Highly contagious disease of newborn piglets but may affect pigs of all ages in susceptible herd. High morbidity and high case-fatality rate in piglets under 10 d of age. Large economic losses. Epidemics of disease occur in susceptible herd. Transmission by oral and aerosol routes. Recrudescence of infection and endemic disease commonly follows epidemics. Infection of pregnant sows results in protection of piglets by secretory 1 gA in milk. Porcine respiratory coronavirus mutant of transmissible gastroenteritis (TGE) virus has reduced incidence of TGE
Epidemic disease: Acute diarrhea, vomiting, dehydration and death in piglets under 10 d of age. Less severe diarrhea in older pigs of all ages
Endemic disease: diarrhea in young pigs 6 d of age and older, including weaned pigs.
The disease is associated with the transmissible gastroenteritis (TGE) virus, a member of the Coronaviridae family1 belonging to the Order Nidoviridales. The nucleotide sequences from 20 TGE virus isolates obtained from eight countries between 1946 and 1996 have been compared.2 The virion is enveloped, large-single stranded RNA genome with positive polarity. There are three major structural polypeptides-200 Kda spike protein (S protein), 30 Kda membrane protein (M) and a minor 10 Kda protein (E).3 These are produced by ORFs 2, 5 and 6. The function of ORF products from 3a and 3b are not known but they have been postulated as an important determinant for virulence.4
The disease occurs in pig-producing areas of North America, Europe and many parts of Asia, principally in the northern hemisphere. During the past three decades, TGE has changed from a sporadic disease as it historically occurred in the midwestern United States to an endemic disease in most countries of the northern hemisphere. In densely swine-populated areas such as the midwestern United States, the disease is one of the major causes of morbidity and mortality in young pigs.5 The disease has not been diagnosed in Australia and New Zealand.6 In 1990, the prevalence of infection in the United Kingdom was low, at 0.6% of sows sampled as being seropositive compared to 3% in 1984.2 No major epidemics have occurred since 1981. In 1984, a seroconversion to the TGE virus occurred in a closed herd in the absence of any clinical disease. The TGE virus was not isolated and it is possible the seroconversion was due to emergence of the porcine respiratory coronavirus (PRCV) throughout Europe and the United Kingdom beginning in 1986.2 The PRCV is a deletion mutant of the TGE virus and its high rate of prevalence has markedly reduced the number of TGE outbreaks in European swine herds. The TGE virus probably co-exists in these herds together with the PRCV. In 1999, a single case was diagnosed in East Yorkshire as a one-off.7 Other isolated outbreaks in herds which were seropositive for PRCV were also reported.8 An outbreak of TGE occurred in the UK in 1996 in which the virus was a variant with an intact spike gene but with a large deletion in the ORF3a which may not be necessary for enteric virulence.9
The prevalence of infection of the TGE virus based on serological surveys of swine herds varies with the size of the herd, the distance between herds and the purchase of breeding stock from non-specific pathogen-free herds.10 Depending on the geographical location, up to 50% of herds may be seronegative, and in 45% of herds the prevalence of infection in sows will vary from 10–80%.10 In the United States in 1990, a national survey of swine herds found that 36% of herds were positive for the TGE virus and 24% of the producers’ herds were vaccinated for the virus.11 By 1997, up to 100% of survey herds and 91% of the sera were positive for both the TGE virus and the PRCV, which indicates a marked increase probably due to subclinical infections.
The disease is highly contagious and affects piglets primarily under 10 days to 2 weeks of age. Pigs over 5 weeks of age often have milder clinical signs. Epidemic TGE occurs when the virus is first introduced into a susceptible herd and is usually of short duration and no longer clinically evident after herd immunity develops. Epidemics of the disease occur most commonly during the winter months. Endemic TGE occurs when the virus persists in a partially immune herd into which susceptible swine are introduced or if the epidemic form is not well managed. Endemic TGE is a common sequel to a primary epidemic in herds of more than 300 sows in which diarrhea occurs in piglets from 6 days of age to about 2 to 3 weeks after weaning.11 Recurrence of clinical TGE often occurs in endemically infected herds about 9 months after the first outbreak as the piglets of susceptible sows are exposed to the virus. Recurrence has been associated with:
Typically, an epidemic in a herd is explosive and dramatic. There is rapid spread and high morbidity of pigs of all ages within 2–3 days but major clinical disease is restricted to pigs prior to weaning and to lactating sows. Case–fatality rates may approach 100% in pigs under 10–14 days of age but are much lower with increasing age, and mortality is low in postweaned and adult pigs. The epidemic commonly terminates in 3–5 weeks with the loss of young susceptible pigs and the development of herd immunity and, generally, the disease does not recur again for a 3–6-year period.
Epidemics of clinical disease occur following the introduction of the virus into a susceptible herd with no previous exposure to the virus. All age groups will become infected and most pigs will be affected clinically to variable degrees. Nursing piglets under 2–3 weeks of age are most susceptible to clinical disease and experience the highest case–fatality rate. Clinical disease disappears when the herd becomes immune. Endemic TGE develops when the virus and clinical disease persists within partially immune herds, caused by continual or periodic introduction of susceptible pigs. In endemic situations, diarrhea is generally observed in pigs from the age of about 6 days until about 14 days after weaning. Overall pig mortality is lower and generally occurs in recrudescent episodes. After weaning, piglets no longer have the protection provided by TGE-specific secretory IgA antibody in milk and are susceptible to infection and clinical disease if the infection rate in the weanlings is high. Thus weanling pigs serve as a major reservoir of infection.
Sow parity may be a risk factor. Parity-1 sows with no previous exposure to the virus may be a risk factor on some farms.10 On other farms, parity-3 sows were at increased risk for unknown reasons. A single boar may be a high-risk animal on some farms.
There is a higher likelihood that sows will be seropositive if the herd size exceeds 500 sows and if more than 25 replacement breeding animals are purchased from non-specific pathogen-free herds.10 A mathematical model of detection and dynamics of disease in Australia indicates that the disease is likely to establish in breeding and finishing swine herds of average size.6 The threshold number of susceptible pigs for establishment of the infection is 90–160. Swine herds most at risk are those with1 large numbers of susceptible pigs,5 continuous breeding of susceptible pigs,6 high numbers of purchased pigs, and2 close contact between feral pigs and susceptible domestic pigs.6 The risk is highest in herds which do not make a rapid diagnosis when there is little or no veterinary involvement in health and disease management. In small farms (containing 15 to 40 sows) outbreaks of TGF are characterized by rapid spread of infection to most animals of all ages but a duration of only 3–5 weeks.12
Climatic factors appear to be important in the occurrence and establishment of the disease. Climate does not yet have significance in the tropics or southern hemisphere and there is evidence that its spread is limited in hot climates. In areas where the disease is endemic, it has a distinct seasonal occurrence with the majority of outbreaks occurring from midwinter to spring, and cyclic occurrence is recorded.13 Virus is labile above 21°C and is very sensitive to sunlight. It is also killed by most disinfectants. The disease tends to occur in area outbreaks in which herds in close proximity are affected within several weeks. Within swine barns, the location of the farrowing crates may be a risk factor if the cold air inlets are directly above the crates.
The use of a continuous flow system of production in a herd is a major risk factor. The constant overlap of farrowing sows in farrowing rooms, overlap of weaned pigs in nursery rooms, and a continuous flow of finishing pigs without adequate cleaning and disinfection between each group of pigs are major risk factors, and perpetuate persistent infection and the endemic form of the disease. An all-in all-out system for each group of pigs reduces the risk of infection between pigs.
The virus does not persist in infected premises for more than a few weeks and is readily destroyed by standard solutions of phenol and formalin, by boiling and by drying but not by freezing. The virus is photosensitive, which may account for the more frequent occurrence of the disease during the winter and spring months. The virus survives freezing, and infected pork scraps or offal may provide a source of infection either directly through feeding of uncooked garbage or possibly indirectly via dogs. Purposeful infection by the feeding of frozen infected piglet intestine to sows to induce immunity may also be a significant source of continued infection of a herd or area.
The genome and the genetic basis for the pathogenesis of the virus have been described.5 Antigenic differences between TGE viruses have been examined and the nucleotide sequences of isolates from various countries have been compared.14
The TGE virus is not antigenically related to the two other porcine coronaviruses, hemagglutinating encephalomyelitis virus and porcine epidemic diarrhea virus but it is related to PRCV.
The PRCV is a deletion mutant of the TGE virus with altered tissue tropism to the respiratory tract, first recognized in Belgium in 1984. It has a partially deleted receptor binding protein. The virus closely resembles the TGE virus antigenically and pigs infected with the PRCV develop a serological response which cannot be distinguished by virus-neutralization tests from the response of pigs infected with the TGE virus.2 In other words it provides cross protection.15 Despite the antigenic relation of PRCV and the TGE virus, they can be differentiated with monoclonal antibodies.16 All PRCV strains have around 600–700 nt deletions within the amino-terminal S gene resulting in the loss of hemagglutination activity17 and two antigenic sites.18 European PRCVs have an identical deletion of 672 nt at the same position, whereas US strains have 621–681 nt deletion located at different positions which suggests that they arose separately. Natural infection of the sow with the PRCV induces natural antibodies which neutralize classic enteric transmissible gastroenteritis. The virus has spread throughout Europe19 and has been identified in the United States20 and Canada.16 Spread of the virus has been explained in part by airborne transmission and infection shows a seasonal pattern, affecting farms during winter and spring. Seroprevalence studies in Belgium indicate that 95% of sows are PRCV positive.21 The infection is widespread in swine herds in Spain22. The risk factors associated with seropositivity in Danish swine herds include: (a) increasing herd size; (b) certain geographical locations; (c) presence of a slurry system with slatted floors; and (d) purchase of pigs.19 The serological status of neighboring herds was also risk related and the closeness of a seropositive herd was associated with an increased risk of a herd becoming serologically positive.
The PRCV replicates in the respiratory tract of pigs but to a very limited extent in the intestines. Its pathogenicity is controversial. Some studies indicate that the virus causes only subclinical respiratory infections while others have linked the virus with field outbreaks of respiratory disease. Experimentally, inoculation of the virus intratracheally into 8-week-old piglets results in clinical respiratory disease and bronchointerstitial pneumonia, and the virus can be recovered from the respiratory tract.23 Some isolates of the virus produce interstitial pneumonia in neonatal piglets with no recognizable clinical respiratory disease.24
The exact mode of transmission of the TGE virus is uncertain. Virus shedding in the feces of infected pigs usually ends at or within a few weeks of recovery, although recovered pigs may harbor the virus in pulmonary or intestinal tissue for periods of more than 100 days. The shedding period is supposed to be 14 days. After weaning, the pig is no longer protected by specific secretory IgA antibody of the sow’s milk and is highly susceptible to infection if the rate of infection is high in the weanling population. The weanling pig is a major reservoir of infection for continuous infection in the herd. Feeder pigs with no clinical signs can be an important reservoir of the virus. The virus has also been isolated from pharyngeal swabs taken from farm-raised sows sent to slaughter.
