foals in most circumstances include rabies, Potomac horse fever (PHF), WEE, and equine viral arteritis (EVA). For these diseases, the following approach is appropriate:

Vaccinate the dam before foaling if the disease is a significant risk to adult horses and a vaccine shown to be safe for use in pregnant mares is available. If the available vaccines are not considered safe for use in pregnant mares, administer boosters before breeding.

Ensure good passive transfer of maternal antibodies.

Start foal vaccination after the risk of maternal antibody interference is no longer present in any foal (typically 9 months to 1 year of age).

Adverse Reactions to Vaccines

Although uncommon, the possibility always exists for adverse reactions (including anaphylaxis) associated with administration of a vaccine; therefore vaccines should be administered by or under the direct supervision of a veterinarian. Adverse reactions should be reported to the vaccine’s manufacturer and to the U.S. Department of Agriculture (USDA) (1-800-752-6255) or the U.S. Pharmacopeia (USP) Veterinary Practitioners Reporting Program (forms may be obtained or reports submitted by calling the USP at 1-800-487-7776). Anaphylaxis constitutes a life-threatening emergency requiring prompt treatment with epinephrine (3 to 5 mL of a 1:1,000 dilution IM or 5 mL of a 1:10,000 dilution slowly intravenously [IV] for a 450-kg horse). Repeated doses of epinephrine can be administered at 15-minute intervals if necessary.

Local irritant tissue reactions occur more frequently, particularly when polyvalent combination vaccines and injectable strangles vaccines are used. These reactions usually are self-limiting, but resolution can be promoted by parenteral or oral administration of nonsteroidal antiinflammatory drugs (NSAIDs), topical application of warm compresses or the cutaneously absorbed NSAID diclofenac (Surpass, Idexx Pharmaceuticals, Greensboro, NC), and gentle exercise. Significant reactions in the neck muscles may make the horse reluctant to lower or raise its head; therefore feed and water buckets should be positioned accordingly. The occurrence of externally visible local reactions can be reduced by administration of the vaccine deep in the semimembranosus and semitendinosus muscles of the hind leg rather than in the neck and by allowing the horse to exercise after vaccination. In addition, horses that repeatedly react to polyvalent vaccines may benefit from administration of an NSAID before vaccination, from administration of the individual antigenic components separately in different sites, from use of a different brand of vaccine, from use of a vaccine that can be administered by a route other than IM, or from use of a vaccine that contains a different adjuvant or no adjuvant at all.

Some horses develop transient, self-limiting systemic signs that may include fever, anorexia, lethargy, colic, diarrhea, tachycardia, and congested mucous membranes after intramuscular administration of vaccines. The systemic signs are perhaps more common with certain vaccines but can be associated with any vaccine.^39,40 It is therefore inadvisable to give horses any injectable vaccine within 2 weeks before a show, performance event, sale, or domestic shipment or within 3 weeks before international shipment. It may also be beneficial to minimize environmental dust when vaccinating horses known to have allergic airway disease or hypersensitivity.³⁹

If unacceptable reactions occur repeatedly, the need for continued annual or more frequent revaccination against individual antigens should be carefully reevaluated, taking into account risk of disease, balanced against the risk of an adverse reaction. Many of the horses that experience adverse reactions have received many doses of many vaccine antigens, repeated over many years. In this situation the vaccination protocol should be “pared down” so that only the most essential antigens are administered and the maximum possible interval between boosters is employed. For diseases such as rabies and tetanus for which resistance can reasonably be correlated with circulating antibody titer, one possible approach to define the maximum or optimal interval between booster doses would be to measure the antibody titer. Unfortunately, this approach is currently limited by paucity of laboratories that offer this type of testing on a routine basis, inexpensively, and with a short turnaround time. Introduction of commercially available ELISA testing for antibodies to the SeM protein of S. equi subsp. equi (Equine Biodiagnostics-Idexx, Lexington, Ky.) and neutralizing antibody testing for WNV (Cornell University, Colorado State University, the University of Florida, and the USDA Animal and Plant Health Inspection Service [USDA/APHIS] National Veterinary Services Laboratory) in recent years has made it possible to refine vaccination protocols for these diseases in horses that experience adverse reactions to vaccination. In addition, testing for antibodies to other pathogens may be available through State Diagnostic Laboratories.

Safety of Vaccines in Broodmares

Consideration of vaccine safety in broodmares must take into account risks to the pregnancy and safety to the fetus. Potential adverse effects of vaccines on pregnancy are difficult to document, even when large numbers of mares are used, unless obvious problems occur. Because fetal organogenesis occurs early in gestation and this period is also characterized by substantial embryonic loss, even in normal mares, it is sound practice to avoid administering vaccines to mares during the first 60 days of gestation unless conditions of imminent risk prevail. Few vaccines carry specific label recommendations for use in pregnant mares, and few published data document the safety of equine vaccines during pregnancy. Of the available fully licensed vaccines, the two EHV-1 vaccines (Pneumabort-K+1b, Fort Dodge, and Prodigy, Intervet) marketed for use in pregnant mares as an aid to prevention of EHV-1 abortion, the vaccine marketed for prevention of type B botulism in foals (BotVax B, Neogen), and the Calvenza line of influenza and EHV vaccines (Boehringer Ingelhein, St. Joseph, Mo.) include directions for use in pregnant mares. In addition, the conditionally licensed vaccine for prevention of RV infection in foals (Equine Rotavirus Vaccine, Fort Dodge) is similarly labeled for use in pregnant mares. Although not specifically labeled for administration during pregnancy, widespread use in practice over many years has failed to document that any of the inactivated vaccines currently marketed for use in horses pose an unacceptable risk to pregnant mares. Therefore pregnant mares are routinely vaccinated with inactivated vaccines directed against tetanus, EEE, WEE, WNV, influenza, EHV-4, strangles, and, to a lesser extent, PHF, rabies, and VEE. Similarly, adverse impacts on pregnancy have not been documented for modified live intranasally administered strangles and influenza vaccines or the modified live parenterally administered EHV-1 vaccine (Rhinomune, Pfizer). In addition, safety of the recombinant WNV and influenza vaccines (Recombitek, Merial) should not be a significant concern because the modified live canarypox vector lacks the ability to infect mammalian cells. In contrast, modified live virus EVA and VEE vaccines and live anthrax spore vaccines should not be used in pregnant mares. Protection of mares against the potential abortigenic effects of EVA infection is therefore best accomplished by completing the primary immunization series before the mare enters the broodmare band and by administering subsequent boosters during the open period before rebreeding.⁴¹

The practice of booster vaccinating mares against multiple diseases to maximize colostral transfer of antibodies to the foal, and the fact that mares in broodmare bands are generally middle aged or older, result in the typical broodmare receiving multiple doses of many vaccine antigens and adjuvants during her lifetime. In addition to stimulating high levels of antibody against a range of antigens for the benefit of the foal, this practice may also predispose these mares to a higher rate of local and systemic adverse reactions, an issue that not only warrants further investigation but may force horse owners and veterinarians to carefully consider strategies for revaccination.

Tetanus

All horses are at risk for developing tetanus, an often-fatal disease caused by a potent neurotoxin elaborated by the anaerobic, spore-forming bacterium Clostridium tetani. Infection of tissues typically occurs via puncture wounds (particularly those involving the foot or muscle), open lacerations, surgical incisions, exposed tissues such as the umbilicus of foals and reproductive tract of the postpartum mare (especially in the event of trauma or retained placenta). C. tetani is present in the intestinal tract and feces of horses, other animals, and human beings, and spores are abundant as well as ubiquitous in soil. Spores of C. tetani survive in the environment for many years, resulting in an ever-present risk of exposure of horses and people on equine facilities. Tetanus is expensive to treat and has a high mortality rate; therefore all horses should be actively immunized using tetanus toxoid as part of the core vaccination program. Active immunization reduces the need to administer tetanus antitoxin, the use of which is associated with risk of inducing potentially fatal serum hepatitis.

Protection against tetanus is mediated by circulating antibodies; toxin binding inhibition (ToBi) antibody titers of >0.2 IU/mL are considered to be protective in the horse.^38,42 The many available vaccines are formalin inactivated, adjuvanted toxoids that are inexpensive, safe, and potent antigens that induce an excellent serologic response and solid, long-lasting immunity when administered according to manufacturer recommendations. Primary immunization involves administration of two doses of toxoid at 3- to 6-week intervals. Titers of specific antibody increase to protective levels within 14 days after administration of the second dose in the primary series and, in adult horses, persist at detectable levels for 12 months or longer, depending on the adjuvant system used in the vaccine.^38,42-44 A recent study documented substantial differences between currently licensed combination tetanus-encephalomyelitis vaccines with regard to the magnitude of the vaccine-induced tetanus-specific IgGb and IgG(T) antibody responses.²⁰ The vaccine containing a Carbopol adjuvant induced substantially higher antibody titers than those containing either saponin or squaline combined with surfactants.²⁰ Revaccination once annually is recommended.

No published challenge studies are available to document the speed of onset or duration of protection induced by tetanus toxoid preparations currently licensed in North America; conclusions regarding their efficacy are therefore based on the serologic response obtained in horses and laboratory animals and on field experience. However, a challenge study conducted in Europe more than 40 years ago found that horses were resistant to challenge 8 days after receiving a single injection of tetanus toxoid, before antibody could be detected in their serum.⁴⁵ A second study demonstrated that a series of three doses of tetanus toxoid induced protection lasting for at least 8 years, and perhaps for life, even when antibodies could no longer be detected.⁴² In contrast, tetanus has been documented in vaccinated horses in North America,⁴⁶ although survival was strongly associated with previous vaccination. Thus it would not be prudent to recommend extension of the annual interval for revaccination with tetanus toxoid, pending publication of data documenting duration of immunity (DOI). Vaccinated horses that sustain a wound or undergo surgery more than 6 months after receiving their previous tetanus booster should be revaccinated with tetanus toxoid immediately at the time of injury or surgery.

Annual revaccination of pregnant mares should be completed 4 to 8 weeks before foaling to protect the mare if she sustains foaling-induced trauma or retained placenta and to enhance concentrations of specific immunoglobulins in colostrum. Colostrum-derived antibodies significantly interfere with the immune response of foals vaccinated with tetanus toxoid until they are approximately 6 months of age.^23,44

Primary vaccination of foals that have received appropriate transfer of colostral antibodies from a vaccinated mare should include three doses of tetanus toxoid beginning at age 6 months or older. The interval between the first two doses of vaccine should be approximately 4 weeks, and the interval between the second and third doses should be 8 to 16 weeks. The three-dose primary series is recommended for foals because a high proportion of foals fail to seroconvert in response to two doses of tetanus toxoid, regardless of whether maternal antibodies are detectable at administration of the first dose.^23,44 For foals born to nonimmune mares, this initial three-dose series can start at 1 to 4 months of age.

Tetanus antitoxin is produced by hyperimmunization of donor horses with tetanus toxoid. Administration of one vial of antitoxin (1500 IU) to unvaccinated horses induces immediate passive protection that lasts not more than 3 weeks.⁴⁴ More prolonged protection may be accomplished with higher doses. In addition to the use of high doses of tetanus antitoxin to treat tetanus, indications frequently cited include administration to newborn foals born to unvaccinated mares and to unvaccinated horses that sustain an injury. In these cases the concurrent administration of tetanus antitoxin and tetanus toxoid at different sites using separate syringes has been advocated, followed by administration of additional doses of toxoid at 4- to 6-week intervals to complete the primary series.⁴⁷ Because a small but significant number of horses experience serum sickness and fatal hepatic failure (serum hepatitis) several weeks after receiving tetanus antitoxin,^48,49 a preferred approach to the unvaccinated horse that sustains a puncture or deep laceration is to thoroughly clean and debride the wound, initiate active immunization by administering tetanus toxoid, and institute a course of antimicrobial treatment with penicillin or alternate antimicrobial that is active against C. tetani.

Equine Encephalomyelitis (Sleeping Sickness)

The equine encephalomyelitis viruses (EEE, WEE, and VEE) belong to the Alphavirus genus of the family Togaviridae. They are transmitted by mosquitoes, and infrequently by other bloodsucking insects, to horses from wild birds or rodents, which serve as natural reservoirs for these viruses. Risk of exposure and geographic distribution of the encephalomyelitis viruses vary by season and from year to year with changes in distribution of insect vectors and wildlife reservoirs. The distribution of EEE has historically been restricted to the eastern, southeastern, and some southern states with recent northward encroachment. WEE has caused minimal disease in horses in North America during the last two decades; however, the virus continues to be detected in mosquitoes and birds throughout the Western states. In the past, outbreaks of WEE have been recorded in the western and midwestern states, with sporadic cases in the Northeast and Southeast United States. Because EEE, WEE, or both are endemic in most areas of North America, vaccination against these diseases should be part of the core vaccination program for all horses. VEE is a reportable foreign animal disease. Epidemics of VEE occur when the virus undergoes genetic change and develops greater virulence for avian and mammalian hosts. These viral variants are able to multiply to high levels in the horse, and then the horse becomes a reservoir for infection in these outbreaks. VEE occurs in South and Central America but has not been diagnosed in the United States or Mexico for many years; therefore routine vaccination of horses in these regions against VEE is not recommended at this time, unless transportation to endemic areas is planned.