Epidemics commonly follow the introduction of pigs into a herd and the carrier pig is a major source of infection. Frequently the disease first appears in older pigs in the herd and then subsequently spreads to newborn pigs and sows in the farrowing area. Spread is much more rapid in a continuous-flow system of production compared to an all-in all-out system, whereby groups of pigs of the same age or production stage are handled as groups and their housing facilities are cleaned and disinfected before and after being occupied. Visitors, their boots, transport vehicles, equipment, and starlings have also been incriminated in the transfer of infection to new locations. Starlings may act as vectors to spread to adjacent farms. The virus can also multiply in house flies (Musca domestica Linneaus) and they may be a vector. Feral pigs are not a significant reservoir for the TGE virus in the southern United States, but are capable of becoming infected and developing virus-neutralizing antibodies against the virus.25 Sub-populations of infected pigs may exist within the herd and although shedding is normally for 14 days it is possible for animals to be infected for 100 days.
Once infection has gained access to a herd, transmission occurs by both oral and respiratory routes. The speed of spread without direct contact indicates that the virus can be spread by aerosol. Respiratory transmission appears significant in adults and replication in the respiratory tract is followed by excretion in nasal secretions and milk within 1 day of infection, and also in feces. Excretion in milk results in rapid transmission to suckling piglets which in turn may excrete large quantities of the virus within 2 days of infection.
Immunity to clinical disease in newborn piglets is dependent on the level of TGE-specific secretory IgA antibody in the colostrum of the sow known as ‘lactogenic immunity’. When pregnant sows are infected orally with the virulent TGE virus, specific IgA precursor cells are sensitized in the intestine. These sensitized cells migrate to the mammary glands and differentiate into plasma cells that secrete IgA class, TGE virus antibody in colostrum and milk. This immune mechanism to induce protective antibody for suckling pigs is termed the ‘gut–mammary gland link’ or the ‘gut-associated lymphoid tissue’ (GALT) system. Following natural infection with TGE during pregnancy, the recovered sow or gilt is capable of protecting her litter against the disease. After farrowing, the colostrum contains antibodies of the IgG, IgM and IgA isotypes derived from serum. After the third day, milk is produced and the only antibody it contains is the IgA antibody, which is synthesized at the mammary gland. The IgA antibodies of immune sows are the most critical in protection of mucosal surfaces such as the gastrointestinal tract, and this immunoglobulin is the most abundant isotype in porcine milk. These IgA antibodies are not induced after the parenteral administration of viral antigens, which explains the relative ineffectiveness of parenteral vaccines. Serum antibody induced by vaccination of the pregnant sow does not provide protection of piglets through colostrum or milk.
Secretory IgA is the predominant antibody class in milk and is responsible for lactogenic protection of pigs and active protection of the intestine. It is stimulated by oral inoculation with non-attenuated, but not attenuated TGE virus. Although high concentrations of IgA and IgG originating from the serum are present in colostrum, IgG does not persist in the milk, whereas IgA does persist because of local mammary secretion. After the first week of lactation, secretory IgA constitutes 50–60% of the total immunoglobulin concentration in swine milk and IgG makes up 20–30%.
Suckling pigs are protected from infection by continued ingestion of antibody of the IgA class secreted in milk. The level of serum IgA antibody as an indicator of immunity to transmissible gastroenteritis can be measured using the indirect immunoperoxidase antibody test. Young pigs, 6 weeks of age, which are exposed to experimental infection with the virus, develop both a humoral and cellular immunity which reach peaks at 21 and 28 days, respectively.
In recent years, less typical forms of the disease have been observed. With continuous farrowing and the continual introduction of susceptible pigs into an infective environment, outbreaks may be considerably prolonged, and this or recrudescence is more likely than when pregnant sows are kept in relative isolation on pasture or elsewhere. Atypical endemic forms of the disease with a low morbidity and mortality and frequently with the onset of clinical disease delayed until piglets are 2–4 weeks of age have been observed and may go unrecognized because of the atypical clinical findings. They are more likely to occur in large continuous farrowing units and may be associated with partial herd immunity and low virulence virus. Some sows do not develop a significant immunity following a single infection and in large herds there may be a sufficient number of these to allow the disease to perpetuate in a low-incidence, endemic form.
A recrudescence of the disease may occur after a period of several months and is thought to be due to inadequate exposure and immunity of some pigs, particularly dry stock during the initial outbreak followed by reinfection from a carrier pig. Recrudescence of clinical disease is usually of much shorter duration than the primary outbreak and commonly lasts only 6–10 days. The periods of recrudescence are commonly precipitated by the simultaneous farrowing of several susceptible gilts in the same farrowing room. Of greater long-term concern is that about 50% of some large herds continue to experience clinical recrudescences for almost 2 years or more. The endemic form of the disease appears to be correlated with herds of more than 100 sows and in herds where finishing pigs were kept. In large herds the virus may spread more slowly and replacement gilts entering the herd may take several months to become infected and to seroconvert. In large herds, the rapid turnover of breeding stock and continuous farrowing and early weaning also contribute to perpetuation of an endemic infection, thus endemic transmissible gastroenteritis can maintain itself by the slow and incomplete spread of the virus among adult pigs, particularly herd replacements. Joint infection with PRRS and TGE did not appear to enhance the clinical effects, shedding or persistence of either virus.26
A herd epidemic of TGE causes economic losses through:
• Increased downtime of the swine enterprise
• Disturbance of the breeding program
• Subsequent reduced growth of young pigs destined for slaughter
The economic losses can be very large. Simulation of the economic losses due to an outbreak of disease in Australia where the disease is exotic estimated a reduction in net revenue of 70% in the 6 months after a moderate outbreak (50% mortality of piglets under 1 week of age), and 100% for a severe outbreak (95% mortality of piglets under 1 week of age).27 An analysis of the economic losses due to the disease in swine farms in some areas in the United States over a 2-year period estimated the average loss to be between 13% and 18% of the average return earned above total production costs. It has been assumed that the growth of surviving pigs was depressed by 10% and their feed conversion by 18%, but pigs surviving or born shortly after an epidemic of TGE are profitable to raise.
The S protein of the viral membrane of TGEv has four major antigenic sites and is the major inducer of neutralizing antibodies. The protein mediates the binding of the virus to the cell surface and the subsequent fusion of the viral and cellular membranes.3 High titers of serum IgG and virus neutralizing antibody to TGEv probably reflect the amount of S spike protein the pig has received.28 Two different ligands have been shown to interact with the S protein and binding to the porcine aminopeptidase N, the cellular receptor for TGEv is essential for infection of the cells.29 The TGEv is also able to recognize sialic acid residues and attach to sialylated macromolecules.30 A second binding site on the N-terminal division of the S protein allows TGEv to interact with terminal sialic acid residues on glycoproteins or glycolipids and to agglutinate RBCs. TGEv also recognizes a porcine intestinal brush border protein called MEP (mucin type glycoprotein and TGEv binds to this mucin produced by goblet cells. A mutant virus that has lost its sialic acid binding capability is not pathogenic as it is unable to attach to goblet cells.31-34 Sialic acid binding activity is a pathogenicity factor for TGEv and it is important to note that the sialic acid binding sites for TGEv and E. coli are different.
The virus infects the upper respiratory tract and the intestines but the major clinical effects are due to intestinal infection.35 Following oral challenge of susceptible piglets, the incubation period may be as short as 24 h. The virus infects mature differentiated columnar epithelial cells of the intestinal villi but not the undifferentiated cells of the crypts.36 Replication occurs within 4–5 h with sloughing of the infected cells and release of virus, and after several replication cycles there is a marked reduction in villous size with villous atrophy. The loss of epithelial cells results in increased migration of undifferentiated cells from the crypts to line the shortened villi. With virulent virus, epithelial cells at all levels of the small intestine are infected with major lesions occurring at the proximal jejunum and to a lesser extent the ileum. The lesser virulence of attenuated strains of virus may be associated with their inability to infect and produce lesions in the villi of the more cranial portions of the jejunum. Gnotobiotic pigs inoculated orally with a TGE vaccine will develop lesions similar to the naturally occurring disease.37
Diarrhea results from a combination of malabsorption and osmotic effects subsequent to the loss of intestinal surface area and disaccharidase activity, and impaired lumen-to-extracellular fluid flux of sodium consequent on the occurrence of undifferentiated cells lining the stunted villi. The virus invades the villus, but not the crypt epithelium of the small intestine within hours after experimental administration. The infected villus cells are quickly shed and replaced by relatively undifferentiated enterocytes. As infected cells are shed, the epithelium proliferates and migration of cells from the crypts accelerates. There are marked abnormalities in ion transport function in the jejunum and ileum at the height of the diarrhea. There is failure of the intestine to actively transport sodium and chloride and there is a defect of the glucose-mediated sodium ion transport. Macromolecular hyperpermeability of the small intestine also occurs but its significance is uncertain.38 Experimentally induced infection of 3-week-old pigs with the virus results in villous atrophy and crypt hyperplasia, and a marked decrease in the secretory response of the villous epithelium to Escherichia coli enterotoxins. The disease is more severe in gnotobiotic pigs that are infected with E. coli in addition to the TGE virus, suggesting that bacterial factors also influence the severity of the diarrhea.
In the experimental disease in 2-day-old pigs, vomiting and diarrhea occur 12–24 h after oral inoculation of the virus and affected piglets are moribund 1 or 2 days later. Before becoming moribund most piglets become lethargic and comatose. In addition to dehydration and metabolic acidosis, there is a severe hypoglycemia due to a combination of inadequate glucose metabolism inherent to neonatal piglets and the acute maldigestion and malabsorption from the diffuse and severe villous atrophy. The high mortality may be due to a combination of dehydration, acidosis and severe hypoglycemia.
The age-dependent resistance to TGE can be explained in part by a decreased susceptibility of the epithelial cells of older pigs to infection and by an increased proliferative capacity of crypt cells with much more rapid regeneration of atrophic villi in pigs over 2 weeks of age. It may be that the virus has developed strategies to evade apoptosis in intestinal enterocytes by producing huge amounts of the virus.39,40
A recent experiment comparing a Korean strain with two US strains showed that the progression of the Korean virus was much slower, i.e. much less virulent, possibly because there was only replication in the ileum and jejunum whereas the US strains also replicated in the duodenum.41 The more virulent strains attack a wider area of enterocytes. Most only attack the villous rather than the crypt enterocytes. An outbreak of reduced virulence TGE was associated with the presence on the farm of three strains of PRCV which had variable sequence changes in ORF3/3a/3b.42
In a primary or epidemic outbreak the clinical findings of typical acute TGE are characteristic. The appearance of the disease is not significantly altered by a concurrent infection with PRRS.43
After an incubation period of 24–48 h there is a sudden onset of vomiting and diarrhea. The diarrhea is profuse and frequent; the feces are watery and usually yellow-green in color. The feces may contain clots of white undigested milk and have an offensive odor. The vomitus is yellow, foamy and slimy. There may be a transitory fever but in most cases the temperature is normal. Depression and dehydration are pronounced, the hair coat is ruffled, and weakness and emaciation progress to death on days 2–5. Some piglets may continue to suck to within a few hours of death; those which survive are severely emaciated and gain weight slowly. The illness may commence as soon as 24 h after birth. It is not uncommon on an individual farm for the disease to become less severe and to spread more slowly with the passage of time. In the outbreak described by Pemberton there was an 80% mortality across two weeks of piglets7 with 10% of 4 to 6-week-old freshly weaned pigs.
In older pigs there may be signs similar to those which occur in piglets but many animals become infected without clinical abnormalities. Diarrhea may occur first in the dry sows.44 In older pigs, recovery is much more likely to occur, the illness lasting for up to 10 days. Lactating sows may or may not be affected clinically. Fever and inappetence occur, with or without diarrhea, and agalactia is a common complication in sows. In endemically affected herds with continuous farrowing and partial sow immunity, the disease is milder with diarrhea affecting piglets about 6 d of age or older and diarrhea in weaned pigs. Brief periods of clinical disease occur in some parts of the herd, mortality is low, and affected pigs subsequently grow poorly.
A severe dehydration with metabolic acidosis and a marked hypoglycemia are common.
The virus can be detected in the mucosal scrapings and feces using an ELISA, immune electron microscopy, fluorescent antibody staining or by the immunoperoxidase test.5 A capture-enzyme immunoassay has also been developed.45 A reversed passive hemagglutination test for detection of the virus in feces is also available. A solid-phase immune electron microscopic technique for detection of the virus in feces is also useful for diagnosis in living animals. The PRCV can be isolated by tissue culture.24
DNA probes can differentiate the porcine respiratory coronavirus from the TGE virus.46 PCRs were described quite early on for identifying TGE.47
In situ hybridization has been described48 and a nested RT-PCR was developed49 which was very sensitive. A multiplex RT-PCR for differentiating PED from TGEv in clinical samples has been described.50 It has also proved possible to use formalin fixed tissue for multiplex PCR, nested-PCR and ISH with 100% conformity.51
Several serological tests can detect and measure antibody to the virus in live animals. The serum neutralization test is sensitive and reliable, but is time-consuming and requires facilities for cell culture techniques. Neutralizing antibodies appear in the serum 7–8 days after infection and persist for at least 18 months. An ELISA is more sensitive than the virus neutralization test and a competitive ELISA differentiates between TGE virus and PRCV.35 A blocking ELISA to differentiate TGE and PRCV has also been described.52
The lesions are confined to the intestine and stomach although in many field outbreaks and in the experimental disease the changes may be minor. The intestinal wall is thin and translucent and the intestine is distended with fluid ingesta. Despite the presence of milk in the intestine there is little evidence of fat absorption in the draining lymphatics. The important histopathological change is atrophy of villi with failure of epithelial cell differentiation in the small intestine. The atrophy is evident 24 h after infection and regeneration occurs 5–7 days later. The marked reduction in the size of intestinal villi may even be detected at low magnification on a stereomicroscope. In the stomach there may be engorgement of vessels and necrosis of the epithelium deep in mucosal crypts. No inclusion bodies are detectable. When secondary pathogens contribute to the disease there may be inflammatory lesions in the intestines. In chronic cases a thickening of the intestinal wall identical with that seen in terminal (regional) ileitis has been described.
The disease, as it occurs in Europe, is characterized by more severe mucosal lesions, often including fibrin exudation. There is also degeneration of the heart muscle and, in some cases, of the skeletal muscle.
A simple test for the presence of intestinal lactase in intestinal washings may assist in the laboratory diagnosis. Examination of frozen sections of jejunum from acutely ill piglets by the fluorescent antibody technique is a rapid and effective method for the detection of virus in tissues. The intestine may be segmentally affected so multiple areas must be sampled. Viral antigen is detectable for only 24–36 h utilizing most fluorescent antibody (FA) conjugates. This makes selection of acute cases critical. Electron microscopy is often utilized to identify the presence of coronavirus but this method is not specific for the TGE virus. PCR methods for TGE virus detection are being developed and immunohistochemical techniques are available for use in formalin-fixed tissues.53,54
The use of apoptotic markers shows that most of the cells are undergoing apoptosis but are not infected with TGEV they are what is called bystander cells.55 It has been previously suggested that apoptosis does not occur in the enterocytes of piglets infected with TGEv.56 An accumulation of interferon alpha producing cells occurs in the GALT of TGEv infected piglets.57 It has been suggested that these are the mucosal counterparts of the dendritic cells recently shown to produce IFN alpha after in vitro viral induction57. The TGEv challenge of pigs produces changes in CD4+/CD8+ cells which rise, natural killer cells and cytotoxic T-cells which also rise together with an increased expression of IL-2 receptors and a decrease in null cell phenotypes.58
• Histology – several segments of jejunum and ileum, stomach (LM, IHC)
• Virology – several segments of jejunum and ileum (FAT), feces (EM).
The epidemiological and clinical characteristics of TGE should make possible a presumptive diagnosis, but confirmation must depend upon the finding of compatible histological lesions, the detection of antigen, transmission experiments and evidence of seroconversion. It is unusual to encounter outbreaks of diarrhea in piglets which appear to be typical of TGE, and either the virus can be demonstrated in the tissues by fluorescent antibody test (FAT) but serum antibodies cannot be detected in the breeding animals, or serum antibodies can be detected in the adults but the virus cannot be demonstrated in the tissues by either immunofluorescence or tissue culture.
Villous atrophy is not pathognomonic for the disease, since it occurs in 3-week-old piglets affected with diarrhea and steatorrhea, in rotavirus infections of piglets, and in some herds for undetermined causes immediately following weaning.
In piglets, TGE must be differentiated from the following:
• Enteric colibacillosis A common disease of piglets under 10 d of age with profuse diarrhea, no vomiting, dehydration and a good response to therapy if treated early
• Clostridium perfringens type C Enterotoxemia occurs in piglets under a few days of age and causes marked depression, diarrhea, dysentery, reddening of the anus and rapid death. Lesions at necrospy are characteristic
• Coccidiosis Affects newborn piglets 5–15 d of age, causing profuse diarrhea, depression, dehydration and unthriftiness. Affected piglets may continue to suck. There is a high morbidity, low mortality and oocysts in the feces
• Rotavirus enteritis Rotavirus causes diarrhea in suckling and weaned piglets with a high morbidity and low mortality. Most affected piglets recover in a few days and epidemics are commonly associated with continuous farrowing
• Porcine epidemic diarrhea A coronavirus-like virus causes diarrhea in pigs similar to TGE, except much less severe with less mortality. Porcine epidemic diarrhea Type I causes diarrhea only in pigs up to 4–5 weeks Porcine epidemic diarrhea Type II causes diarrhea in pigs of all ages. The morbidity may reach 100% but mortality is low. The disease may start in the finishing pigs and spread rapidly to pregnant sows and their nursing piglets. The diarrhea may persist in the 6–10-week-old pigs and seronegative gilts introduced into the herd may become infected and develop a profuse diarrhea lasting a few days
• Vomiting and wasting disease Vomiting and wasting disease affects piglets under 10 d of age in epidemics similar to TGE. However, vomiting is characteristic, diarrhea is not a feature and laboratory differentiation is necessary.
In adults (gilts, sows and boars, TGE must be differentiated from the disease listed below.
There is no specific treatment. Because dehydration and metabolic acidosis are severe and hypoglycemia occurs, treatment with fluids and electrolytes containing glucose is indicated. Because there is loss of intestinal villi and the enzyme lactase, the ideal treatment would be to reduce the intake of milk for up to 5 days and administer a glucose–glycine–electrolyte solution orally every few hours to maintain hydration. However, removal of affected piglets from the sow is impractical and not recommended. Oral fluid therapy should improve the survival rate, affected piglets recovering in a few days following treatment. In experimentally induced TGE, removal of the milk diet and the use of an oral glucose–glycine–electrolyte solution plus a 5% dextrose solution given intraperitoneally at the rate of 25 mL/kg BW once daily decreased the severity of the diarrhea, dehydration and metabolic acidosis but did not prevent or improve significantly the renal failure and severe hypoglycemia. A newborn piglet weighing 1.25 kg has an energy expenditure of about 170 kcal/d (711 kJ) if maintained at 30°C (86°F); 30 mL of a 5% dextrose solution supplies 1.5 g of glucose for a total of about 5.6 kcal/d (the gross energy of glucose is 3.74 kcal/g). Because the volume of 5% dextrose solution injected daily into piglets should not exceed 8% of their body weight, it is unlikely that the hypoglycemia can be prevented or treated.
The use of natural human interferon given orally to piglets 1–12 days of age affected with the disease increased survival rates compared to placebo-treated piglets.59
Control of the disease is complex because it is so highly contagious and because of the dynamics of infection between the different age groups of animals within large swine herds. While there is considerable information available about the biology of the virus and the nature of the disease, there is little documented reliable information about the control in a swine-breeding herd. Most of the recommendations for control are empirical and based on clinical experience without any controlled field trials to evaluate different control strategies. What follows are guidelines for the control of TGE based on some principles which are based on the some characteristics of the virus and the disease:
• The disease is highly contagious and spreads rapidly between groups of pigs in a herd. Most epidemics last 6 weeks
• Newborn piglets are highly susceptible to disease if the sow’s milk does not contain specific TGE secretory IgA antibody
• Infection of pregnant sows with the virulent virus results in protective immunity for their piglets. Recovered sows are immune, usually do not harbor or shed the virus, and need not necessarily be culled
• Weaned pigs are a major reservoir of infection in farrow-finish herds
• Vaccination of pregnant sows with any of the available vaccines is not as effective as natural infection in providing protection for piglets
• The disease is controlled either by elimination of the virus from the herd or by controlled natural immunization and use of the all-in all-out system of production.