Available vaccines are formalin inactivated, adjuvanted, bivalent whole-virus products containing EEE and WEE (Encevac with Havlogen, Intervet; Encephaloid Innovator, Fort Dodge; Cephalovac EW, Boehringer Ingelheim), or trivalent products that also contain VEE (Cephalovac VEW, Boehringer Ingelheim). Veterinarians and horse owners often use combination products containing other antigens, such as tetanus, influenza, WNV, or EHV for primary or booster immunization of horses against encephalomyelitis viruses. Although correlates for protection against EEE, WEE, and VEE are not well established, circulating antibodies are assumed to be important because infection is acquired by vascular injection (mosquito bites) and current inactivated vaccines appear to have good efficacy.^50,51 A study evaluating the serologic response of horses to commercial encephalomyelitis-tetanus combination vaccines showed that the EEE neutralizing antibody responses to Encevac T (Intervet, Carbopol adjuvant) and Equiloid (Fort Dodge, squaline and surfactant adjuvant) were of greater magnitude and persistence than responses to Cephalovac EWT (Boehringer Ingelheim, saponin adjuvant).²⁰ However, no comparative randomized challenge studies have been performed using these vaccines to document whether differences in serologic responses equate to differences in efficacy. Early testing of bivalent (EEE/WEE) vaccines was performed by intracranial challenge with either EEE or WEE; the formalin inactivated preparations demonstrated 100% protection.

Primary immunization of unvaccinated adult horses is accomplished by administering two doses of inactivated vaccine 3 to 6 weeks apart. In areas where EEE is not a threat and mosquito vectors are active for less than 6 months of the year, annual revaccination in the spring, before the peak insect vector season, is recommended. In areas such as the Gulf States where EEE is endemic and mosquitoes are active virtually year-round, many veterinarians prefer to revaccinate horses semiannually to ensure more uniform protection throughout the year. Inactivated encephalomyelitis vaccines are considered to be safe for use during pregnancy; therefore booster vaccination of pregnant mares 4 to 8 weeks before foaling is routinely recommended to enhance colostral concentrations of specific immunoglobulins. Neutralizing antibodies to WEE and EEE are transferred passively to foals through colostrum and decline with an estimated half-life of 33 and 20 days, respectively. MDAs appear to confer protection and are detectable in the serum of many foals from vaccinated mares for at least 3 months and up to 7 months, depending on the postnursing titer.^34,52-54

Several studies have shown that MDAs exert a profound inhibitory effect on the ability of foals to mount serologic responses to inactivated bivalent WEE/EEE vaccines, which likely accounts for some of the reported cases of vaccine failure and resultant clinical EEE in vaccinated horses, particularly those less than 2 years of age.^{30,34,35,52,53} Studies have shown that 3-month-old foals born to immune mares consistently failed to mount a serologic response to two doses of inactivated bivalent WEE/EEE vaccine and the majority had not responded even after administration of a third dose.^35,37 Whereas many 6-month-old foals failed to seroconvert after administration of two doses of vaccine, most responded after administration of a third dose.³⁵ Based on these data, inclusion of a third dose in the primary series, 8 to 16 weeks after administration of the second dose, is strongly recommended for primary immunization of foals and yearlings.

WEE has a lower mortality rate than EEE, and prevalence of WEE in many western states is sufficiently low that the risk of foals acquiring infection during their first year of life is also low. Therefore primary vaccination of foals of vaccinated mares in areas where mosquitoes die off in the winter and the risk of infection is low is best completed when foals are 5 to 6 months of age or older in order to minimize the potential for MDA interference. Because foals born in the late spring and summer months are still less than 6 months of age by the time the mosquito season comes to an end in many regions, primary vaccination of these foals can be delayed until the spring of the yearling year. In contrast, EEE is a highly fatal disease that poses a significant risk to foals during their first year of life, particularly in the Gulf States, where competent vectors are present year-round.^30,53,55 Therefore most veterinarians in these regions recommend commencing primary vaccination of foals at 3 to 4 months of age using a three-dose primary series followed by a fourth dose before the onset of the next mosquito season and semiannual boosters thereafter, to maximize the chances of overcoming the inhibitory effects of MDAs and inducing protection.⁵³

West Nile Virus

In the few years since WNV infection was first diagnosed in horses in the northeastern United States in 1999, it has spread across the entire North American continent and is now considered to be endemic in all mainland areas of North America and Mexico, where it has become an important consideration in the differential diagnosis of horses with signs of neurologic disease. As of June 2007 the disease had been confirmed in almost 25,000 horses in the United States, approximately 35% of which had died or been euthanized. Approximately 40% of horses that survive acute illness caused by WNV exhibit residual effects, such as gait and behavioral abnormalities, 6 months postdiagnosis.⁵⁶

WNV, a member of the family Flaviviridae, is transmitted by mosquitoes and infrequently by other bloodsucking insects to horses, human beings, and a number of other mammals from avian hosts, which serve as natural reservoirs for these viruses. Horses and humans are considered to be “dead-end” hosts of the WNV and therefore do not contribute to the transmission cycle. The virus is not directly contagious from horse to horse or from horse to human. Similarly, indirect transmission via mosquitoes from infected horses is highly unlikely because horses do not experience a significant level of viremia.⁵⁷ Risk of infection and death appears to increase with increasing age; however, the disease has been confirmed in foals as young as 3 weeks of age. Although cases have been seen virtually year-round in the southeastern United States, the risk of acquiring infection is highest during those months in which mosquito activity peaks, typically July, August, September, and October in most areas of North America. WNV infection is a core disease against which all horses residing in the continental United States and Canada should be vaccinated.

As of June 2007, three fully licensed vaccines (West Nile—Innovator, Fort Dodge Animal Health; Recombitek, Merial; and PreveNile, Intervet) were marketed for use in horses in North America. West Nile—Innovator is an inactivated whole virus vaccine that contains a metabolizable oil adjuvant.⁹ This vaccine is available as either a monovalent (single component) or as a multivalent vaccine containing other encephalitis virus antigens (EEE and WEE). Recombitek is a Carbopol-adjuvanted canarypox-vectored recombinant modified live vaccine,^11,12,17 and PreveNile is a nonadjuvanted chimeric yellow fever—vectored vaccine.^15,58 A fourth vaccine, a plasmid DNA vaccine with a metabolizable oil adjuvant (Fort Dodge Animal Health) was licensed in 2005 but had not been marketed as of June 2007.¹³ All four vaccines have met USDA requirements for safety in tests, each involving more than 640 horses.

Needle and mosquito challenge models have shown that West Nile—Innovator, Recombitek, and the plasmid DNA vaccine all significantly reduce the magnitude of viremia in experimentally infected, vaccinated horses compared with unvaccinated control horses for as long as 12 months after primary vaccination with two doses of vaccine.^9,11,13,59 Although viremia was reliably induced in unvaccinated control horses in these challenge models, clinical disease was not. Therefore West Nile—Innovator and Recombitek are labeled as aids to the prevention of viremia caused by WNV infection. In contrast, an intrathecal challenge model that reliably induced severe clinical disease was used to test the efficacy of PreveNile in studies for licensure.^15,16 In this model a single dose of PreveNile prevented clinical disease as well as viremia in 4- to 6-month-old horses challenged 1 year after vaccination; therefore PreveNile is labeled for protection against viremia and as an aid in the prevention of disease and encephalitis caused by WNV.^15,16 Subsequently, Recombitek was shown to induce a high level of clinical protection when tested using this rigorous intrathecal challenge model in a placebo-controlled study in which horses were challenged 14 days after completion of a two-dose vaccination series.¹⁷ The comparative efficacy of West Nile—Innovator, Recombitek, and PreveNile has now been tested in a randomized, blinded, placebo-controlled intrathecal challenge study in which groups of five or six horses ≥6 months of age were challenged intrathecally 28 days after completion of the two-dose (West Nile—Innovator and Recombitek) or one-dose (PreveNile) primary vaccination series.⁵⁹ In this study, all six unvaccinated control horses developed grave neurologic signs postchallenge, whereas all vaccinated horses survived and none developed detectable viremia. Clinical disease was prevented in 100% of PreveNile-vaccinated horses, 80% of Recombitek-vaccinated horses, and 33% of Innovator-vaccinated horses. These findings support the results of field studies that provide clear evidence that, when used according to manufacturer recommendations, both West Nile—Innovator and Recombitek reduce the risk of disease and death after natural challenge, although clinical disease may not be fully prevented.^60-62

Directions for primary immunization using West Nile—Innovator and Recombitek include administration of two doses of vaccine 3 to 6 weeks apart (consult the specific label). Optimal protection cannot be expected until 2 weeks after administration of the second dose, although Recombitek has been shown to induce significant protection as early as 26 days after administration of the first dose when tested in both the mosquito challenge and intrathecal challenge models.^12,17 Primary immunization with PreveNile requires one dose. A challenge study in yearlings showed that 83% (five of six) were protected when challenged intrathecally 10 days after vaccination with one dose, indicating that onset of immunity is rapid (Vaala W, personal communication, 2007). Rapid onset of immunity is an important feature when faced with the challenge of protecting naive horses that are being introduced into an endemic area, as is the case when horses from Europe and other nonendemic countries are imported into North America.

Vaccine manufacturers recommend revaccination of previously vaccinated horses on an annual basis, or more frequently when local conditions are conducive to a prolonged period of potential exposure to infected mosquito vectors. Annual revaccination is best completed in the spring (late February through early April), before the onset of the insect vector season. In areas such as the southeastern states where the mosquito season is prolonged, revaccination twice annually, once in the spring and again in the late summer or early fall (late July through early September) has been advocated in the past to maximize protection, although the rationale for semiannual vaccination against WNV has not been tested in controlled studies.

None of the licensed vaccines currently marketed in the United States carry label recommendations for administration to pregnant mares; therefore it is recommended that mares be vaccinated before breeding whenever possible. It is well recognized, however, that pregnant mares are at risk for acquiring infection from infected mosquitoes. Consequently it has become accepted practice by many veterinarians to administer vaccines to pregnant mares on the reasonable assumption that the risk of adverse consequences of WNV infection far exceeds the reported adverse effects of use of vaccines in pregnant mares. Thousands of doses of West Nile—Innovator vaccine have been administered safely to pregnant mares, and a published study failed to document vaccine-associated adverse effects in a large population of pregnant mares.⁶³ Although the Recombitek vaccine is a live vectored vaccine, the canarypox vector is incapable of replication in mammals and does not induce a viremia that could infect a fetus. In addition, a canarypox-vectored influenza vaccine available in Europe is licensed for use in horses during pregnancy; therefore the vectored WNV vaccine is unlikely to be associated with an increased risk of adverse effects in pregnant mares. Similarly, data currently under review by the USDA from studies involving a large number of pregnant mares suggest that PreveNile will likely also be shown to be safe for use in pregnant mares (Vaala W, personal communication, 2007). As with other vaccines, it is sound practice to avoid administering West Nile vaccines to mares during the first 60 days of gestation unless conditions of imminent risk prevail.

Booster vaccination of previously primed pregnant mares 4 to 8 weeks before foaling appears to induce a strong anamnestic serologic response that provides their foals with passive colostral protection lasting at least 3 to 4 months.³⁷ In contrast, a significant proportion of naive pregnant mares failed to seroconvert when the primary series of WNV—Innovator vaccine was administered during the second half of gestation, perhaps reflecting pregnancy-associated downregulation of Th2 responses.³⁷ This observation adds further justification to the recommendation that when inactivated West Nile—Innovator vaccine is used, the primary series is best completed before breeding. In a similar study, pregnancy did not appear to suppress the response of mares to primary immunization with Recombitek (Wilson WD and colleagues, unpublished observations, 2007).

In contrast to findings with many other vaccines in the foals of immune mares, MDAs do not block the response of foals as young as 3 months of age to vaccination with either the inactivated or the recombinant vaccine.³⁷ Although this finding is somewhat surprising for the inactivated vaccine, it might reasonably have been expected for the recombinant vaccine because the canarypox vector system accomplishes transfection of cells and expression of the major E-peptide and M-peptide antigens of WNV on the surface of antigen presenting cells (APCs) in association with major histocompatibility complex (MHC) class I and class II antigens. These peptide antigens are therefore not free in the tissues and circulation to be neutralized by MDAs.

Primary vaccination of foals from properly vaccinated mares can be started by administration of the first dose of either West Nile—Innovator or Recombitek as early as 3 to 4 months of age, followed by a second dose approximately 1 month later, then a third dose 8 to 16 weeks after the second dose. This third dose increases the likelihood that foals with high MDA levels, which may have attenuated the response to the first dose of vaccine, will become primed and protected. Even in foals that have no maternally derived WNV antibodies after nursing, the third dose of inactivated vaccine in the primary series induces significantly higher and more persistent levels of antibody than do two doses. A booster should be administered during the spring of the yearling year, after which the recommendations for vaccination of adult horses should be followed. Primary vaccination of foals from unvaccinated, unexposed mares should commence at 3 months of age or younger (as early as 1 month of age), depending on month of birth and seasonal level of activity of mosquito vectors in the area. The three-dose primary vaccination protocol previously outlined should be followed. Revaccination should be performed before the onset of the next mosquito season.

The influence of MDAs on the response of foals to PreveNile has not been established, but the product is labeled for administration of a single priming dose to foals 5 months of age or older. A second dose of vaccine should be administered before the onset of the next mosquito season. Considering the rapid onset of immunity induced by this vaccine, protection can likely be accomplished at a similar age to that induced when the primary series with the inactivated or canarypox-vectored vaccines is started at 3 to 4 months of age. Preliminary data suggest that the plasmid DNA WNV vaccine may also circumvent the potentially interfering effects of MDAs.¹³

Horses that have recovered from clinical WNV infection will likely be protected for the remainder of their lives and should not need to be revaccinated unless changes in their immune status, as might occur with prolonged corticosteroid administration, alter their susceptibility to infection.⁶⁴

It is remarkable that in little more than 6 years after WNV disease was first encountered in the Americas, four vaccines with documented efficacy based on challenge studies have been licensed for the benefit of horses, including three that apply the most modern technologies available for either animals or humans at this point in time.

Rabies

Rabies is an infrequently encountered neurologic disease of equids resulting from inoculation of the rabies virus through the bite of infected (rabid) wildlife. Wildlife species that serve as the natural reservoirs for infection with this rhabdovirus differ among regions of North America but include raccoons, foxes, skunks, and bats. Horses most often sustain bites on the muzzle, face, or lower limbs. The rabies virus then migrates via nerves to the brain, where it initiates rapidly progressive encephalitis. Even though the incidence of rabies in horses is low, the disease is invariably fatal and has considerable public health significance. All horses kept in areas where rabies is endemic in the wildlife population are at risk and should be vaccinated as part of the core vaccination program. Therefore it is recommended that horses be vaccinated against rabies by, or under the direct supervision of, a veterinarian using one of the three inactivated, tissue culture—derived products currently licensed for use in horses (Rabvac 3, Fort Dodge; RM Imrab 3, Merial; and Rabguard TC, Pfizer). These vaccines are potent immunogens that induce strong serologic responses that peak within 28 days after intramuscular administration of a single dose.

Although correlates for protection against infection with rabies virus in horses are not well defined, it is logical to assume that protection correlates with titers of circulating antibody. In humans, postvaccination antibody titers are used to predict protection. In dogs, however, postvaccination serologic test results were not found to be completely predictive of resistance to challenge exposure during tests performed with certain inactivated vaccines.⁶⁵ Challenge studies demonstrating efficacy are required for licensing of all rabies vaccines, including those labeled for use in equids in the United States; however, published results are not available. The challenge studies are conducted by the vaccine manufacturers as outlined in the Code of Federal Regulations (CFR) from the USDA. These studies indicate a DOI of 12 months, and a minimum of 80% of vaccinated animals must be resistant to severe challenge with rabies virus.

For primary immunization, label directions on inactivated rabies vaccines licensed for use in horses suggest administration of one dose to horses age 3 months or older followed by a second dose 1 year later. Thereafter, annual revaccination is recommended. Although none of the licensed vaccines carries a specific label approval for use in pregnant mares, it is important to acknowledge that only a limited number of equine vaccines are specifically licensed for use in pregnant mares, and veterinarians do administer inactivated rabies vaccines to pregnant mares. Alternatively, veterinarians may recommend that mares be vaccinated against rabies before breeding in order to reduce the number and type of vaccines given in the period before foaling. Because rabies antibodies persist in serum for a prolonged period, foals born to mares that are revaccinated while open acquire substantial titers of rabies antibody after ingesting colostrum.

Documentation of rabies in reportedly vaccinated horses, most of which were less than 2 years of age, has brought into question the efficacy of label recommendations for primary vaccination of foals against rabies.⁶⁶ Recent studies in our laboratory have shown that the serologic response of most 3-month-old foals from antibody-positive mares is completely blocked, even when a two-dose primary vaccination series is used. Although the response to the first dose of vaccine is typically blocked in 6-month-old foals from antibody-positive mares, these foals appear to seroconvert after administration of a second dose 4 weeks later. Primary vaccination of foals from vaccinated mares should therefore be delayed until they are 6 months of age or older and should include 2 doses of inactivated vaccine administered approximately 4 weeks apart, followed by a third dose at 1 year of age. For foals from unvaccinated mares the primary vaccination series can be started according to manufacturers’ recommendations as early as 3 months of age and may consist of only one dose, although a two-dose series will likely induce more durable immunity.

Equine Influenza

Infection of the respiratory tract of horses with the orthomyxovirus influenza A/equine/2 (H3N8), remains one of the most common causes of rapidly spreading outbreaks of respiratory disease, despite the widespread practice of frequently revaccinating horses with inactivated vaccines by intramuscular injection. The influenza A/equine/1 subtype (H7N7) has not been recognized as a cause of clinical disease for many years and is likely extinct in nature. Influenza is endemic in the equine populations of the United States and much of the world, with the notable exceptions of New Zealand and Iceland. Rapid national and international transportation of horses facilitates spread of the virus. Concentrating young horses at racetracks, training facilities, boarding stables, breeding farms, shows, or similar athletic events increases the risk of infection, as does a low serum concentration of specific antibody.⁶⁷ Older horses are generally less susceptible to infection but may become ill when partial protection is overwhelmed by exposure to horses excreting large amounts of virus. Explosive outbreaks occur at intervals of several years when the immunity of the equine population wanes and sufficient antigenic drift has occurred to generate a new viral strain. In contrast to herpesviruses, equine influenza virus is not maintained in asymptomatic carrier horses and does not circulate constantly, even within large groups of horses. Rather, the disease is introduced sporadically by a symptomatic or asymptomatic infected horse. This epidemiologic finding and the rapid elimination of the virus by the equine immune response suggest that infection can be avoided by preventing entry of the virus into an equine population (e.g., by quarantine of newly arriving horses for at least 14 days) and by appropriate vaccination.⁶⁸

Equine influenza virus is highly contagious and spreads rapidly through groups of horses in aerosolized droplets dispersed by coughing. Contaminated buckets, grooming or feeding equipment, tack, and transport vehicles may serve as fomites because the virus can survive for hours on such objects. Severity of clinical signs of influenza, which include nasal discharge, fever, lethargy, anorexia, cough, and myalgia, depends on the degree of existing immunity and other factors. Infected horses shed virus for up to 10 days in their nasal secretions. Inactivated vaccines do not induce sterile immunity; therefore recently vaccinated horses can become infected, shed virus, and contribute to interepidemic persistence of infection within the equine population and propagation of infection during outbreaks.¹⁹

Immunity to the same (homologous) strain of H3N8 virus after natural infection persists for more than a year and involves both local and systemic humoral and cellular mechanisms These include induction of large amounts of virus-specific neutralizing IgG and secretory IgA antibody in nasal secretions, high levels of circulating IgG antibodies, and genetically restricted antigen-specific cytotoxic T lymphocytes (CTLs) that kill infected cells.^69-73 Memory CTLs can be detected in peripheral blood for at least 6 months after infection, and solid immunity persists even when circulating antibody titers have declined to low or undetectable levels.^70,71,74,75 Similarly, protection induced by the licensed modified live intranasal influenza vaccine (Flu-Avert IN, Intervet) is presumably mediated through induction of local immune responses in the respiratory tract, because this vaccine does not typically induce high levels of circulating antibody.^6,8 With the possible exception of ISCOM vaccines, inactivated vaccines administered by intramuscular injection have limited potential to induce CTL or nasal secretory IgA responses and induce only low levels of neutralizing antibody in nasal secretions.^69,75,76 The degree of protection induced by inactivated influenza vaccines is highly correlated with postvaccination titers of circulating antibody, predominantly of the IgGa and IgGb subisotypes, as measured by HI or single radial hemolysis (SRH) tests.^67,77-80 SRH levels ≥100 mm² are considered to be at least partially protective; however, levels >140 mm² are required for successful prevention of disease.⁷⁹ The partial protection induced by inactivated vaccines is of limited duration (up to approximately 7 months, depending on the vaccine) and is manifested as a reduction in clinical signs and attenuation of viral shedding in horses exposed to infection.^68,69

The magnitude of the serologic response to inactivated influenza vaccines depends on many factors, the most important of which are the quality and quantity (mass) of the viral antigen and the choice of the adjuvant.^79,81,82 Carboxypolymer-based compounds (carbomer, Carbopol) and ISCOMs are contained in some of the most efficacious inactivated influenza vaccines, whereas some commonly used adjuvants such as alum have been associated with induction of unproductive immune responses.^69,81 History of previous vaccination or infection, interval since the last dose of vaccine, antibody titer at the time of vaccination, age, maternal antibody status, and relatedness of the vaccine strain to circulating field strains of influenza virus are other important determinants of efficacy, at least for inactivated influenza vaccines.^77,83,84 Antigenic drift of the A/equine/2 subtype has resulted from point mutations in the genes encoding the amino acid sequences of the hemagglutinin (H) and neuraminidase (N) glycoprotein antigens on the surface of the virus. The result is emergence of viral strains representing two antigenic lineages, American and Eurasian, of the H3N8 virus. Further antigenic drift within each lineage has generated variants that, as with the prototypic strain A/equine/2/Miami 63, are named according to the location and year in which they were first isolated. Antigenic drift, by generating antigenically heterologous viruses, reduces the degree and duration of protection conferred by previous infection or vaccination because of the specificity of immunoglobulins, and it allows horses with high titers to become infected and develop clinical signs of disease if the vaccine strain is not closely related to the drifted infectious field strain.⁸⁵ Although antigenic drift of equine influenza viruses is slower than that of human influenza viruses, it is recommended that inactivated equine influenza vaccines include viral antigens from isolates obtained within the most recent 5 years and, ideally, representatives of both the American and Eurasian lineages. An expert surveillance panel meets annually to recommend strains that should be included in influenza vaccines in subsequent years (www.equiflunet.org.uk). In order to comply with federal regulations for licensing and marketing of vaccines, any change of a vaccine, such as including the most recently isolated influenza virus, usually leads to costly and time-consuming evaluation of the revised product. Consequently, viral antigens contained in inactivated vaccines typically lag more than the recommended 5 years behind the antigenic drift of field viruses, resulting in suboptimal protection. Even though Flu-Avert IN contains only a 1991 H3N8 strain of North American lineage, it has been shown to be protective against challenge with Eurasian strains and recently isolated North American strains.

The short-lived immunity after vaccination with inactivated equine influenza vaccines was the impetus for past recommendations for frequent revaccination, at intervals as short as 2 months. However, too short an interval between revaccination may compromise efficacy because influenza vaccination in a horse with a high antibody titer inhibits development of an optimal anamnestic response.⁸⁶ An additional consideration that potentially limits the efficacy of influenza vaccines is the phenomenon termed “original antigenic sin,” whereby horses exposed to a drifted field A/equine/2 virus will mount an anamnestic immune response directed more strongly against the strain with which they were vaccinated initially than against the drifted field virus.⁸²

A considerable amount of published efficacy data, based both on challenge studies and on field epidemiology studies, has been available for many years in Europe to support the use of influenza vaccines. In contrast, information regarding the efficacy of influenza vaccines marketed in North America has remained sparse until recently. Furthermore, studies conducted in North America during the late 1990s showed that the inactivated influenza vaccines in use at the time failed to provide much benefit in terms of reducing the risk of infection and clinical disease during field outbreaks.^19,67 Serologic testing performed during these and other studies indicated that vaccine failure was caused by failure of the influenza vaccines in use at the time to induce protective antibody titers.^19,67,87

Fortunately, vaccine manufacturers in North America have responded to the challenge of producing more efficacious equine influenza vaccines during the last few years by incorporating more relevant recent viral strains, by increasing antigenic mass of relevant strains, by eliminating the seemingly irrelevant H7N7 strain, by modifying adjuvant systems, and by introducing novel technologies. An important advance occurred in 1999 when Heska Corporation marketed an attenuated live, cold-adapted influenza vaccine (Flu-Avert IN, Intervet, Millsboro, Del.) for intranasal administration. This vaccine, which contains a Kentucky/1991 strain of North American lineage, was found to be highly efficacious in blinded, controlled challenge studies conducted 5 weeks, 6 months, and 1 year after administration of a single dose to naive horses.⁸ Subsequently Flu-Avert IN was shown to cross-protect against European H3N8 strains, as well as against North American strains isolated during the late 1990s and early 2000s, and to induce a rapid onset of protection within 7 days of administration of a single dose to naive horses.^6,88 Although horses challenged 1 year after administration of a single dose showed a significant, but only partial, reduction in severity of clinical signs and virus shedding, a more marked reduction in clinical signs and viral shedding was found when the challenge was performed 6 months after vaccination.⁸ Based on these results, revaccination at 6-month intervals is recommended. Field experience indicates that this regimen induces solid clinical protection after natural challenge. Currently Flu-Avert IN is licensed for use in nonpregnant horses 11 months of age or older, primarily because this was the youngest age of the horses used in the challenge studies for licensing. Horses may shed small amounts of vaccinal virus for several days after vaccination with Flu-Avert IN, but the amount of virus shed is so low that in-contact horses will not generally become infected or immunized with vaccinal virus shed by recently vaccinated horses, and the likelihood of reversion to virulence is extremely low.⁷

Recently updated inactivated influenza vaccines have demonstrated good efficacy in challenge studies. Inactivated influenza vaccines containing one or more relevant H3N8 strains are currently marketed by Boehringer Ingelheim (Calvenza EIV) and Fort Dodge Animal Health (Fluvac Innovator). These and other companies also market a large number of multicomponent combination vaccines that contain the same inactivated influenza antigens as are in their single-component products but also contain tetanus, WEE and EEE virus, EHV, or WNV antigens. Calvenza EIV is adjuvanted with Carbopol and is the only inactivated vaccine currently licensed in North America that contains antigens from H3N8 viruses of both the American and European lineages (Kentucky/95 and Newmarket/2/93).