The highly contagious nature of the disease makes the immediate control of an outbreak in a herd virtually impossible. Epidemics usually last about 6 weeks, during which time many piglets die and the herd eventually becomes immune. Successful control depends on planning and implementation of certain strategies which must be understood and implemented by the producer, and monitored by the veterinarian. Failure of the producer to fully understand or accept the diagnosis and apply the principles of control will result in failure to control the disease and the persistence of an endemic form of the disease in the herd.11 Several strategies are used to control the infection pressure and to enhance immunity where possible.
Isolation of sows due to farrow.
To avoid further new infections of newborn piglets, sows due to farrow within 2–3 weeks should be isolated under strict hygienic conditions. However, this is usually impractical in most intensive swine production enterprises where isolation facilities are usually not available. The disease is so highly contagious that isolation is ineffective. There should be no movement of pigs between the farrowing or nursery rooms. An all-in all-out movement of pigs, especially in the farrowing rooms and nurseries, with complete cleaning and disinfection between groups is established (see all-in all-out practices discussed later).
Discontinuation of selling and purchasing breeding stock.
Once a diagnosis of TGE has been confirmed in a breeding herd which sells breeding stock, all sales should be discontinued. Likewise, all purchases of breeding stock from other herds should be discontinued for a few months until the epidemic has subsided and future production plans of the herd, including disease control, are reviewed.
Partial depopulation and culling.
If possible and feasible, all weaned pigs ready for finishing units should be moved off the farm to contract finishing units. This allows a general clean up of facilities, a break in the production cycle and an intensive all-in all-out system. All cull pigs should be destroyed to prevent a reservoir of pigs actively shedding the virus.
Planned exposure to virulent virus.
To minimize the duration and severity of the outbreak, all pregnant sows due to farrow more than 3–4 weeks ahead should be given an inoculum of virulent TGE virus obtained from virus-infected intestines, ideally from piglets in which the disease began within the last 12–24 h. The piglets should be submitted for necropsy and TGE confirmed by a diagnostic laboratory. It cannot be assumed that all piglets which die in an epidemic of TGE will be infected with the virus. The intestines of the confirmed cases should be homogenized in special media, centrifuged and supernatants poured into capsules and frozen for storage. The contents of the capsules are thawed and poured onto the feed of the sows.60 The inoculum is given daily for 3 days. Preparation and use of the inoculum insures adequate uniform inoculation of sows, compared to earlier recommendations to feed feces and intestines of piglets which died of the disease to the pigs. More inoculum can be prepared by inoculating weaned piglets in isolation and collecting their small intestines 1–2 h after onset of diarrhea, which is usually 16–21 h after inoculation. The boars are also fed the inoculum. An alternative to the inoculum is to mix the intestines from two affected piglets in 25 l water and feed 50 mL of the solution daily for 3 days.61
If there is sufficient time for immunity to develop, the piglets born 3–4 weeks later will be protected through the colostrum and milk which will contain the TGE virus-specific IgA antibodies. Piglets sucking such sows are resistant to infection while sucking, but become fully susceptible if transferred to a non-immune sow. Natural infection by mouth produces a high level of secretory antibody particularly IgA in the colostrum and milk whereas vaccination produces a good IgG response but a much lower IgA response. The newer recombinant vaccines have also been shown to be immunogenic but are still not able to produce lactogenic immunity.
An alternative to the feeding of infectious material to pregnant gilts and sows is vaccination using the available vaccines. The gilts, sows, replacement stock, boars and newborn piglets are vaccinated according to the indications of the vaccines used. However, the efficacy of the vaccines is questionable.
Biosecurity and acquisition of replacement breeding stock.
Following recovery from an epidemic in a herd, replacement breeding stock should be introduced as a group at one time and exposed to animals in the herd, monitored for clinical disease and tested. Serological testing using paired sera at 30 and 60 days after entry will indicate seroconversion to the virus. The usual precautions to prevent transmission of infection between units of the herd and between herds are necessary, including:
Washing hands and changing into clean outerwear or showering or changing into clean outerwear after being in contact with TGEv infected pigs was sufficient to prevent mechanical transmission of TGEv to susceptible pigs.62
All-in all-out management system.
The all-in all-out management and production system is based on the principle of handling, feeding and housing pigs in small subgroups as they move through the various stages of production. These subgroups either remain free of certain infectious agents, if absent, or all animals in the group become infected and immune to the infectious agents which are present in some pigs and transmitted to others in the subgroup but not to other subgroups. With this system, breeding gilts and sows are handled and bred as subgroups, kept in the gestation units as subgroups, farrowed as subgroups, and nurse their pigs as subgroups. The pigs are weaned as subgroups at the same time, the weaned pigs are placed in the nursery facilities as a subgroup at the same time and all of the pigs are moved out of the nursery to the finishing facilities at the same time. The pigs are handled in finishing units as subgroups and all pigs are marketed as a subgroup. At each stage of production the housing facilities should be cleaned and disinfected following removal of the pigs, and left vacant for a few days before a new subgroup of pigs is introduced into previously cleaned rooms. The system avoids the mixing of pigs back and forth between groups and ages, which is often done to maintain uniformity of size and age of pigs. During an epidemic, the use of a strict all-in all-out system in the farrowing and nursery units will aid in the control of clinical disease. About 2 months after the epidemic and the absence of clinical disease, sentinel seronegative pigs 2–4 months of age can be introduced to each part of the herd and monitored serologically for evidence of viral activity.
Complete depopulation and repopulation or establishment of new herd.
In some situations where the disease cannot be controlled, complete depopulation of the herd is the best option. This should be followed by repopulation with breeding stock derived from specific pathogen-free herds or minimal disease-free herds which are known to be free of the virus. Serological testing can be used to test the animals before they are moved into the facilities. The establishment of new swine herds now commonly depends on the acquisition of breeding stock from disease-free herds.
In many instances they do not provide a reliable complete protection for suckling pigs against a challenge exposure.63 However priming piglets with PRCV was very beneficial in providing resistance to TGEv and also gave a much better maternal antibody response.64
Because of the effectiveness of acquired immunity following natural infection, vaccination of the pregnant sow would appear to be the method of choice for control of the disease. However, the available vaccines have not been efficacious enough to be a reliable control strategy. Circulating VN antibodies acquired actively or passively, provide insufficient protection against clinical disease and parenteral vaccines have been relatively ineffective. Protection against the disease requires the presence of secretory IgA antibody, either actively or passively acquired, in the intestine (see immune mechanisms).
Several live-attenuated and inactivated virus vaccines are available for use in pregnant sows and neonatal pigs. Vaccines for oral and intranasal administration were developed on the basis that vaccination by the oral or intranasal route would induce the production of secretory IgA antibody. However, these vaccines have not been efficacious.
The vaccination of pregnant sows with attenuated strains of the TGE virus by either the parenteral or oral routes does not provide sufficient lactogenic immunity to protect their piglets against the virulent TGE virus. Some litters sucking vaccinated sows may achieve partial protection in which the onset of diarrhea is delayed, the diarrhea is less severe, and the case–fatality rate is decreased. Villous atrophy is inhibited to varying degrees in pigs sucking immunized sows, depending in part on the antibody titer in the colostrum and milk.
The severity of the losses in a vaccinated herd after exposure to the virus will vary, depending on:
After natural infection or experimental oral infection of pregnant sows with a virulent strain of the TGE virus, lactogenic immunity is highly protective for piglets and neutralizing antibodies in milk are mainly associated with the IgA fraction. Vaccination of sows orally with a non-attenuated vaccine provides greater levels of lactogenic protection than does orally or parenterally administered attenuated virus vaccine.65 In vaccinated sows, the levels of colostral antibody correlate with the percentage of survivability of their piglets when challenge exposed at 3–5 days, whereas the serum antibody to TGE does not. There is also a significant relationship between milk antibody and percentage survivability when pigs were challenge exposed at 5, but not at 3 days of age. There is a need to develop an attenuated virus strain which is completely avirulent for pigs, but which also replicates sufficiently in the small intestine of sows after oral administration and induces secretory IgA antibody. It appears that no strains of the virus have been identified which are sufficiently attenuated and safe for pigs while yet able to provide a sufficient immune stimulus in the intestine of the sow. The Nouzilly strain, which is a mutant of the TGE virus resistant to acidity and proteases of the digestive tracts of adult pigs, is being evaluated as a vaccine.66
If vaccines are used, it is generally recommended that the two vaccinations, 14 days apart, be given during the last trimester of pregnancy. Vaccines are available for vaccination of neonatal piglets, and weaner and finishing pigs but there is insufficient published information available on the efficacy of the vaccines based on randomized clinical trials using controls under field conditions.
Experimentally, a recombinant TGE virus S glycoprotein subunit vaccine given subcutaneously or intramammarily to pregnant sows induced colostral and milk IgG, but not IgA antibodies to the virus.67 Piglets born from vaccinated sows were challenged at 4–5 days of age with the virulent virus and the morbidity was 100% with a mortality ranging from 20–80%.67 The same vaccine given subcutaneously to 11-day-old piglets induced virus-neutralizing antibodies. This is consistent with the well-known observation that secretory IgA antibody in the milk is necessary for protection in piglets. Compared to VN antibodies, antibodies of the secretory IgA class are more effective at neutralizing the TGE virus because they are at higher titers in milk, more resistant to proteolytic enzymes, and bind to gastrointestinal enterocytes. Protective immunity to transmissible gastroenteritis correlates with milk whey secretory IgA antibody titer to the TGE virus when pigs are challenge exposed with the virulent virus at 3–5 days of age.
There is considerable cross-protection between the TGE virus and the PRCV.68 There is indirect evidence that a bronchial-associated lymphoid tissue (BALT)– mammary gland link similar to the gut-associated lymphoid tissue (GALT)–mammary gland link described for the TGE virus may exist in pregnant, multiple PRCV-exposed sows. In herds infected with the PRCV, multiple exposures of pregnant sows are associated with higher IgA and IgG antibody titers to TGE virus in milk and these titers contribute to protection against the TGE virus.68 The immunization of pregnant gilts with the PRCV induces lactogenic immunity and partial protection of piglets from challenge with the TGE virus.69 An overall survival rate of 70% was found for piglets nursing PRCV-infected gilts, compared to a 16% survival rate for piglets nursing control gilts. The highest degree of protection occurs in sows primed with the PRCV, then given a booster vaccination with the TGE virus 2 weeks later.70 Infection of pigs with the PRCV primes the systemic and mucosal humoral immune system against the TGE virus, so that subsequent challenge with the TGE virus results in a secondary antibody response and in a decreased duration of excretion of virus.71 Protective immunity to TGE virus infection can also be induced in piglets exposed to the PRCV at 2–6 days of age.72
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Vomiting and wasting disease occurs in suckling piglets1 and is characterized by vomiting and constipation with subsequent ill thrift. An acute encephalomyelitis may also occur following infection with the same virus.2 A third possibility is inapparent infections in adults.3
A coronavirus with hemagglutinating properties is the cause of both vomiting and wasting disease and an encephalomyelitis of piglets. The physical properties of the virus have been described.4,5 The same isolate may be capable of causing either syndrome following experimental infection.6
Vomiting and wasting disease was reported from Canada in 19587 and subsequently from the United States,8 United Kingdom,9 Europe,10-12 and Australia.13 Following its occurrence in Canada, a hemagglutinating virus was isolated from suckling piglets showing encephalomyelitis14 and it has been subsequently demonstrated that both syndromes are associated with the same agent.6,9 The encephalitic form has subsequently been reported in the United States.15 Although clinical disease is comparatively rare, serological surveys suggest that infection is widespread even in countries that have not experienced the disease or where it has been absent for some years.15-17 The infection is widespread in breeding swine herds.18 Piglets suckled by immune sows will obtain maternal antibodies which disappear by 15 weeks of age. Active immunity begins between 8 and 16 weeks of age. Disease presumably occurs when the virus gains access to a susceptible herd but the reason for the age limitation is unknown. Transmission is by the oral or respiratory route.6
Both syndromes tend to occur in outbreak form affecting several litters within a short period of time. Morbidity approaches 100% and the case–fatality rate is high. The disease occurs in suckling pigs as early as 2 d of age but generally not in pigs older than 3 weeks of age. The encephalitic form tends to involve younger piglets than the gastroenteric form. The clinical course of the encephalitic form is usually 2–3 d whereas pigs with vomiting and wasting may survive several days to several weeks. The course of the outbreak is usually 2–3 weeks and subsequent litters are not affected. Area outbreaks may occur.
The pathogenesis is uncertain. Vomiting and wasting disease and encephalomyelitis are probably two clinical extremes of the same disease, and both syndromes may be observed in the same outbreak.6,15 Differences in pathogenicity of the strain of virus, and age and litter susceptibility may influence the form that the disease takes. Virus may be isolated from the brain of pigs with acute encephalomyelitis but in cases of chronic vomiting and wasting disease, neither virus nor encephalomyelitis is demonstrable. It is possible that the vomiting and wasting syndrome is centrally mediated but that during the course of chronic illness the virus is eliminated and the inflammatory lesions resolved.6 Localization of antigen in the stomach wall has been demonstrated in vomiting and wasting syndromes.19 After experimental oral inoculation, the first site of multiplication is probably the nasal mucosa and then the virus replicates preferentially in the tonsils, lungs and small intestines.19 This is followed by spread to the central nervous system via the sensory nuclei of the trigeminal and vagus nerves with subsequent spread to the brainstem, cerebrum, and cerebellum. The cause of the vomition is probably due to the replication of the virus in the ganglion distale vagi. The other sign wasting is due to disturbance of the vomiting centre with persistent vomiting resulting in malnutrition.
In most cases these infections are not apparent clinically or go unrecognized and continue to cause morbidity.
In many instances the clinical signs are similar to those of poliencephalomyelitis associated with the porcine enteroviruses. Usually pigs under 2 weeks of age are affected. Mortality in young pigs is high but in older pigs they may survive and become stunted.
Vomition of yellow-green vomitus is the first sign and is accompanied by anorexia and thirst. Ineffective attempts to drink are characteristic. The temperature is usually normal or slightly elevated except for transient febrile reactions (up to 40.5°C; 105°F) for 24 h in the early stages in some pigs, and the feces are usually hard and dry. Diarrhea may occur but is not severe and occurs mostly in the older piglets. Vomiting may continue for some days but, in all affected pigs, there is severe, rapid emaciation and dehydration. They may continue in this state for some weeks and eventually die, apparently of starvation, or are destroyed by the owner.
The encephalitic form is manifest initially by depression. Piglets continue to suck for the first day but rapidly become inappetent and there is rapid loss of condition. Hyperesthesia, incoordination and muscle tremor with occasional vomiting is followed in 48–72 h by an acute encephalitis with paddling convulsions and death. Sows may show inappetence and mild fever for 1–2 d at the onset of the outbreak.
Infection may be demonstrated by the isolation of virus in primary tissue culture and growth detection is made by hemagglutination, fluorescent antibody or electron microscopic techniques.
The demonstration of rising antibody titers in surviving piglets is also satisfactory for diagnosis. Virus neutralization and hemagglutination inhibition tests are more sensitive than agar gel immunodiffusion.20
Gross necropsy findings are generally negative. Non-suppurative encephalomyelitis with perivascular cuffing, gliosis and neuronal death are present in pigs dying with clinical signs of encephalitis but not necessarily in pigs with vomiting and wasting disease.
• Histology – formalin-fixed samples of above tissues, plus stomach and celiac/cranial mesenteric ganglia (LM)
• Virology – brain (including medulla and cerebellum), Gasserian and paravertebral ganglia (ISO)
• Serology techniques for RT-PCR and nested PCR have been described.21
There is no effective treatment. As yet there are no effective vaccines. The control of the disease must depend on prior exposure of sows to infection at least 10 d before farrowing, if necessary by purposeful exposure. The piglets will be protected by colostral antibody. Sows that have had affected litters will be immune to the disease and should not be discarded from the herd.
1 Alexander TJL, Saunders CN. Vet Rec. 1969;84:178.
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4 Garwes DJ, et al. Nature Lond. 1975;275:508.
5 Pensaert MB, Callebout PE. Arch Virusforsch. 1974;44:35.
6 Mengeling WL, Cutlip RC. J Am Vet Med Assoc. 1976;168:236.
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13 Forman AJ, et al. Aust Vet J. 1979;55:503.
14 Grieg AS, et al. Can J Comp Med. 1962;26:49.
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19 Andries K, Pensaert MB. Am J Vet Res. 1980;41:1372.
Common cause of diarrhea in newborn farm animals, most commonly in calves but also in lambs, kids, piglets and foals. Rotaviruses ubiquitous in environment and 50–100% of adults seropositive. Spread by feces. Protection dependent on specific antibody in colostrum in intestinal lumen. Bovine coronavirus is also pneumotropic and causes respiratory disease
Fluid-filled intestine, dehydration. Villous and crypt atrophy
Diagnostic confirmation. Many diagnostic tests to identify viruses in fecal samples
Several families of viruses cause diarrhea in neonatal farm animals, and occasionally in adults. They include: Reoviridae, Coronaviridae, Toroviridae, Parvoviridae.
Rotaviruses of family Reoviridae are primary causes of diarrhea in calves, lambs, kids, piglets and foals. All members of the group of viruses share a common morphology and previously known as the reovirus-like viruses. Rotaviruses of human infants, calves, pigs, and foals are morphologically indistinguishable from each other and from the virus of infant mice; the lamb rotavirus is similar to both calf and pig viruses.
Atypical rotaviruses known as pararotaviruses have been isolated from pigs, cattle and lambs. They are morphologically similar to rotaviruses but do not possess the common group antigen and are characterized by their failure to be detected in serological tests such as immunofluorescence or ELISAs, which are based on the presence of the common group antigen.
Rotaviruses are assigned to serogroups with group members sharing distinctive common antigens. Currently, seven serogroups – A through G – are recognized.1 Rotavirus serogroups are further classified into serotypes based on specificity of the outer capsid proteins VP7 (G types) and VP4 (P types). At least 14G and 12P serotypes of Group A rotavirus are recognized.2 Group A rotaviruses are the most frequently detected viruses as a cause of diarrheal disease in farm animals and humans. Group B rotaviruses have been isolated from adult cows with diarrhea.3
Coronaviruses cause acute diarrhea in calves and piglets and perhaps foals.78 The coronavirus-like virus of pigs is similar to, but distinct from, the virus of TGE and is the cause of porcine epidemic diarrhea type II.
The family Toroviridae includes the Berne virus of horses and the Breda virus, which has been isolated from cattle.4
Parvoviruses have been associated with diarrhea in calves but are not significant pathogens in cattle.5
While the rotavirus and coronavirus are the most common causes of viral diarrhea in newborn farm animals (other than TGE of piglets), the adenovirus and small viruses resembling astroviruses and caliciviruses have been isolated from diarrheic calves but their etiological significance is uncertain.
Multiple mixed viral infections are being recognized more frequently as diagnostic techniques are improved. Both rotavirus and coronavirus may occur in the same diarrheic calf with or without the presence of enteropathogenic E. coli.
The general aspects of the epidemiology of viral diarrheas of newborn farm animals are described here followed by the specific epidemiological features in calves, lambs and kids, piglets, and foals. The rotavirus is used as a model.
The rotavirus is ubiquitous in the environment of domestic animals and serological surveys indicate that 50–100% of adult cattle, sheep, horses and pigs have antiviral antibody.
The rotaviruses from one species can infect members of only some other species and experimental infection of pigs, calves, and lambs with human rotavirus is possible. The calf rotavirus can infect pigs. However, the significance of interspecies infection under field conditions has not been evaluated. Cross-infection between species is not a property shared by all rotaviruses.
There are seven serogroups of rotaviruses recognized A–G,2 and Group A rotaviruses are divided into serotypes defined by surface antigens which elicit neutralizing antibodies.
The intestinal tract is the site of multiplication of the rotavirus and the virus is excreted only in the feces. Infected feces may contain as many as 1010 virus particles/g. Because rotaviruses are stable in feces and relatively resistant to commonly used disinfectants, it is extremely difficult to prevent gross contamination of animal housing once infection has been introduced. The adult animal is the primary source of infection for the neonate. The survival of the bovine rotavirus in air and on surfaces is directly influenced by the level of relative humidity.6 The rotaviruses in general survive well in an aerosol state and medium-range of relative humidity and air may be one of the vehicles for dissemination of the virus.
An important epidemiological characteristic of rotavirus and coronavirus infections in newborn farm animals is that protection against disease is dependent on the presence of specific colostral viral antibody in the lumen of the intestine of the newborn. Colostral serum antibody in the newborn does not directly protect neonates against enteritis. The protective effect of colostrum depends on its antibody titer, the amount ingested and the amount available in the lumen of the intestine. The daily oral administration of colostrum containing specific antibody or hyperimmune serum to lambs will protect them against experimental enteritis. The protection is against clinical disease and not necessarily infection. Calves, lambs and piglets may still excrete virus in the feces while they are protected against enteritis by the presence of colostral antibody in the intestinal lumen. The protection lasts only as long as colostral antibody is present within the lumen of the intestine, which explains why rotaviral diarrhea occurs commonly after 5–7 d of age. Survival from rotavirus diarrhea in calves may be dependent on a high level of serum colostral immunoglobulin.