The initial two doses of this vaccine are administered IM; subsequent doses may be administered IM or intranasally. It is proposed, but not proved, that administration of booster doses by the intranasal route may provide a stronger local mucosal immune response. This vaccine is licensed for use in horses older than 6 months of age, including pregnant mares. Fluvac Innovator contains a KY/97 H3N8 strain in a metabolizable oil (MetaStim) adjuvant.

In late 2006, Merial was granted a North American license to market an injectable canarypox-vectored recombinant equine influenza vaccine that has been used with success in Europe for several years. This vaccine, named Recombitek Equine Influenza Virus vaccine, has been shown to induce strong protection in challenge studies and shows great potential to have a positive impact on influenza prevention in North America.¹⁴ The vaccine incorporates the HA gene from the Kentucky/94 and Newmarket/2/93 H3N8 strains into the same vector delivery platform as the efficacious WNV virus vaccine (Recombitek) and contains a carbomer polymer adjuvant in the diluent.¹⁴ Consequently, this vaccine invokes a broad array of humoral and cellular immune responses. Challenge studies document onset of protection as soon as 2 weeks after completion of a two-dose primary series and persistence of solid protection for at least 5 months. Administration of a booster dose at 5 months induced a strong anamnestic response that provided solid protection persisting for at least 12 months.¹⁸ Preliminary evidence suggests that this canarypox-vectored influenza vaccine will be able to circumvent the inhibitory effect of maternal antibodies, an issue that significantly affects primary immunization of foals using inactivated influenza vaccines.⁸⁹ Recombitek Equine Influenza Virus vaccine is licensed for vaccination of healthy horses as young as 5 months of age.

Vaccination of Foals

The antibody status of a mare at the time of foaling is the main determinant of the postnursing circulating antibody titer in her foal and therefore has a profound impact on the ability of the foal or weanling to respond to influenza vaccines administered during the first year of life. Foals born to seronegative, unvaccinated mares respond appropriately to influenza vaccines; therefore primary vaccination can commence at 3 months of age or younger if significant risk of exposure to influenza exists. In contrast, maternal antibodies have been shown to completely block the serologic response of foals to a primary immunization series composed of two or more doses of inactivated influenza vaccines when the first dose is administered when the foal is younger than 6 months of age.^23-2935 Interference from MDAs may persist until 9 months of age or beyond for foals with high antibody titers postnursing; therefore primary vaccination of foals from immune mares should be delayed as long as possible, and preventive measures should focus on preventing introduction of infected horses.* Studies in Newmarket, United Kingdom, have shown that influenza virus infection is rare in thoroughbred yearlings before they enter training, suggesting that the risk of influenza is low in horses younger than 1 year of age born to mares in herds that are well vaccinated.^84,91,92 Therefore there appears to be little justification to vaccinate young foals from vaccinated mares against influenza, as was recommended in the past.^54,93,94

The intranasal modified live vaccine (Flu-Avert IN) is licensed for vaccination of horses 11 months of age or older. Whereas this vaccine has been shown to be safe in foals as young as 2 months of age,⁹⁵ published data regarding the potential for MDAs to interfere with the response are lacking. Unpublished observations suggest that MDA interferes with the response of foals aged 3 to 6 months, whereas foals with maternal antibody vaccinated at 7 months of age were protected against virulent challenge (Holland and Chambers, personal communication, 2000). Pending publication of well-controlled studies, it is recommended that if the first dose of Flu-Avert IN vaccine is administered before 11 months of age, a second dose should be administered at 11 months of age or older.⁹⁶ The European-licensed live canarypox-vectored recombinant influenza is labeled for use in pregnant mares and foals as young as 4 months of age.⁷⁵ The North American—licensed Recombitek Equine Influenza Virus vaccine has been shown to be safe in foals as young as 4 months, but the minimum age recommended for vaccination of foals from immunized dams is 5 months. Effective priming has been documented after administration of the first dose of the vectored vaccine to foals aged 10 to 20 weeks that had detectable MDAs at the time of vaccination.⁸⁹ If the foal experiences failure of passive transfer of maternal antibodies or if the mare is seronegative for influenza, vaccination can commence at 4 months of age but should include an additional dose in the primary series.

The decision whether to vaccinate in an outbreak is dependent on many factors, the most important of which are the age, vaccination status, and size of the population of horses at risk; the elapsed time since onset of the outbreak; the rapidity with which a diagnosis can be confirmed; the layout of the physical facilities; and availability of personnel. Rapid (same-day) diagnosis of influenza should be pursued during outbreaks of contagious respiratory disease and can be accomplished using the highly sensitive and specific polymerase chain reaction (PCR) test or antigen-capture ELISA. Outbreaks of influenza at racetracks and similar large facilities typically take 1 month or more to spread through the entire population; therefore sufficient time exists to enhance immune protection of many at-risk horses while implementing other management strategies to minimize disease spread.⁸⁸ It is prudent to booster vaccinate those horses that have been on a regular influenza vaccination program but have not been revaccinated within the previous 3 months. It is also important to induce protection as quickly as possible in horses that have not previously been vaccinated. Of the vaccines currently available, Flu-Avert IN induces protection most rapidly, within 7 days of administration of a single intranasal dose; therefore this is currently the product of choice for vaccination of naive horses and those of unknown vaccination status in the face of an outbreak.⁸⁸ There is no evidence to suggest that any adverse effects occur when Flu-Avert IN is administered to horses that are incubating infection, although vaccination of horses that are already clinically ill is not recommended. Preliminary evidence suggests onset of immunity within 14 days of administration of one dose of the canarypox-vectored vaccine; therefore use of this vaccine would likely also prove useful in controlling outbreaks.

Equine Herpesvirus (Rhinopneumonitis)

The respiratory tract is the primary route of infection for both EHV-1 and EHV-4, both of which cause respiratory tract disease that varies in severity from subclinical to severe and is characterized by fever, lethargy, anorexia, nasal discharge, and cough.¹⁰⁰ Seroepidemiologic studies indicate that the vast majority of foals become infected with EHV-1 and EHV-4 during the first few months of life, but the clinical disease syndromes resulting from these infections are not always well defined, perhaps reflecting the modulating effect of MDAs.^101-103 Recurrent or recrudescent clinically apparent infections are seen in weanlings, yearlings, and young horses entering training, especially when horses from different sources are comingled.^100,104 In contrast, surveillance studies involving racehorses document that seroconversion to both EHV-1 and EHV-4 occurs sporadically during the course of a racing season, but these seroconversions are often not clearly associated with outbreaks of respiratory disease that follow an epidemiologic pattern consistent with an infectious agent.^105,106 EHV-1 and EHV-4 are spread by direct and indirect (fomite) contact with nasal secretions, by aerosolized secretions from infected horses, and, in the case of EHV-1, by aborted fetuses, fetal fluids, and placentas associated with abortions. Management practices are therefore of primary importance for control of clinical disease caused by EHVs.

Viremia occurs frequently after infection with EHV-1, potentially leading to paralytic neurologic disease (myeloencephalopathy) secondary to vasculitis of the spinal cord and brain, abortion of virus-infected fetuses, or birth of infected nonviable foals. In contrast, manifestations of infection with EHV-4 (rhinopneumonitis) are generally confined to the respiratory tract because EHV-4 does not typically infect endothelial cells or produce a cell-associated viremia.¹⁰⁷ As with herpesvirus infections in other species, horses typically fail to clear primary infections with either EHV-1 or EHV-4, the result being that most horses in the population remain latently infected with both viruses.^100,103,108 Latently infected horses do not show clinical signs but may experience recrudescence of infection, with or without clinical signs, an increase in antibody titer, and shedding of the virus when stressed. Consequently, many horses have detectable levels of virus-neutralizing (VN) antibody to both EHV-1 and EHV-4 in their serum.^101,108 These features of the epidemiology of herpesvirus infections seriously compromise efforts to control these diseases and explain why outbreaks of EHV-1 or EHV-4 can occur in closed populations of horses. Whereas most mature horses have developed some immunity to EHV-1 and EHV-4 through repeated natural exposure and do not typically show respiratory signs when they become reinfected, horses do not appear to become resistant to the abortigenic or neurologic forms of infection with EHV-1, even after repeated exposure.¹⁰⁹ In fact, mature horses previously exposed are more likely to develop the neurologic form of the disease than are juvenile horses.^110,111

Correlates for protection against EHV-1 and EHV-4 infection have been investigated extensively but are not yet clearly defined. Infection with EHV-1 induces a strong humoral response, but protection from reinfection is short-lived and is not achieved until the horse has experienced multiple infections with homotypic virus.^100,107 No clear relationship exists between protection from EHV-1 infection and concentrations of circulating antibody induced by vaccination or infection, but the duration and amount of virus shedding from the nasopharynx is reduced in animals with high levels of circulating neutralizing antibody.¹⁰⁷ Mucosal immunity and cell-mediated responses likely play a role at least as important as circulating neutralizing antibodies in protection against EHV-1 infection,¹¹² because the presence of MHC class 1—restricted CTL precursors in peripheral blood is correlated with protection.¹⁰⁷ Because EHV-4 replication is largely confined to epithelial cells of the upper respiratory tract, it is likely that mucosal immunity is important in protection.¹⁰⁷ Whereas circulating antibodies alone do not prevent EHV-4 infection, high levels of vaccine-induced circulating VN antibody markedly reduce virus shedding and clinical signs after challenge infection.^107,113-115

Various killed vaccines are available, including those licensed only for protection against respiratory disease; currently all contain a low antigen load, and two (Pneumabort-K+1b, Fort Dodge Animal Health, and Prodigy, Intervet) that are licensed for protection against both abortion and respiratory disease contain a high antigen load. Performance of the killed low—antigen-load respiratory vaccines is variable, with some vaccines outperforming others. Performance of the killed high—antigen-load abortion and respiratory vaccines is superior, resulting in higher antibody responses and some evidence of cellular responses to vaccination. This factor may provide good reason to choose the high—antigen-load abortion and respiratory vaccines when the slightly higher cost is not a decision factor. A single manufacturer provides a licensed modified live EHV-1 vaccine, which to date has not been compared directly with high—antigen-load respiratory and abortion vaccines. This modified live vaccine has been shown to offer superior clinical protection and reduce viral shedding in a comparison with a single killed low—antigen-load respiratory vaccine.¹¹⁶ Vaccination with either EHV-1 or EHV-4 can provide partial protection against the heterologous virus, and vaccines containing EHV-1 may be superior in this regard.

The principal indication for use of EHV vaccines is prevention of EHV-1—induced abortion in pregnant mares and reduction of signs and spread of respiratory tract disease (rhinopneumonitis) in foals, weanlings, yearlings, and young performance and show horses that are at high risk of exposure. Many horses do produce postvaccinal antibodies against EHV, but the presence of those antibodies does not ensure complete protection. Consistent vaccination appears to reduce the frequency and severity of herpesvirus-induced disease. Although convincing evidence is lacking, field experience suggests that, whereas the incidence of sporadic EHV-1—induced abortion in individual mares has not changed, the incidence of abortion storms caused by EHV-1 has declined significantly since the introduction and widespread use of EHV-1 vaccines in the United States.^108,109 Outbreaks of abortion and associated perinatal foal death, however, do continue to occur on occasion in herds of vaccinated mares.