Protection against bovine rotavirus in calves with a high titer of maternal virus neutralizing antibodies is due to both lactogenic immunity and the intestinal serum-derived antibody.7
Many of the epidemiological characteristics of neonatal calf diarrhea associated with the rotavirus and coronavirus must be considered in the context of ‘acute undifferentiated diarrhea of newborn calves’ because mixed infections are more common than single infections.
In cattle, serogroups A, B and C are most commonly reported.2 The bovine group A rotavirus was first isolated in the United States in 1969 as the cause of outbreaks of diarrhea in beef calves in Nebraska. Since then the virus has been recovered from calves affected with diarrhea and normal calves worldwide.2,8 Diarrhea due to Group A rotavirus occurs in calves from 1–2 weeks of age.
The prevalence of infection with Group A rotavirus is high. An overall prevalence of approximately 30% in normal neonatal animals and 40% in diarrheic animals is common.8 Within 14 d of age, 94% of dairy calves may be positive for Group A rotavirus and 80% may be positive for Group B.9 Antibody to rotavirus is present in the serum of 90–100% of young adults, and clinically normal calves with high serum antibody may excrete the virus. The explanations include:
• Calves may be protected from disease but excrete the virus during the colostral feeding period
• Calves may be infected at a subclinical level but still excrete the virus although they may have been denied colostral antibody.
Non-Group A rotaviruses, which have Group B characteristics, have been found in normal and diarrheic calves in India.10
The prevalence of infection with Groups B and C bovine rotaviruses is less than A. Approximately 70% of adult cattle are seropositive to Group B and 50% of adult cattle are seropositive to Group C.2 Serotypes G6 and G10 Group A rotaviruses are the most prevalent serotypes in dairy and beef calves with diarrhea in the United States.11 Unusual Group A rotaviruses have been associated with epidemics of diarrhea in 3-month-old calves, which age is unusual.11 The predominant G serotypes of bovine rotaviruses from outbreaks of diarrhea in dairy and beef calves in Australia have been determined.13
Rotavirus strains belonging to G10P11 constitute the largest proportion of bovine rotaviruses in cattle throughout India.14 This has major zoonotic implications because this strain is related to those found in newborn children in India.
The prevalence of subclinical infection may be greater than that indicated by isolation of the virus from feces. Rotavirus–immunoglobulin and coronavirus–immunoglobulin complexes may be present in the feces of 44% and 70% of adult cattle, respectively, while the free rotavirus and coronavirus may be absent or present in only 6% of fecal samples, respectively. Clinically normal cows can shed the virus for several weeks in the presence of fecal and serum antibody. Repeated bovine rotavirus infection and re-excretion can occur in calves several months of age, even in the presence of serum antibodies. Clinically normal calves may also shed the virus and there may be histological evidence of lesions of the small intestine due to rotavirus infection.
In a longitudinal survey of calves in a herd over two successive calving periods, the rotavirus was first detected in calves at about 6 d of age. Diarrhea or excretion of abnormal feces was associated with rotavirus infection in 58% of infected calves while in the remaining 42% infection was subclinical. The overall prevalence rate of infection can vary from 16–80% among beef and dairy calves. In a random sample of dairy herds in Ohio in the United States, the prevalence of bovine Group A rotavirus detected in the feces of calves was 16% at an average of 14 d. Of all calves with liquid feces, 28% were shedding rotavirus, in comparison with 25% with semi-liquid feces, 23% of those with pasty feces, and 11% of those with firm feces.
Group C rotavirus, designated the Shintoku strain, has been isolated from adult cattle in Japan with diarrhea. A significant antibody response to the isolate was detected in convalescent-phase sera of cows which excreted the virus and no antibody response to bovine Group A was observed.
Rotavirus was detected in the feces of 43% of neonatal diarrheic dairy calves in Spain.15 A concurrent infection was detected in 58% of the rotavirus infected calves, and the most common mixed infection was rotavirus-Cryptosporidium. The detection rates of the other enteropathogens with rotavirus infection were 20% for coronavirus, 85% for Cryptosporidium, 17% for F5+ E. coli and 2% for Salmonella. As age of calf increased, the detection rates of other enteropathogens decreased.
The factors which influence rotavirus infection and its clinical severity include:
Calves are most susceptible to rotavirus diarrhea between 1–3 weeks of age. This age occurrence is related in part to the rapid decline in specific colostral antibody to the rotavirus.
The mortality is highest in the youngest animals which have received insufficient colostrum and are subjected to severe weather conditions.
The occurrence and distribution of the virus in diarrheic and normal calves have been studied. The rotavirus has been found before, during and after the onset of diarrhea. It has been found along with the coronavirus and adenovirus in diarrheic calves. This serves to emphasize that while the virus can be considered as a primary pathogen in calves, the results of field and laboratory investigations indicate that multiple mixed pathogen infections are probably more common than single pathogen infections. There are also differences in the virulence between isolates. Earlier work suggested that most cases of naturally occurring rotavirus diarrhea in calves were also infected with E. coli. However, no attempt was made to determine if the E. coli possessed the virulence characteristics described under colibacillosis.
While the rotavirus has been most commonly associated with outbreaks of diarrhea in beef calves raised in groups outdoors, it has also been recovered from dairy calves raised together in large groups in large dairy herds. The morbidity rate in beef herds has varied from herd to herd and from one year to another.
The survival of the bovine rotavirus in air and on surfaces is directly influenced by the level of relative humidity.6 The rotaviruses in general survive well in an aerosol state and medium range of relative humidity and air may be one of the vehicles for dissemination of the virus.
There are differences in virulence among the bovine rotaviruses, which may explain the variability in the severity of disease in natural outbreaks and must be considered when developing vaccines.
Rotaviruses possess two outer capsid proteins: VP4 and VP7. The neutralization specificity related to VP7 is the G serotype and that associated with VP4 is referred to as P serotype. On the basis of G types, at least 14 serotypes of Group A rotaviruses have been described in animals and humans. The Group A rotavirus has been classified as 14 G serotypes, 11 P serotypes, and 20 P genotypes.
Specific G and P types have been associated with specific animal species. For example, human rotaviruses most commonly belong to G types 6, 8, and 10 and have P types. As more rotaviruses have been characterized from diverse locations worldwide, the host species specificity of P and G types has become less distinct. G types 6, 8, and 10, once thought to be specific to cattle, have been found in humans.
Determination of the serotype specificity is important for development and evaluation of more efficacious vaccines but it is complicated by the dual serotype specificity of the rotavirus. Strains of bovine Group A rotavirus have been isolated from beef herds which had been vaccinating the pregnant cows with the commercial vaccine containing Group A rotavirus.16 The serological and genotypic characterization of strains responsible for the diarrhea were a bovine rotavirus reassortant with a P gene different from the vaccine virus. In the United States, commercially available rotavirus vaccines for cattle contain only strain NCDV-Lincoln.
G and P genotypes of Group A rotaviruses have been found circulating in calves in Brazil,17 and in diarrheic calves born to cows vaccinated against the NCDV (P[1], G6) rotavirus strain.18 The prevalence of the rotavirus G and P types in diarrheic calves in Japan changes in certain areas periodically.19 A Group A rotavirus with G serotype 8 has been isolated from diarrheic calves in Japan.20 A bovine Group 3 rotavirus has been identified and characterized which causes age-dependent diarrhea in cattle in the UK.21
In Sweden, in beef and dairy herds, several bovine rotavirus G-types (G3, G6, and G8, and G10) were detected.22 In beef herds, G6 predominated, while G10 predominated or was the only G-type in dairy herds. In Sweden, the same strains of the virus persist for several years which is probably associates with the limited exchange of cattle between dairy and beef suckled herds which are characteristically small.
It is now becoming clear that in contrast to natural recovery from infection, which results in high titers of P-specific neutralizing antibodies, parenteral administration elicits primarily G-specific neutralizing antibodies. Thus, failure in passive protection with a monovalent vaccine for prevention of rotavirus-associated diarrhea in neonatal calves may be less than optimal due to diversity of P and G types occurring in nature.
Natural subclinical infections are common in calves in the second week of life, which raises doubts about rotavirus pathogenicity. Experimentally, the clinical outcome of infection is dependent on both age and rotavirus isolate. Age-dependent resistance to infection was not found. Bovine rotaviruses differ in virulence for calves in the second week of life and older calves are susceptible to rotavirus infection and disease.23
The calf rotavirus can be experimentally transmitted to piglets and has been isolated from natural outbreaks of diarrhea in piglets. The isolation of a rotavirus from neonatal deer affected with diarrhea in a zoo in Australia raises some interesting epidemiological possibilities.
In some herds the disease starts at a low rate of 5–10% in the first year, increases to 20–50% in the second, and to 50–80% in the third year. In other herds, explosive outbreaks affecting 80% of the calves have occurred in the first year. The case–fatality rate has also been variable – in some herds as low as 5%, while in others as high as 60%. The mortality rate probably depends upon the level of colostral immunity in the calves, the incidence of enteric colibacillosis, and the level of animal husbandry and clinical management provided in the herd.
The virus is excreted by both calves and adult cattle in large numbers (up to 1010/g of feces) and excretion may last for several weeks. Even under open-range conditions, there is a rapid spread of the virus throughout calves which come into frequent contact with each other, particularly during the calving season. Calves are infected after birth from the dam’s feces or from other infected diarrheic calves. Pregnant cows excrete the rotavirus intermittently throughout pregnancy, from one calving to the next, and provide a direct source of infection for the newborn calf. Both subclinically infected and diarrheic calves infected by rotavirus can be a source of infection for other in-contact calves.
Newborn calves are protected from the rotavirus only during the first few days after birth, when colostrum contains specific rotavirus antibody which is active in the lumen of the intestine. This correlates well with the peak incidence of rotavirus diarrhea which is at 5–7 days of age, and coincides with a marked drop in colostral immunoglobulins by the third day after parturition and an incubation period of 18–24 h for the disease to occur. The levels of serum and colostral antibody are lower in first-calf heifers, which explains the higher morbidity and mortality in their calves. The serum colostral immunoglobulins of the calf may also be transudated from the serum into the intestine and complement the role of colostral and milk antibodies in the lumen of the intestine.
Bovine coronavirus causes diarrhea in both dairy and beef calves worldwide ranging in age from 1 day to 3 months of age but mostly between 1 and 2 weeks of age. Disease is more common during the winter months, which may reflect enhanced survival of the virus in a cool, moist environment. The virus is ubiquitous in cattle populations and the majority of adult cattle are seropositive. The coronavirus may be present in both diarrheic and healthy calves; the incidence rates range from 8–69% and 0–24% for diarrheic calves and healthy calves, respectively.