Of the vaccines currently licensed for use in pregnant mares in North America, only inactivated monovalent EHV-1 vaccines (Pneumabort-K+1b, Fort Dodge Animal Health, and Prodigy, Intervet) containing abortigenic strains of EHV-1 carry a label claim for preventing abortion, whereas at least one bivalent EHV-1/4 vaccine is licensed for prevention of abortion in Europe (Duvaxyn EHV-1/4, Intervet). One of the vaccines available in North America, Pneumabort-K+1b, incorporates both the 1p and 1b subtypes of EHV-1 to reflect the documented increase in the proportion of EHV-1 abortions caused by the 1b subtype that occurred during the 1980s as compared with earlier years.¹¹⁷ Pregnant mares should be vaccinated during the fifth, seventh, and ninth months of gestation. Many veterinarians also recommend a dose during the third month of gestation. Similarly, vaccination of mares with an inactivated EHV-1/EHV-4 vaccine at the time of breeding and again 4 to 6 weeks before foaling is commonly practiced to enhance concentrations of colostral immunoglobulin for transfer to the foal. However, no published reports document the effectiveness of this approach in raising titers of specific antibody in mares that have already been vaccinated against EHV-1 three times during the previous 5 months. Vaccination of barren mares and stallions with either a bivalent EHV-1/4 vaccine or a monovalent EHV-1 vaccine before the start of the breeding season, and thereafter at 6-month intervals, is recommended, with the goal of increasing herd immunity in an attempt to reduce viral shedding and challenge to pregnant mares on breeding farms.¹⁰⁸

The modified live virus EHV-1 vaccine (Rhinomune, Pfizer) has been used as an aid to prevention of EHV-1 abortion by some practitioners for many years,¹¹⁸ even though this vaccine is not currently labeled for this use. However, several recent developments have created a renewed interest in the potential for use of modified live virus vaccines for protecting horses against manifestations of EHV-1 and EHV-4 infection. Sequencing of the EHV-1 genome has made it possible to document the nature of the mutation encoding for attenuation, mediated through truncation of the gp2 glycoprotein, of the KyA strain.¹¹⁹ Similar studies may soon yield information regarding the mutation underlying attenuation of the RAC-H strain from which Rhinomune was derived.

Because currently available inactivated vaccines do not block infection with EHV, the most we can hope for when using inactivated vaccines is reduction of severity of clinical signs and attenuation of virus shedding to help protect herdmates. Challenge studies in weanlings aged 5 to 8 months have clearly demonstrated the efficacy of an inactivated whole virus EHV-1/4 vaccine in reducing clinical manifestations and virus shedding induced by virulent EHV-1 challenge administered 2 weeks after completion of the two-dose primary series.¹¹⁴ Efficacy was clearly correlated with vaccine-induced antibody levels at the time of challenge in this study.¹¹⁴

Specific antibodies against both EHV-1 and EHV-4 are passed in colostrum.^{32,33,36,120,121} Field studies with EHV-1 modified live vaccines indicate that colostral antibodies exert a profound inhibitory effect on serologic responses to vaccination up to at least 5 months of age.^32,122,123 However, a cytotoxic cellular immune response to both EHV-1 and EHV-4 was induced in a substantial percentage of foals vaccinated with an EHV-1 modified live vaccine in the presence of maternal antibody, even though humoral responses were often absent.¹²⁴ It is uncertain whether these responses would provide protection against natural challenge. Recent studies with two different commercially available inactivated bivalent EHV-1/4 vaccines and one inactivated EHV-4/influenza vaccine showed that the majority of foals from EHV-vaccinated mares do not mount a detectable neutralizing antibody response to vaccines administered at 3 and 4 months of age, even when three doses are administered in the primary series.^33,35,36 An increased proportion of foals responded when vaccinated with a three-dose series starting at 5 or 6 months of age, but a substantial number still failed to seroconvert.^35,36 Some foals with low or undetectable levels of SN antibody at the time of vaccination failed to mount a serologic response, suggesting that low levels of antibody, below the lower limit of detection of the SN test based on EHV-1 antigen, are capable of inhibiting the serologic response to inactivated EHV-1/4 vaccines.³⁶ The failure of a large proportion of foals less than 6 months of age to mount serologic responses to inactivated EHV-1/4 vaccines and the influence of antibody titer at the time of vaccination on failure to respond has been confirmed using sensitive gD and gG ELISAs in studies on commercial stud farms in Australia.¹²⁵ In parallel studies, these researchers concluded that mares were the source of infection for foals and that intensive use of inactivated EHV-1/4 vaccines on breeding farms in Australia had minimally affected the infection rate of young foals and weanlings with EHV-1 and EHV-4.^101,103,126

Considering the uncertainty regarding the role of EHV-1 and EHV-4 as causes of clinically important respiratory disease, the lack of published data regarding the efficacy of available vaccines in preventing infection and establishment of latency, and results of a recent study documenting the poor serologic responses of naive horses to a number of killed low—antigen-load EHV respiratory vaccines currently marketed in North America,²⁰ there appears to be little rationale to support the common practice of frequent revaccination of foals, weanlings, yearlings, and young performance horses against EHV-1 and EHV-4.⁵ Furthermore, an obvious dilemma in designing a vaccination strategy to prevent EHV-1 and EHV-4 infection in foals and weanlings is that if primary immunization is delayed until 6 months of age or older to reduce the likelihood of MDA interference, foals are likely to encounter field infection before the three-dose primary series can be completed. Thus it is unreasonable to expect a high degree of efficacy for vaccination programs designed to protect foals and weanlings against EHV infection using available vaccines. Despite these uncertainties, many practitioners elect to vaccinate against both EHV-1 and EHV-4. Under these circumstances, a reasonable compromise would be to start foal vaccination at 4 to 6 months of age using two doses of an inactivated bivalent vaccine or an EHV-1 modified live vaccine administered 3 to 4 weeks apart, followed by administration of a third dose 8 to 12 weeks later. Revaccination at 4- to 6-month intervals thereafter using either an inactivated bivalent vaccine or a modified live EHV-1 vaccine appears appropriate for yearlings and young performance or show horses that experience contact with other horses. Frequent vaccination of nonpregnant mature horses, except those on breeding farms, with EHV vaccines is generally not indicated.

Available vaccines make no labeled claim to prevent the myeloencephalopathic form of EHV-1 infection (EHM). However, recent outbreaks of EHM in populations of horses in several regions of North America have prompted many racing jurisdictions and managers of equine facilities and events to impose EHV-1 vaccination requirements for incoming and resident horses in the hope that EHV-1 infection and development of EHM can be prevented. The efficacy of this approach remains to be proven. In fact, frequent revaccination of mature horses to prevent the neurologic form of EHV-1 is not clearly justified in most circumstances because EHM is a relatively rare disease from a population standpoint and most mature horses have previously been infected with EHV-1 and are latent carriers. Currently available vaccines do not reliably block infection, development of viremia, or establishment of latency, and EHM has been observed in horses vaccinated against EHV-1 regularly at 3- to 5-month intervals with inactivated or modified live vaccines.^{110,111,127,128} Furthermore, vaccination has been cited by some as a potential risk factor for development of neurologic EHV-1, although evidence to support this opinion is far from conclusive.¹²⁹

The genetic basis underlying the apparent increased likelihood that some EHV-1 isolates will cause EHM has only recently been described and involves a single point mutation in the DNA polymerase (DNApol) gene.¹³⁰ This mutation results in the presence of either aspartic acid (D) or an asparagine (N) residue at position 752. More than 80% of EHV-1 isolates associated with EHM are of the D₇₅₂ form, whereas less than 20% are of the N₇₅₂ form.¹³⁰ Isolates of the D₇₅₂ form have been designated “neuropathogenic strains” in recent publications, lay articles, and laboratory PCR result reports, whereas N₇₅₂ isolates have been designated as “wild-type,” “abortigenic,” or “nonneuropathogenic.” The latter terminology is unfortunate because both the D₇₅₂ and the N₇₅₂ isolates are capable of inducing all syndromes (i.e., respiratory disease, abortion, neonatal death, and EHM).

A challenge study performed almost 30 years ago to test the efficacy of Pneumabort K in preventing abortion and a recent study to test the efficacy of Rhinomune against challenge with a “neuropathogenic” strain of EHV-1 provided some evidence that these vaccines may have a place in control of outbreaks of EHM.^113,116 Of interest, the Army 183 EHV-1 strain used as the challenge virus in the Pneumabort K efficacy study has now been shown to carry the D₇₅₂ mutation, as has the Findlay ’03 strain used in the Rhinomune study. However, the low numbers of horses used in these studies, the failure of either vaccine to prevent infection or significantly reduce the level of viremia, the lack of statistical significance of results pertaining to prevention of neurologic signs, and the well-known difficulties encountered in accomplishing a consistent and reproducible challenge model for neurologic EHV-1 infection justify caution in interpretation. However, the significant reduction in viral shedding observed in vaccinated horses provides reasonable justification for booster vaccination of unexposed horses that are at risk for infection in order to reduce viral shedding in the event that they do become exposed to EHV-1. Through enhancement of herd immunity, it is hoped that the level of infectious virus circulating in the at-risk population will be reduced and that, in turn, the risk that individual horses in the population will develop disease will be reduced.¹²⁸ This approach also relies on the assumption that the immune system of most mature horses has already been “primed” by prior exposure to EHV-1 antigens through field infection or vaccination and can therefore be “boosted” within 7 to 10 days of administration of a single dose of vaccine. Although the validity of this approach has not been critically evaluated for the prevention of EHV-1 neurologic disease, its implementation seems rational when faced with one or more horses with confirmed clinical EHV-1 infection (any form) at a particular facility. Whereas booster vaccination of horses that are likely to have been exposed already is not recommended, it is rational to booster vaccinate unexposed horses, as well as those that must enter the premises, if they have not been vaccinated against EHV-1 during the previous 90 days. Use of the Rhinomune modified live vaccine or one of the inactivated EHV-1 vaccines known to stimulate high circulating titers of neutralizing antibody appears justified for this purpose. Horse owners must develop an understanding of the concept of boosting herd immunity to help protect the individual horse rather than focusing on the as yet unattainable expectation that the veterinarian can reliably protect an individual horse from developing potentially fatal EHM by administering one of the vaccines currently marketed as aids to prevention of clinical manifestations of EHV-1 infection. Ultimately, enforcement of strict biosecurity measures and hygiene practices is likely to be more effective than widespread vaccination in reducing the risk of acquiring infection.

Streptococcus equi subsp. equi Infection (Strangles)

Strangles is a highly contagious disease caused by the bacterium S. equi subsp. equi. Strangles primarily affects young horses (weanlings and yearlings), although horses of any age can become infected if not protected by previous exposure to the organism or by vaccination. The organism is transmitted by direct contact with infected horses or subclinical carriers or indirectly by contact with water troughs, feed bunks, pastures, stalls, trailers, tack, or grooming equipment contaminated with nasal discharge or pus draining from lymph nodes of infected horses. The organism survives for several weeks in the environment, particularly in aquatic locations and when protected from exposure to sunlight and disinfectants, and can be a source of infection for new additions to the herd. Because S. equi is a clonal organism, there is minimal antigenic variation among different isolates, even though isolates vary in their pathogenicity.

Most horses develop a solid immunity during recovery from strangles, which persists in over 75% of animals for 5 years or longer,¹³⁷ indicating that induction of durable protection through vaccination is biologically feasible if the protective antigens can be identified and presented in an appropriate manner.¹³⁸ Although the basis for acquired resistance to strangles is not completely understood, the finding that recovered horses rapidly clear intranasally inoculated S. equi despite not making circulating antibody to its surface proteins indicates that to be highly effective a strangles vaccine must stimulate local nasopharyngeal tonsillar immune clearance responses and that serum antibody is of lesser importance.¹³⁹ This conclusion is further supported by the finding that ponies with high levels of circulating antibody to multiple unique surface-exposed and secreted proteins after systemic vaccination remained susceptible to challenge with S. equi.¹³⁹ The cell wall M protein of S. equi (SeM) is recognized in the acquired immune response to S. equi infection, a response that involves both production of local antibodies in the nasopharynx and circulating opsonophagocytic antibodies.^140-142 The predominant opsonophagocytic antibodies are of the IgGb subisotype but also include IgGa and IgA, whereas IgGb and later mucosal IgA predominate in nasopharyngeal secretions.^140,143

Strangles vaccines licensed for use and marketed in North America include two inactivated, adjuvanted M-protein cell wall extracts (Strepvax II, Boehringer Ingelheim, and Strepguard with Havlogen, Intervet, prepared by extraction with hot acid or mutanolysin plus detergent, respectively) and one attenuated live vaccine (Pinnacle IN, Fort Dodge) derived from an unencapsulated mutant of S. equi for intranasal administration.¹⁴⁴ Infection of horses with S. equi continues to cause troublesome outbreaks of strangles throughout North America, despite the availability and widespread use of these vaccines, indicating that their efficacy is suboptimal.¹⁴⁵ M-protein vaccines induce a good opsonophagocytic antibody response in serum but a minimal mucosal IgA response, which likely accounts for the incomplete protection observed when they are used in the field.^140,146 However, data do exist to document that vaccination using injectable SeM vaccines reduces the attack rate and severity of strangles in herds with endemic infection.^146-148 The live intranasal vaccine has been shown to induce a relevant mucosal immune response and partial or complete protection but may do so without inducing a strong serologic response.^145,149 Because vaccinal organisms in the intranasal vaccine must reach the inductive sites for immunity in the pharyngeal and lingual tonsils, accurate vaccine delivery is critical to vaccine efficacy.