The virus can be shed by up to 70% of adult cows despite the presence of specific antibodies in their serum and feces. The peaks of shedding are during the winter months and at parturition in North America. Calves born to carrier cows are at a higher risk of diarrhea. Subclinical persistence and recurrent infections are also common in both neonatal and older calves and virus excretion from these animals may maintain a reservoir of infection.
Vaccination of the cows with a modified live rotavirus–coronavirus E. coli combination vaccine does not influence seasonal shedding, but in vaccinated cows the incidence of shedding does not increase at parturition as it does in non-vaccinated cows. The bovine coronavirus isolates all belong to a single serotype as polyclonal sera have detected only minor antigenic variations.
The bovine coronavirus is also a pneumotropic virus which can replicate in epithelium of the upper respiratory tract. In dairy calves, initial infections occur when the calves are 1–3 weeks of age but there are multiple episodes of shedding of viral antigens or seroconversion when the calves are several weeks of age. Clinical signs of respiratory disease occur between 2 and 16 weeks of age but are mild. A more severe lower respiratory tract infection causing minor lung lesions has been reported but is also not severe enough to warrant treatment. Such infections are probably common in closed herds with recurrent subclinical infections occurring in older calves. Persistence of infection or reinfection of the upper respiratory tract with the virus is also common. The amount and specificity of bovine coronavirus maternal antibodies in calves at the time of infection with the virus may interfere with the development of an active antibody response in serum and mucosal secretions. The fecal–oral route is the presumed method of transmission but aerosol transmission may also occur.
The bovine coronavirus has been isolated from wild ruminants with diarrhea.24 Feces from diarrheic sambar deer, one waterbuck and one white-tailed deer in wild animal habitats contained coronavirus particles which were identified as bovine coronavirus. Gnotobiotic and colostrum-deprived calves inoculated with the isolates developed diarrhea and shed coronavirus in their feces and nasal discharge.24 Thus wild ruminants may harbor coronavirus strains transmissible to cattle.
The bovine coronavirus of winter dysentery in adult cattle is closely related to the coronavirus causing diarrhea in young calves.25 There is no evidence for serologic or in vivo antigenic differences between these two bovine coronaviruses.
The parvovirus has been associated with outbreaks of postweaning diarrhea in beef calves but it pathogenicity is uncertain. Seroprevalence studies found 49–83% of adult cattle seropositive to the virus over a 2-year period.5
Breda virus, a member of the genus Torovirus, has been isolated from the feces of neonatal calves with diarrhea in Iowa, Ohio, and several areas in Europe, and in Canada.26 In Ohio, the virus was detected in 9.7% of fecal samples from cattle with diarrhea; it occurred in 26% of the total calf samples.27 It is a common virus in the feces of calves with diarrhea on farms in Ontario. In veal calf operations in Ohio, 24% of calves shed the virus during the first 35 days after arrival and the shedding of the virus was associated with diarrhea. Calves shedding additional pathogens were more likely to have diarrhea than those shedding less than one pathogen.28 Calves which were seronegative or had low antibody titer to the Breda virus on arrival were more likely to shed the virus than those calves which were seropositive on arrival.
More than 88% of adult cattle are seropositive to the Breda virus.4 More than 90% of newborn calves have high maternal antibodies to the virus which wane at a few months of age, followed by active seroconversion between 7 and 24 months of age.
Noroviruses, formerly known as Norwalk-like viruses have been recognized as the most common pathogens involved in outbreaks of acute non-bacterial gastroenteritis in humans. The Norovirus genus, is one of four genera in the Caliciviridae family. Norovirus-specific DNA has been detected in the fecal samples of diarrheic calves in Michigan and Wisconsin.29 The complete genomic sequence of an enteropathogenic bovine calicivirus isolated from Nebraska has been characterized and its enteropathogenicity documented.30 The seroepidemiological prevalence of the Jena virus, a bovine enteric calicivirus, is 99% in some selected cattle populations in Germany.31 In The Netherlands, the Norwalk-like virus (NLV) in endemic in veal calf operations and in a selected dairy herd.32 The highest number of NLV positive veal calf farms were found in the regions with the highest number of veal calf farms. The virus is endemic in cattle populations and genetically distinct from the Norwalk-like virus in humans.
The rotavirus has been isolated from the feces of lambs under 3 weeks of age with diarrhea. The disease is sporadic and no particular epidemiological characteristics have been described. Rotaviruses have been found in the feces of diarrheic lambs born in lambing sheds where about 1300 lambs were born on one sheep ranch and 3000–5000 on another ranch within 30 d. Clinical signs were noticed as early as 12–16 h after birth.33 Morbidity approached 100% and mortality 10%. The virus was not classified as Group A but probably Group B.
In a series of fecal samples from diarrheic and normal lambs and goat kids from 1–45 d of age, the prevalence of Group A rotavirus infection in diarrheic lambs was very low (2.1%).34 Group A and B rotaviruses were detected in 8.1% and 13.5% of diarrheic goat kids. Group C rotaviruses were detected in four normal goat kids. An association of diarrhea and infection was demonstrated only for Group B rotaviruses.
Atypical rotaviruses, possibly Group B, have been isolated from the feces of diarrheic goat kids.35 Affected kids were 2–3 d of age and the disease was severe with marked dehydration, anorexia, and prostration.
The experimental disease in lambs is mild and characterized by mild diarrhea, abdominal discomfort and recovery in a few days. The mortality in lambs is much higher when both the rotavirus and enteropathogenic E. coli are used.
Group A rotaviruses are a common cause of diarrhea in nursing pigs from 1–5 weeks of age with peak occurrence from 1–3 weeks of age, and weanling pigs at 3–5 weeks of age and within 3–5 d of weaning. Groups A, B and C occur in diagnostic surveys with about 90% belonging to Group A.36 Group C rotavirus has also been found to be the cause of enzootic neonatal diarrhea in a minimal disease herd. Multiple rotavirus G serotypes and P types have been detected in swine.37 There is little or no cross-protection between porcine rotaviruses with distinct G and P types, but viruses which share common G and P types induce at least partial cross-protection in experimental studies.38 Variant serotypes of porcine rotavirus such as G3 may cause severe outbreaks of diarrhea in piglets. Subclinical infections are common, and age resistance to rotavirus infection may not occur.
In an infected herd, piglets become infected between 19 and 35 d of age, and the virus cannot be detected in piglets under 10 d of age, presumably as a result of protection by lactogenic antibody. It is suggested that in an intensive piggery, with a constant shedding of viruses in feces of sows before and after farrowing leading to continuing cycle of rotavirus infection with a build-up of host immunity against a circulating strain in the pig population, a virus such as CRW-8 probably could undergo changes through mutations over a period of time leading to antigenic drift.
In piglets, rotaviral diarrhea is most common in pigs which are weaned under intensive management conditions and the incidence increases rapidly from birth to 3 weeks of age.36 There is no age-dependent resistance up to 12 weeks of age. The disease resembles milk-scours, or 3-week scours of piglets. Mortality due to rotavirus varies from 7–20% in nursing pigs and 3–50% in weaned pigs depending on the level of sanitation. In the United States, the peak incidence occurs in February and a moderate rise occurs in August–September.36
A case–control epidemiological study examined the relationship between Group A rotavirus and management practices in Ontario over a 5-year period.39 In rotavirus-positive herds, herd size was larger and weaning age was younger compared with rotavirus-negative herds.39 Pigs raised in all-in all-out nurseries were 3.4 times more likely to have a positive Group A rotavirus diagnosis than in pigs in a continuous flow system. Pigs in the all-in all-out system were weaned at an earlier age.
The sow is the source of infection. Seropositive sows can shed rotavirus from 5 d before to 2 weeks after farrowing, when piglets are most susceptible to infection. There are increased secretory IgA and IgG antibodies to rotavirus in the milk of sows after natural rotavirus infection or following parenteral inoculation of pregnant or lactating sows with live attenuated rotaviruses.38 The early weaning of piglets at a few days of age or at 3 weeks of age results in the removal of the antibody supplied by the sow’s milk.
Continuous transmission of the virus from one group to another is an important factor in maintaining the cycle of rotaviral infection in a piggery. The virus can be found in dust and dried feces in farrowing houses which have been cleaned and disinfected. This suggests that the environment is also an important source of infection. The porcine rotavirus can survive in original feces from infected pigs for 32 months at 10°C.40 Gilts and sows shed virus antigen prior to farrowing and during lactation, which makes it next to impossible to eliminate the infection from a herd. As sows increase in age they develop increasing levels of lactogenic IgA rotavirus antibodies but do not transfer increasing levels of protection to their piglets.
Different electrophoretypes of Group A rotavirus and different groups of rotaviruses may occur at the same time in a single piggery, which must be considered when developing vaccines. The subgroups of group A porcine rotaviruses have been classified and there are differences in virulence of isolates. Most isolates from outbreaks of diarrhea belong to Group A while a small percentage are atypical rotaviruses. Some porcine rotaviruses are related antigenically to human rotavirus serotypes 1 and 2. Porcine rotaviruses which display the typical bovine P[1], P[5], P[11], G[6], and G8 genotypes have been detected in pigs which indicates the high frequency of rotavirus transmission between cattle and pigs.41 The various G and P types of the virus have been examined and compared in Poland and the US.42
Atypical rotaviruses and other enteroviruses are often present in preweaning and postweaning diarrhea in swine herds and should be considered as potential pathogens.46 Some atypical rotaviruses are associated with villous epithelial cell syncytia in piglets with enteritis. Single and mixed infections of neonatal piglets with rotaviruses and enteroviruses have been described. Combined rotavirus and K99+ E. coli infection causes an additive effect when induced experimentally in gnotobiotic pigs.32 The inoculation of calici-like viruses into gnotobiotic piglets can result in diarrhea and villous atrophy. Diarrhea in unweaned piglets 1–3 weeks of age has been associated with a combined infection of rotavirus and Isospora suis. The combined effect of a dietary change at weaning and rotavirus infection in gnotobiotic piglets is a temporary villous atrophy and there is no evidence of persistent atrophy of the small intestine.
The porcine epidemic diarrhea virus is a coronavirus-like virus which causes diarrhea in pigs, similar to TGE except much less severe and with less mortality. Two clinical forms of the disease have been described: porcine epidemic diarrhea types I and II. Porcine epidemic diarrhea type I causes diarrhea only in pigs up to 4–5 weeks of age. Porcine epidemic diarrhea type II causes diarrhea in pigs of all ages. The morbidity may reach 100% but mortality is low. The disease may start in the finishing pigs and spread rapidly to pregnant sows and their nursing piglets.44 The diarrhea may persist in the 6 to 10-week-old pigs and seronegative gilts introduced into the herd may become infected and develop a profuse diarrhea which lasts a few days.