Vaccination against S. equi is not routinely recommended for pleasure or performance horses kept in low-risk situations, but it is a consideration for horses that are resident on, or being transported to, premises such as breeding farms where strangles is a persistent endemic problem or where a high risk of exposure is anticipated. The bacterial modified live vaccine is generally preferred over inactivated injectable vaccines for primary vaccination of foals and weanlings and for routine use in older horses that are at high risk for infection. On breeding farms, efforts should be concentrated on preventing infection of foals and weanlings by booster-vaccinating broodmares 4 to 6 weeks before foaling to maximize colostral content of antibodies. Whereas the intranasal vaccine has been shown to be safe for use in mares at all stages of pregnancy and can be used in mares in the face of an outbreak, it does not reliably stimulate high levels of circulating antibody. For this reason, intramuscularly administered inactivated SeM products are preferred for prefoaling booster immunization of mares. Antibodies of the IgG and IgA class recognizing the SeM are passively transferred to the foal through colostrum and are also present in the milk of immune mares.¹⁵⁰ Antibodies of predominantly the IgGb isotype are absorbed from colostrum and redistribute to the nasopharyngeal mucosa.¹⁴³ These IgGb antibodies, along with the SeM-specific IgA antibodies that are present in milk and passively coat the pharyngeal mucosa of nursing foals, provide protection to most nursing foals up to the time of weaning.^142,143,150 Resistance of nursing foals to strangles during the first few months of life appears to be mediated by IgGb antibodies in nasal secretions and milk and not by IgA.¹⁵⁰ Serologic (ELISA) responses to M-protein vaccines are poor in foals, most likely owing to the inhibitory effect of maternal antibodies.

Whereas the intranasal modified live vaccine may be less susceptible than the inactivated extract vaccines to MDA interference, this issue has not been investigated, and the manufacturer does not recommend administration of this vaccine to horses less than 9 months of age. Considering that on farms where strangles is endemic foals often become infected around the time of weaning, at 4 to 8 months of age, it is difficult to protect them if vaccination is delayed until 9 months of age. Therefore a reasonable compromise on breeding farms where the risk of strangles infection is high and mares are on a regular vaccination program would be to begin primary vaccination of foals using the intranasal live vaccine as early as 4 months of age. The recommended two-dose primary series administered 2 to 3 weeks apart should be followed by a third dose 3 to 4 months later and boosters at 6- to 12-month intervals thereafter, depending on risk of infection. The intranasal vaccine has been administered to foals as young as 5 or 6 weeks of age during outbreaks. If a vaccine is used in this manner, a third dose of the vaccine should be administered 2 to 4 weeks before the foal is weaned to optimize protection during this high-risk period. Although there are few reports of adverse effects attributable to use of the intranasal strangles vaccine in young foals, the inability of foals to mount an adequate mucosal IgA response during the first month of life and the potential for interference by maternal antibodies suggest that foals are unlikely to fully benefit from intranasal strangles vaccine administered before 4 months of age. When an inactivated M-protein vaccine is used for primary vaccination of foals, it is recommended that the initial series begin at 4 to 6 months of age, using three doses administered at 3- to 6-week intervals, followed by semiannual boosters for as long as high-risk conditions prevail.

Strangles vaccines should be administered only to healthy, nonfebrile horses free of nasal discharge and should not be administered to those that are known to have had recent direct exposure to clinically ill animals.¹³⁸ However, outbreaks of strangles generally persist for several months to more than 1 year, particularly on breeding farms where each foal crop adds new susceptible animals to the population. Therefore strangles vaccines are frequently administered in the face of an outbreak as an adjunct to management practices designed to bring outbreaks under control, and it is not always possible to accurately determine the exposure status of each horse. Under these circumstances the likelihood of preventing strangles is greatest for horses that have not yet been exposed and can be kept isolated from infected horses until 2 weeks after the vaccination protocol can be completed. Horses that have been vaccinated previously will generate a response more rapidly than will naive horses. Similarly, the intranasal modified live vaccine is preferred over inactivated vaccines for immunization of naive horses in an outbreak because it is likely to generate a protective immune response more rapidly.

Injectable strangles vaccines tend to cause local reactions at the site of injection more often than do other equine vaccines. Injection in the gluteal muscles is not recommended because gravitational drainage along fascial planes can prove troublesome in the event that an abscess develops at the injection site. In addition, purpura hemorrhagica, a serious and sometimes life-threatening systemic immune complex (Arthus-type) vasculitis manifested as edema with or without petechial hemorrhages on mucosal surfaces, has been observed with low frequency in the weeks after administration of strangles vaccines. Inactivated extract vaccines are implicated more often than the intranasal modified live vaccine, but all strangles vaccines have the potential to induce purpura. The antigen present in immune complexes is SeM, along with antibodies of the IgA class. Because a high serum IgG titer against S. equi appears to be associated with an increased risk of developing purpura, routine testing for specific IgG antibodies using a commercially available ELISA test has been recommended as a means of preventing vaccine-associated purpura.¹⁴⁵ Horses with titers of 1:1600 or greater in the SeM ELISA and those known to have had strangles during the previous year should not be vaccinated.¹⁴⁵

The bacterial modified live vaccine for intranasal administration will cause injection site abscesses if inadvertently injected IM. To avoid inadvertent contamination of other vaccines, syringes, and needles, it is advisable and considered good practice to administer all parenteral vaccines before handling and administering the intranasal strangles modified live vaccine. Other reported adverse responses after administration of the intranasal modified live vaccine include nasal discharge, submandibular or retropharyngeal lymphadenopathy with or without abscessation, limb edema, internal abscesses (bastard strangles), and purpura hemorrhagica. The overall frequency of adverse events is low but appears to be higher than reported to the manufacturer (4.8 per 10,000 doses). On the other hand, the majority of reported adverse events, including the development of nasal discharge, lymph node abscesses, and purpura hemorrhagica, occur in horses on farms with endemic or epidemic strangles. Therefore it is often uncertain whether the adverse event was caused by the vaccine or by a wild strain of S. equi.

Equine Monocytic Ehrlichiosis (Potomac Horse Fever)

Equine monocytic ehrlichiosis, also known as Potomac horse fever, is caused by Neorickettsia risticii (formerly Ehrlichia risticii). Originally described in 1979 as a sporadic disease affecting horses residing in the northeastern United States near the Potomac River, the disease has since occurred in horses in 43 states in the United States, three provinces in Canada (Nova Scotia, Ontario, and Alberta), South America (Uruguay, Brazil), Europe (the Netherlands, France), and India. The disease does not appear to be directly contagious, and it now appears that accidental ingestion of aquatic insects harboring metacercariae infected with N. risticii is at least one mode of transmission.¹⁵⁵ PHF is seasonal, occurring between late spring and early fall in temperate areas, with most cases in July, August, and September at the onset of hot weather. The disease may affect individual horses sporadically or cause outbreaks involving multiple horses. Foals appear to be at low risk for the disease. If PHF has been confirmed on a farm or in a particular geographic area, it is likely that cases will occur in future years. Documentation of the involvement of operculate freshwater snails and aquatic insects such as caddisflies and mayflies in the life cycle of N. risticii has permitted formulation of focused control measures directed at minimizing exposure of horses to the habitats occupied by these species during the summer and fall months when disease risk is highest in endemic areas.¹⁵⁵ Risk reduction is best accomplished by denying horses access to river banks, creek beds, and irrigation ditches, as well as pastures that have recently been flooded or flood-irrigated.

Recovery after natural infection with N. risticii induces a strong antibody response and durable protection from reinfection lasting 20 months or longer. However, the presence of antibodies does not necessarily correlate with protection, and cell-mediated responses likely play a crucial role.¹⁵⁶ A β-propiolactone inactivated host cell—free N. risticii vaccine protects mice against homologous challenge.¹⁵⁷ Several inactivated PHF vaccines for intramuscular administration (Mystique, Intervet; Potomavac, Merial; PotomacGuard, Fort Dodge; PHF-Gard, Pfizer; and Equovum PHF, Boehringer Ingelheim) are licensed for use in horses with the label claim that they aid in prevention of PHF. Two of these are also available combined with a rabies vaccine. None carry a label claim for prevention of abortion. The high rate of serious complications and mortality associated with this disease has been considered adequate justification for vaccinating horses residing in or traveling to endemic areas. In a series of studies in which ponies were challenged IV with N. risticii approximately 4 weeks after completion of the two-dose primary vaccination series using a formalin-inactivated, aluminum hydroxide—adjuvanted vaccine (PHF-Vax, Schering-Plough), Ristic and colleagues (1988) reported that 78% of experimentally infected ponies were protected against all clinical manifestations of disease except fever, and 33% were protected against all signs, including fever.¹⁵⁸ A published noncontrolled field study involving the same vaccine documented induction of serologic responses in most vaccinated horses and a substantial reduction in disease prevalence, morbidity, and mortality compared with data collected in a previous year when horses were not vaccinated.^156,159

In contrast to the results of the studies cited above, an epidemiologic investigation involving a large number of horses failed to demonstrate any clinical or economic benefit from annual vaccination with currently available vaccines in New York State.^160,161 Failure of a substantial number of individual horses to mount an immune response to inactivated PHF vaccines, heterogeneity of N. risticii isolates, the presence of only one N. risticii strain in vaccines, and much more rapid waning of immunity after vaccination than after natural infection likely account for the observed failure of vaccines to provide protection against field infection.^156,162 Despite the lack of documented efficacy of approved vaccines to prevent infection in the field setting, many practitioners who work in endemic areas believe that severity of disease is attenuated and mortality is reduced in vaccinated horses when vaccines are administered at 4- to 6- month intervals, with administration of one booster timed to precede the anticipated period of peak challenge.

If vaccination is elected, a primary series of two doses should be administered 3 to 4 weeks apart. Manufacturers recommend revaccination at 6- to 12-month intervals; however, some veterinarians encourage a revaccination interval of 4 months in order to achieve a reasonable likelihood of protection. Because the disease has a distinct seasonal pattern, revaccination in the late spring, approximately 1 month before the first cases are expected, followed by a second dose 4 months later appears to be a reasonable approach for strategic immunization to maximize the chances of protection during the period of peak challenge. Available vaccines are licensed for use in stallions and pregnant mares and can be administered to gestating mares 4 to 8 weeks before foaling to maximize passive transfer of specific antibodies to foals through colostrum. Whereas approximately 67% of foals from antibody-positive mares were antibody negative by 12 weeks of age, antibody was detectable in 33% of foals up to 5 months of age. On the basis of these findings, the low risk of clinical disease in young foals, and the apparent susceptibility to infection of two foals vaccinated earlier than 12 weeks of age, primary vaccination of foals from antibody-positive dams should begin with a two-dose primary series starting 5 months of age or older, followed by administration of one subsequent booster dose 8 to 12 weeks later.¹⁵⁹ However, the efficacy of this recommended regimen requires further study. If the primary series of two vaccinations is initiated before 5 months of age, additional doses should be administered at monthly intervals up to 5 months of age to ensure that an immunologic response is achieved. Vaccination of foals in endemic areas is further complicated by the distinct seasonal incidence of disease in July, August, and September, a time when most foals are aged between 2 and 6 months and may be subject to maternal antibody interference with vaccination.

Botulism

Botulism is a neuromuscular paralytic disorder caused by one of eight distinct neurotoxins (A, B, Ca, Cb, D, E, F, G) produced by Clostridium botulinum, a soil-borne, spore-forming, saprophytic, anaerobic, gram-positive bacterium.¹⁶³ Botulinum toxins are among the most potent biologic toxins known and act by blocking transmission of impulses at motor endplates, resulting in weakness progressing to paralysis, inability to swallow, and frequently death. Of the seven serogroups (A through G) of C. botulinum, types A, B, C, and D have been reported to cause disease in horses, with types B and C being responsible for most cases.¹⁶³ Three forms of botulism—toxicoinfectious botulism (shaker foal syndrome), forage poisoning, and wound botulism—have been observed in horses. Forage poisoning results from ingestion of preformed toxin produced by decaying plant material or animal carcasses present in feed, whereas “wound botulism” results from vegetation of spores of C. botulinum and subsequent production of toxin in contaminated wounds. Shaker foal syndrome results from toxin produced by vegetation of ingested spores in the intestinal tract. Currently toxicoinfection with C. botulinum type C is being investigated as a cause of equine grass sickness, a largely fatal, pasture-associated dysautonomia affecting horses mainly in Great Britain, continental Europe, and Australia, with reports of isolated cases in the United States. Almost all cases of shaker foal syndrome are caused by type B. Shaker foal syndrome is a significant problem in foals aged 2 weeks to 8 months in Kentucky and in the mid-Atlantic seaboard states and occurs sporadically in other areas.^164-166

A toxoid vaccine (BotVax-B, Neogen Corporation, Tampa, Fla.) directed against C. botulinum type B is licensed for use in horses in the United States. Its primary indication is prevention of the shaker foal syndrome via colostral transfer of antibodies induced by vaccination of the mare. For primary vaccination, mares should be vaccinated during gestation with a series of three doses administered 4 weeks apart, scheduled so that the last dose will be administered 4 to 6 weeks before foaling to enhance concentrations of specific immunoglobulin in colostrums (i.e., months 8, 9, and 10 of gestation). Subsequently, mares should be revaccinated annually with a single dose 4 to 6 weeks before foaling. A similar type B toxoid is available to protect foals in endemic areas in Australia.¹⁶⁷

Passively derived colostral antibodies appear to protect most foals for 8 to 12 weeks, although foals from properly vaccinated dams can develop botulism.^165-167 Insufficient production of specific antibody by the dam in response to the vaccination, failure of passive transfer of specific immunity to botulinum toxin, overwhelming toxin production, and loss of passive immunity by the time exposure to the toxin occurs may be reasons for vaccine failure. The clinician should therefore be aware of the status of MDA transfer of each foal.

Maternal antibodies do not appear to interfere with the response of foals to primary immunization against botulism¹⁶⁸; therefore a primary series of three doses of vaccine administered 4 weeks apart can be started when foals in endemic areas are 2 to 3 months of age or older. Other horses can be immunized using a primary series of three doses of vaccine administered at 4-week intervals, followed by annual revaccination. Currently there are no licensed vaccines available for preventing botulism caused by C. botulinum type C or other subtypes of toxins, and cross-protection between the B and C subtypes does not occur; therefore routine vaccination against C. botulinum type C is not currently practiced. A type C toxoid approved for use in mink was administered to horses under special license to protect them during an outbreak of forage poisoning caused by contaminated alfalfa cubes in southern California in 1989.

Horses and foals with clinical botulism may be treated with botulinum antitoxin administered IV. Antitoxin is not effective against toxin that has been translocated to motor endplates. Therefore clinical signs may progress for 12 to 24 hours after administration of the antitoxin or until all internalized toxin has attached to motor endplates. The dose of botulinum type B antitoxin recommended for treating a foal is 30,000 IU and for an adult is 70,000 IU. Foals of unvaccinated mares born in or being moved to endemic areas may benefit from transfusion with plasma from a vaccinated horse or from administration of C. botulinum type B antitoxin. The efficacy of these practices needs further study. Vaccination with type B toxoid as described previously is an alternative to passive immunization.

Equine Viral Arteritis

EVA is a contagious disease of equids caused by equine arteritis virus (EAV), an RNA virus that is found in the horse populations of many countries. EAV is the prototype virus in the family Arteriviridae of the genus Arterivirus, order Nidovirales. Although all horse breeds appear to be equally susceptible to EAV, the prevalence of infection, as determined by seroconversion, is much higher in some breeds, notably standardbreds and warmbloods, than in others. Despite the high seroprevalence of infection in standardbreds, clinical disease is rarely observed in this breed, indicating that subclinical infection is common.^41,169 Conversely, thoroughbreds and most other breeds have a low seroprevalence of infection but are more likely to show fulminant clinical signs when they become infected. Most primary EAV infections are subclinical or asymptomatic. Clinical signs, if they occur, typically develop 3 to 7 days postinfection and vary in severity, both within and between outbreaks, but may include some or all of the following: fever; anorexia; depression; dependent edema involving the limbs, prepuce, scrotum, mammary glands, or ventrum; localized or generalized urticaria; supraorbital or periorbital edema; conjunctivitis; lacrimation; and serous or mucoid nasal discharge. EAV is of special concern because abortion is a frequent sequela to infection in the unprotected pregnant mare. In addition, EAV can cause life-threatening pneumonia or pneumoenteritis in young foals, and infection of the postpubertal colt or stallion may establish a long-term carrier state.^41,170 Transmission most frequently occurs through direct or aerosol contact with virus-infective respiratory secretions, leading to widespread dissemination of the virus among susceptible horses in close proximity. Indirect transmission, although less significant, can occur through contact with virus-infected fomites. Venereal transmission from infected carrier stallions to mares via semen during natural breeding or artificial insemination with fresh, chilled, or frozen semen can play a significant role in introduction and spread of infection on or between breeding farms or other equine facilities. The virus can persist in the reproductive tract of stallions for many years and possibly result in lifelong infection.

Historically, large-scale outbreaks of EVA have been relatively infrequent. However, the number of confirmed occurrences appears to be increasing, likely as a result of increased global movement of horses, increased accessibility of carrier stallions, and increased use of shipped cooled or frozen virus-infected semen. Outbreaks can be associated with serious economic consequences, as clearly exemplified by the 2006 multistate outbreak in quarter horses that was propagated by widespread shipment of semen from the index cases, two inapparently infected carrier stallions in New Mexico. Because the carrier stallion is widely accepted as the natural reservoir of EAV and the source of diversity among naturally occurring strains of the virus, identification of these individuals through serologic testing, followed by PCR testing or virus isolation from semen, forms the cornerstone of eradication measures. Vaccination also constitutes an important means of controlling spread and minimizing the consequences of infection.

A modified live vaccine based on an attenuated strain of EVA virus was developed by researchers in Kentucky in 1969.¹⁷¹ This vaccine (Arvac, Fort Dodge Animal Health, Fort Dodge, Iowa) was first used extensively in the field during the 1984 outbreak of EVA in Kentucky and proved to be safe and effective in bringing the outbreak under control.⁴¹ Subsequently this vaccine was developed further and licensed for use in North America. Vaccination of stallions, nonpregnant mares, and prepubertal colts has been shown to be a safe and effective means of controlling EVA. Strategic use of the modified live vaccine has formed the cornerstone of a highly successful program to control EVA in the Kentucky thoroughbred breeding population for many years.⁴¹

The indications for vaccination against EVA are as follows:

To protect stallions against infection and subsequent development of the carrier state.

To immunize seronegative mares before they are bred with EAV-infective semen.

To curtail outbreaks in nonbreeding populations. Vaccination in the face of an EVA outbreak in concentrated populations of performance horses at racetracks has been successful in controlling horizontal disease dissemination within 7 to 10 days.

Primary immunization with the modified live vaccine involves intramuscular administration of a single dose, with a booster administered annually thereafter. VN antibodies are induced within 5 to 8 days after modified live virus vaccination and persist for at least 2 years.^41,172 Revaccination induces high VN antibody titers that persist for several breeding seasons.¹⁷² Although the current modified live vaccine is highly attenuated and has been shown to be safe and effective in stallions and nonpregnant mares, a small proportion of first-time—vaccinated horses develop mild febrile reactions and transient lymphopenia after vaccination with the modified live vaccine, and vaccine virus may be isolated sporadically from the nasopharynx and buffy coat for 7 days but occasionally up to 32 days after vaccination.^41,172-174 Vaccinated stallions do not shed vaccine virus in either semen or urine.¹⁷²

Primary vaccination provides sustained clinical protection against EVA but does not prevent reinfection and subsequent limited replication and shedding of field strains of virus.¹⁷⁵ However, in vaccinates the frequency, duration, and amount of viral shedding via the respiratory tract are significantly less than observed with natural infection. Vaccinated mares may shed field virus transiently after being bred to carrier stallions; therefore isolation of these individuals for 21 days after breeding is recommended.⁴¹

Annual revaccination of breeding stallions 28 days before the start of breeding season is highly recommended as a means of preventing establishment of the carrier state.⁴¹ Annual revaccination of mares being bred to carrier stallions should occur at least 21 days before breeding. The modified live vaccine is not recommended for use in pregnant mares, especially during the last 2 months of gestation, or in foals less than 6 weeks of age, except in emergency situations when there is a high risk of exposure. Apparent fetal infections with modified live vaccine after vaccination of pregnant mares have been documented, but only rarely.^172,176

Foals born to seropositive mares become seropositive after ingesting colostrum. MDAs decay with a mean half-life of approximately 32 days, with the result that foals become seronegative between 2 and 7 months of age.^177,178 Maternal antibodies are unlikely to interfere with the response to vaccine administered at 7 months of age or older.¹⁷⁷ However, when foals less than 6 months of age are vaccinated during conditions of high risk, they should be revaccinated after 6 months of age. Establishment of the carrier state appears to depend on the high levels of androgens circulating in intact stallions and can be prevented by vaccinating colts, preferably before puberty, before they are used for breeding.⁴¹ Vaccination of prepubertal colts at 6 to 12 months of age is therefore central to effective control of the spread of EAV infection and should be strongly encouraged in breeds such as standardbreds and warmbloods in which EVA is prevalent and on facilities on which risk of infection is high. Persistent infection has never been documented in a stallion that was properly vaccinated with the licensed modified live vaccine before exposure.⁴¹

Rotaviral Diarrhea

Equine RV, a nonenveloped RNA virus, is one of the most important causes of infectious diarrhea in foals during the first few weeks of life and often causes outbreaks involving the majority of the foal crop on individual farms.^185-187 Older foals and adult horses are more resistant to infection. Equine RV is transmitted via fecal-oral contamination and causes diarrhea by damaging the tips of villi in the small intestine, resulting in cellular destruction, maldigestion, malabsorption, and diarrhea. The genus Rotavirus is one of five genera of the family Reoviridae and is divided into seven serogroups (A through G) based on differences in the inner capsid protein, VP6.^187,188 All equine RV isolates to date are in group A, which is further subdivided using neutralizing antibodies to the VP4 and VP7 outer capsid proteins into P (protease-sensitive, VP4-positive) and G (glycoprotein, VP7-positive) serotypes, respectively.¹⁸⁸ Five P serotypes (P1, P6, P7, P12, and P18) and eight G serotypes (G1, G3, G5, G8, G10, G13, G14, and G16) have been identified and characterized in horses.^189-191 Most equine RV isolates from all parts of the world are, however, of the P12 and G3 serotype and include 2 subtypes (A and B).¹⁹² A number of RV isolates remain untyped, so it is possible that other equine RV serotypes, and perhaps other serogroups, are active in the equine population.

An inactivated RV A vaccine (Equine Rotavirus Vaccine, Fort Dodge Animal Health, Fort Dodge, Iowa) containing the G3, P12 serotype (H2 strain) in a metabolizable oil-in-water emulsion is conditionally licensed in the United States and is indicated for administration to pregnant mares in endemic areas as an aid to prevention of diarrhea in their foals caused by infection with RVs of serogroup A. Foal vaccination is not indicated because there are no data to suggest that vaccination of the newborn foal with inactivated RV A vaccine has any benefit in preventing or reducing the severity of infection. Label recommendations call for a three-dose series of the vaccine to be administered to mares during each pregnancy at 8, 9, and 10 months of gestation. This protocol has been shown to induce significant increases in serum concentrations of neutralizing antibody in vaccinated mares and in the concentrations of antibodies of the IgG, but not IgA, subclass in the colostrum and milk of vaccinated mares.^193,194 It is essential that the newborn foal receive an adequate amount of good-quality colostrum so that it absorbs sufficient anti-RV antibodies. After nursing, the concentration of passively derived RV-specific antibody of the IgG subclass in the serum of foals up to 90 days of age from vaccinated mares is significantly higher than that measured in serum of foals born to unvaccinated mares.^193,194 A field study showed this vaccine to be safe when administered to pregnant mares and provided circumstantial evidence of at least partial efficacy. An approximately twofold higher incidence of rotaviral diarrhea was found in foals from unvaccinated mares compared with those from vaccinated mares, although this difference did not prove to be statistically significant.¹⁹³ Similarly, a controlled field study in Argentina in which an inactivated aluminum hydroxide—adjuvanted vaccine containing the SA11 (G3P2), H2 (G3P12), and Lincoln (G6P1) strains was administered to 100 mares at 60 days and again at 30 days before foaling demonstrated a substantial reduction in the incidence and severity of rotaviral disease in foals from vaccinated mares compared with foals from unvaccinated mares.¹⁹⁵ As MDA titers wane at approximately 60 days of age, foals may develop rotaviral diarrhea. However, the severity of diarrhea is generally milder and of shorter duration than occurs in foals that become infected during the first 30 days of life.

Challenge studies involving two inactivated RV vaccines administered in a similar manner to pregnant mares in Japan showed that their foals were not completely protected against infection but had a substantial reduction in severity of clinical signs after challenge.¹⁸⁹ The major correlate for protection against rotaviral infection appears to be mucosal immunity, predominantly mucosal IgA, in the gastrointestinal tract. Studies of the immunoglobulin isotype responses of mares and of antibodies passively transferred to their foals after parenteral vaccination of their dams with inactivated RV vaccines indicate that this approach is unlikely to provide foals with intestinal mucosal protection in the form of IgA.¹⁹⁴ Consequently it is not surprising that current protocols do not provide complete protection. In addition, because the conditionally licensed vaccine available in the United States contains only the G3 serotype of the A serogroup, it cannot be expected to protect against infection with all field strains.

Equine Protozoal Myeloencephalitis

EPM is a multifocal neurologic disease caused by the apicomplexan parasites Sarcocystis neurona and, less often, Neospora hughesi. Serologic studies indicate that exposure to S. neurona occurs in most regions of North America, and in some areas seroprevalence exceeds 50%. Prevalence of clinically apparent neurologic disease caused by S. neurona and N. hughesi is much lower than the prevalence of antibodies, indicating that many horses become infected and mount an immune response that is effective in clearing infection before substantial damage occurs in the central nervous system. It is not known whether all seropositive horses have experienced neural infection or whether the immune response in these individuals is successful in clearing parasites before neural invasion occurs. The life-cycles of S. neurona and N. hughesi have not been determined definitively, although opossums are a definitive host for S. neurona and horses are likely dead-end hosts that inadvertently become involved in the life cycle.¹⁹⁶

There is widespread exposure of horses in North America to S. neurona and a high level of owner concern (in some cases hysteria) within the equine industry, leading to the perception that EPM is of high economic importance. This, coupled with inadequate diagnostic techniques for antemortem confirmation of EPM and the suboptimal effectiveness of current treatment and control protocols, led the USDA to grant a conditional vaccine license to Fort Dodge Laboratories in 2000. This vaccine is an inactivated whole-parasite S. neurona vaccine with a metabolizable oil adjuvant (EPM Vaccine, Fort Dodge Animal Health) that has met USDA requirements for quality assurance and purity in the manufacturing process. The criteria for safety were also met in a field study involving vaccination of more than 700 horses. The manufacturer met the requirement for documenting “a reasonable expectation of efficacy” by demonstrating seroconversion in vaccinated horses using a plaque reduction assay to measure neutralizing antibodies. Subsequent studies in which indirect fluorescent antibody testing (IFAT) and immunoblot (IB) tests were used to measure humoral responses, and intradermal skin testing and peripheral blood mononuclear cell proliferation assays were used to assess cell-mediated immunity (CMI), documented seroconversion and sensitization of CMI in a high proportion of vaccinated horses.^197,198

Development of a clinically relevant experimental model for S. neurona infection has proven to be difficult; therefore the efficacy of this vaccine has not been determined in experimental challenge studies or in prospective controlled double-blind field studies. Because antibody to S. neurona is detectable in the cerebrospinal fluid (CSF) as well as blood of some horses postvaccination,¹⁹⁷ prospective field efficacy studies will be difficult to complete because one of the criteria now used to confirm a diagnosis—the presence of antibodies detectable by IB testing or IFAT in CSF not contaminated with blood—will be rendered invalid in vaccinated horses. This vaccine has not gained widespread use, even though it may ultimately prove to be effective in preventing EPM. However, such use has inevitably generated controversy within the veterinary and scientific communities. In addition, one of the most useful aspects of currently available serologic tests, the finding of a negative IB test or IFAT result to rule out a diagnosis of EPM, will be invalidated in vaccinated horses. The vaccine manufacturer has indicated that a modified IB procedure currently being tested may be effective in differentiating vaccinated horses from those that have experienced natural exposure. It is hoped that answers to these questions and concerns will be revealed in the future.

Anthrax

Anthrax is a serious and rapidly fatal septicemic disease caused by proliferation and spread of the vegetative form of Bacillus anthracis in the body. B. anthracis is acquired through ingestion, inhalation, or skin penetration through contamination of wounds by soil-borne spores of the organism. Anthrax is encountered only in limited geographic areas where moist alkaline soils, particularly those with high organic content, favor survival, germination, and sporulation of the organism. Vaccination is indicated only for horses pastured in endemic areas.

The only vaccine currently licensed for vaccination of livestock, including horses, contains viable live Sterne’s strain 34F2 nonencapsulated spores in saponin (Anthrax Spore Vaccine, Colorado Serum Company, Denver, Colo.). A primary series consisting of two doses of that vaccine should be administered subcutaneously 2 to 3 weeks apart followed by annual revaccination. Mild to moderate swelling at the injection site is common, and adverse systemic reactions may occur occasionally, particularly in young and miniature horses. Little objective information is available regarding use of this vaccine in horses, but clinical evidence suggests that it provides protection; however, vaccination of pregnant mares is not recommended.¹⁹⁹ Because the vaccine is a live bacterial product, appropriate caution should be used during storage, handling, and administration to prevent accidental inoculation of people and to maintain vaccine potency. Concurrent administration of antimicrobial drugs that are effective against B. anthracis is contraindicated if the vaccine is to function as intended.

Challenge

The level of disease challenge and the degree of protection continually fluctuate. Because of biologic variability, the degree of protection is different in every vaccinated animal. The same is true of the level of exposure to a pathogen. Overwhelming challenge can override immunity and lead to disease even in well-vaccinated animals.²⁰¹

Timing of Disease

On many farms certain diseases occur at consistent times. The timing may give some insight into stresses that occur in the management of the cattle. Correcting these stresses can have a positive impact on vaccination and lessen animals’ susceptibility to disease. This type of history also is helpful in determining the timing of vaccinations, a concept that often is underused in veterinary medicine. Knowing when a problem historically has occurred allows vaccinations to be scheduled when they will induce maximum immune responses in preparation for expected challenges.

Assessing Vaccine Efficacy

The efficacy of a vaccine can be extremely difficult for the practitioner to assess. Traditionally, serologic data showing prevaccination and postvaccination titers have been equated with protection. For many diseases, however, the correlation is poor between the antibody measured and the protection generated by the vaccine in the animal.²⁰² Recently, cell-mediated immune function tests have been added to show a more complete stimulation of the immune response after vaccination.²⁰³ Although these tests provide more information about the vaccine, they still do not answer the basic question of how well a vaccine really protects. This question can be answered only by well-designed challenge studies. There are many examples of well-designed studies involving both viral^204,205 and bacterial^206,207 agents. To assess a challenge study, the following information is needed:

Unfortunately, the challenge model is not well established for many diseases. Field trials are even harder to assess but are valuable for judging the effectiveness (i.e., efficacy in a particular situation) and efficiency (i.e., cost-effectiveness) of a vaccine²⁰⁸ (Boxes 48-1 and 48-2). Several good references on field trial analysis are available.^209,210 Recently the Center for Veterinary Biologics (CVB) began giving vaccines different labels depending on the strength of the efficacy data submitted to the Center in the licensing trials.

Autogenous Vaccines

In addition to the vaccines licensed by the USDA, several companies will make autogenous vaccines for use by veterinarians and cattle owners. These vaccines do not fall under any particular USDA/APHIS guidelines and usually are derived from cultures (e.g., viral or bacterial) isolated from specimens submitted by the particular farm. Such vaccines can be used only on that particular facility and cannot be sold for use on other farms. These vaccines are not tested for efficacy or safety, and the components found in the vaccines may vary from batch to batch; this adds some element of risk when they are used. Nevertheless, this type of vaccine may be an option to consider when federally licensed vaccines are not available for a specific farm problem.

Maternal Antibody Interference Revisited

It is an accepted belief that maternal antibodies can block immune responses from vaccination. This belief has been based on a procedure of vaccination followed by a titer evaluation in the vaccinates. Many studies have shown that vaccinated animals may not display increased antibody levels if high levels of maternal antibody to that antigen are present. However, recent studies have shown that both the formation of B cell memory responses and cell-mediated responses can be stimulated in spite of high maternal antibody for the same antigens.^216-218 Seropositive calves vaccinated at a young age with modified live bovine herpesvirus type 1 (BHV-1), parainfluenza type 3 (PI-3), and/or BRSV vaccines have shown higher antibody responses on revaccination than control calves vaccinated only at the second date. These young vaccinates typically do not show increased antibody responses after the first vaccination in the presence of high maternal antibody. Cell-mediated immune responses, as indicated by antigen-specific T cell blastogenesis, have been demonstrated in the face of high maternal antibody levels²¹⁹ when attenuated BRSV and BHV-1 vaccines were used. Similar responses have been reported in laboratory animals as well.^220,221 One study also demonstrated higher levels of protection at challenge if calves were vaccinated with a modified live BRSV vaccine.²¹⁸ It is clear from these studies that maternal antibody interference with vaccines is not as absolute as once thought. The animal’s immune status, the specific antigen, and the presentation of that antigen should be considered when designing vaccination programs in which maternal antibody may be a factor.

Impact of Stress

Stress affects the immune system of all cattle, as can a number of other factors. The release of corticosteroid that occurs during the birthing process has a dramatic impact on the newborn’s immune system. Newborns also have a higher number of suppressor T cells than do adults.² These factors and others dramatically diminish systemic immune responses for the first week of life.²²² Other stressors should be avoided at vaccination time to maintain the integrity of the immune system. Procedures such as castration, dehorning, weaning, and movement need to be considered as stressors in cattle, and all have the potential to diminish immune system functioning temporarily.^223-225

Systemic vaccinations should be avoided during high-stress times because of these diminished responses and because vaccination at such times may even have undesired effects.

Booster Importance

It is important to follow the label directions for administering vaccines. Many inactivated vaccines and some modified live BRSV vaccines require a booster before protection is complete. The first time an inactivated vaccine is administered, the primary response occurs. This response is not very strong, is fairly short-lived, and is predominantly composed of IgM antibodies (Fig. 48-1). The response seen after a booster vaccination is called the secondary, or anamnestic, response. This response is much stronger, is of longer duration, and is primarily composed of IgG antibodies.^201,210 If the booster is given too early, the anamnestic response does not occur, and if too much time elapses before the booster is given, it acts as an initial dose, not as a booster.

Fig. 48-1 Anamnestic response seen after a booster dose is administered to vaccinates.

With most modified live virus vaccines (except for some BRSV vaccines), the primary vaccination also stimulates the secondary response without the need for a booster because the virus or bacterium is replicating in the animal.

Adverse Reactions

Adverse reactions are a risk with any vaccination. These reactions can be categorized as one of the following two primary types of hypersensitivity.^{201,203,226-231}-

One of the more common reactions seen in dairy cattle has been associated with the endotoxin and other bacterial components found in most gram-negative vaccines.^230-233 Currently, there are no requirements for monitoring or reporting the amount of endotoxin found in cattle vaccines, and the level of endotoxin may vary dramatically among vaccines and among serials of the same vaccine. Furthermore, the potency of endotoxin varies among different gram-negative bacteria. This type of reaction is seen primarily in Holsteins because of a genetic predisposition and may be seen after administration of any gram-negative bacterin. The signs vary depending on the farm’s or the individual’s sensitivity to gram-negative bacterial components. The number or potency of the gram-negative fractions in vaccinations administered simultaneously also are instrumental in causing these reactions. As a general rule, no more than two gram-negative vaccines should be administered to dairy cattle on the same day because of the possibility of adverse reactions, which may include anorexia and transient decreases in milk production, early embryonic death, abortion, and gram-negative bacterial shock (endotoxic shock), which requires treatment with flunixin or ketrofen, steroids, antihistamines, and fluids.

Site reactions are common sequelae to many vaccines. These granulomas usually are caused by overreaction to the adjuvants, but they may also be directly aimed at the antigen or antigens. This has been a major focus of beef quality programs and has generated a push to have all vaccines labeled and to have them administered subcutaneously to avoid damaging the muscle.

Read the Labels

The vaccine label is a wealth of information that is approved by the CVB. The CVB evaluates the supportive efficacy data supplied by the vaccine manufacture and decides whether the vaccine can be licensed or not. They also determine what type of efficacy claim can appear on the vaccine labels and in advertising. Included are dosage, route of administration, precautions, timing, indications, storage indication, withdrawal period, and shelf life. As found in Veterinary Services Memorandum 800.202, June 2002, the CVB also does some preliminary rating of vaccine efficacy by granting one of five protection statements. According to the memorandum, one of five levels of protection may be granted (in order of highest efficacy to lowest): Prevention of infection, Prevention of disease, Aid in the prevention of infection, Aid in disease control, Other miscellaneous claims. The label can be a good starting point for comparing vaccines.

Summary

Designing a vaccination program requires a good history of the individual farm and a basic understanding of the immune system. Vaccines that should be considered for routine or optional use in various classes of pastured beef cattle, feedlot cattle, and dairy cattle are listed in Boxes 48-3 to 48-12. The vaccines chosen should be supported by good, solid efficacy studies (and by effectiveness and efficiency studies if possible) to ensure that the product can fulfill the needs of the farm or ranch (Table 48-11). Management decisions may be made that do not maximize the potential of the product chosen, and realistic expectations of all products should be well explained to the producer before the vaccines are administered. The owner should be involved in the vaccine decision-making process, and all information on the product should be shared.

Table 48-11 Recommendations for Use of Some Bovine Disease Vaccines

The establishment of good baseline immunity of replacement heifers and the foundation vaccination program can have dramatic effects on the health and profitability of the herd; therefore such programs must be well planned.

BOVINE HERPESVIRUS TYPE 1: INFECTIOUS BOVINE RHINOTRACHEITIS VIRUS