A porcine respiratory coronavirus with close antigenic relationship to the transmissible gastroenteritis virus has been identified as enzootic in the United Kingdom and some European countries.45 A Canadian isolate of the virus inoculated into 8-week-old piglets caused polypnea and dyspnea and diffuse bronchioloalveolar lesions.46 Seroprevalence studies in Spain reveal that 100% of large herds and 91% of small herds had animals with antibodies.45 Only mild or inapparent respiratory signs occur and the growth of finishing pigs may be temporarily affected.
Although rotavirus infection is a common cause of diarrhea in foals, there are surprisingly few epidemiological and microbiological data. Group A rotaviruses are a major cause of diarrhea in foals up to 3 months of age.47 Most equine rotaviruses are distinct from those of other species with a distinctive electropherotype and subgroup reaction. Rotavirus serotype G3 and subtypes predominate in horses.47 In Germany, G3 P[12] is the predominating type of rotavirus in the horse.48
The virus can be isolated from the feces of healthy foals and from diarrheic foals in outbreaks of diarrhea. Outbreaks of the disease occur on horse farms with a large number of young foals where the population density is high. A case–control study of foal diarrhea in the United Kingdom over a 3-year period revealed rotavirus was a significant pathogen associated with diarrhea in foals.49 The other common pathogens were Clostridium perfringens, Strongyloides westeri, and Cryptosporidium spp. A survey of the enteric pathogens in diarrheic thoroughbred foals in the United Kingdom and Ireland revealed a prevalence of 37% rotaviruses, and 8% in normal foals. In a United States survey of diarrhea in foals on 21 thoroughbred horse farms, rotavirus was detected only in diarrheic foals. A higher percentage of foals born to visiting mares developed diarrhea, compared to foals born to resident mares. Serological surveys indicate the presence of the rotavirus antibody in almost all of the mares whose foals are infected with the virus.50
The rotavirus infects mature brush border villous epithelial cells in the small intestine and to a lesser extent in the large intestine. The infected cells are sloughed, leading to partial villous atrophy and the atrophic villi are rapidly recovered with relatively undifferentiated crypt cells which mature over a few days and result in the healing of the lesion. The activity of the mucosal β-galactosidase (lactase) in the brush border of the villous epithelium is less than that found in normal animals, which results in decreased utilization of lactose. This reduction in enzymes is associated with immature enterocytes on the villi during rotavirus infection. In vitro studies have suggested that lactase may be the receptor and uncoating enzyme for rotavirus, which may explain the high degree of susceptibility of the newborn with high levels of lactase. The net effect of the morphological and functional changes in the intestine is malabsorption resulting in diarrhea, dehydration, loss of electrolytes and acidosis. The diarrhea in milk-fed calves with the experimental disease ceases if the milk is withdrawn and replaced with glucose and water, which is similar to transmissible gastroenteritis. The d-xylose absorption test can be used to measure the degree of malabsorption in calves infected with rotavirus.
The pathogenesis is similar in calves, lambs, pigs and foals. Lesions occur within 24 h after infection, villous epithelial cells of the small intestine are infected and become detached, and regeneration occurs within 4–6 d after the onset of the diarrhea. The intestinal villi usually return to near normal within about 7 d after recovery from the diarrhea. However, calves and pigs may require 10–21 d to fully recover to a normal growth rate following rotavirus infection. Experimental rotaviral infection in 3-week-old piglets results in diarrhea, anorexia and vomiting. Villous atrophy of the small intestine is the most severe lesion which returns to normal in 6 d. Infection and clinical disease developed in the presence of serum-neutralizing antibody obtained from seropositive sows.
While it has been generally accepted that lactose malabsorption is an important factor in the pathogenesis of diarrhea, the experimental infection of gnotobiotic lambs with rotavirus did not result in lactose intolerance as assessed by the measurement of reducing substances in the feces or by the clinical effects and blood glucose levels after a lactose load. Lactose intolerance could be demonstrated by using extremely high doses of lactose, three to four times the normal dietary intake. Thus, lactose-containing feeds such as milk are not necessarily contraindicated in rotavirus diarrhea.
A combined infection with rotavirus and enterotoxigenic E. coli may result in a more severe disease than produced by rotavirus infection alone, particularly in calves several days of age when the rotavirus normally produces a mild disease and when calves are resistant to enterotoxigenic colibacillosis. The intestinal lesions of villous atrophy are also more severe and extend into the colon in dual infections. Naturally occurring cases of the dual infection in calves are considered to be more severe than single infections. Under field conditions more than one enteropathogen is likely to be involved in the pathogenesis of the diarrhea.
Experimentally, in gnotobiotic 1-day-old calves, concurrent infection with bovine virus diarrhea virus and bovine rotavirus results in a more severe enteric disease than that associated with either virus alone.51 The BVDV potentiated the effect of the rotavirus. Severe lymphoid depletion was associated with BVDV infection regardless of the concurrent rotavirus infection. The clinical findings of induced combined BVDV and rotavirus infections in neonatal calves at 8 to 9 days of age are much more severe and the duration of diarrhea much longer than in rotavirus infection alone.52
The pathogenesis of coronaviral enteritis in calves is similar to the rotavirus infection. The villous epithelial cells of the small and large intestines are commonly affected. The crypt epithelium is also affected, which makes regeneration of villous epithelial cells much longer, which in turn results in persistent diarrhea for several days and death from dehydration and malnutrition. Experimental infection of calves with virulent bovine coronavirus results in depletion of lymphocytes in the mesenteric lymph nodes and Peyer’s patches, low levels of immunoglobulins and generalized immune suppression.53 Experimental infection with the attenuated virus results in lower levels of intestinal immunoglobulin titers than with the virulent virus. Experimentally, newborn calves are capable of mounting an intestinal immune response to bovine coronavirus and vaccine failures may be the result of overattenuation of the virus.53 The pathophysiological changes due to coronavirus-induced diarrhea in the calf have been described and are similar to the changes which occur in acute diarrheal disease in the calf associated with other enteropathogens.
This virus replicates in the villous epithelial cells of both the small and large intestine and clinically resembles TGE of piglets. There is no evidence that rotavirus infection in piglets is accompanied by increased permeability of the intestine to macromolecules.54
This virus causes degeneration of the villous epithelial cells of the proximal part of the small intestine leading to villous atrophy, a reduction in disaccharidase activity and xylose malabsorption. In gnotobiotic calves experimentally infected with the Breda virus, the villous epithelial cells of the ileum and colon are affected, including the dome epithelial cells.
Experimental infection of calves with the parvovirus results in lymphopenia and viremia and damage to the small intestinal crypt epithelium and the associated mitotically active lymphoid tissues. Villous atrophy occurs because of failure of replacement of villous epithelial cells. By 5 d after inoculation there was evidence of repair of the intestinal lesions. Following experimental challenge the tonsillar tissues, intestinal mucosa and mesenteric lymph nodes all become infected. Subsequent spread also results in greater involvement in the large intestine and the upper jejunum, Peyer’s patches and mesenteric lymph nodes.
The naturally occurring disease usually occurs in calves over 4 d of age and is characterized by a sudden onset of a profuse liquid diarrhea. The feces are pale yellow, mucoid and may contain flecks of blood. Recovery usually occurs in a few days. Explosive outbreaks occur and up to 50% of calves from 5–14 d of age in the affected population may develop the disease. If enterotoxigenic E. coli are present, the disease may be acute; dehydration is severe and deaths may occur. Multiple mixed infection with E. coli, coronavirus, and Cryptosporidium spp. are common in calves over 4 d of age and thus it may be impossible to describe a typical case of uncomplicated naturally occurring rotavirus or coronavirus-like diarrhea. There is a tendency for viral diarrhea in newborn calves to occur in explosive outbreaks; the calves are usually not toxemic, but the character of the diarrhea cannot be differentiated clinically from that associated with the other common enteric pathogens of newborn calves.
A coronaviral enteritis affecting calves from 1–7 d of age has been described, but there are no distinguishing clinical characteristics. The diarrhea may be persistent for several days, followed by death in spite of fluid therapy and careful realimentation with milk. The feces are voluminous, mucoid and slimy, and may be dark-green or light-brown in color.
Experimentally, newborn gnotobiotic lambs develop diarrhea 15–20 h following oral inoculation and show dullness and mild abdominal discomfort. There are only a few documented descriptions of naturally occurring rotaviral diarrhea in newborn lambs. Affected lambs under 3 weeks of age develop a profuse diarrhea and the case–fatality rate is high. It is not clear if outbreaks of uncomplicated rotaviral diarrhea occur in newborn lambs.
Rotaviral diarrhea may occur in nursing piglets from 1–4 weeks of age and in pigs following weaning.15 The disease in nursing piglets resembles milk-scours or 3-week scours. Most of the pigs in the litter are affected with a profuse liquid to soft diarrhea with varying degrees of dehydration. Recovery usually occurs in a few days unless complicated by enterotoxigenic E. coli or unsatisfactory sanitation, overcrowding and poor management. The disease is often most severe in herds in which there is continuous farrowing with no period of vacancy for cleaning and disinfection in the farrowing barn. The disease may also occur in pigs a few days after weaning and may be a major factor in postweaning diarrhea of piglets weaned at 3 weeks of age or earlier in the case of weaning pigs at 1–2 d of age.
Porcine epidemic diarrhea type I affects piglets only up to 4–5 weeks of age and is characterized by profuse watery diarrhea, high morbidity and low mortality.
Porcine epidemic diarrhea type II causes a profuse fluid diarrhea in pigs of all ages, including nursing piglets. Explosive outbreaks may occur and the morbidity may reach 100%. Mortality is usually restricted to piglets under 3 weeks of age.
Affected foals appear depressed, fail to suck and become recumbent. The temperature ranges from 39.5–41.0°C (103–106°F) and the respiration may be rapid and shallow. There is a profuse, watery, non-fetid diarrhea which results in severe dehydration and electrolyte imbalances. Recovery following treatment usually occurs within 2–4 d. Death may occur within 24 h after the onset of diarrhea.
Fecal samples (20–30 g) should be collected from affected animals as soon possible after the onset of diarrhea and submitted to the laboratory in a chilled state. Samples of intestinal mucosa from several sections of the small and large intestine should be submitted chilled for virus detection and possible isolation.
Because multiple mixed viral and bacterial infections are common, the request for a laboratory diagnosis must include consideration of all of the common pathogens. The viruses are much more difficult to detect than bacterial enteropathogens. In herd outbreaks, fecal samples from several affected animals and some normal animals should be submitted. The rotavirus will usually be present in both normal and diarrheic animals, which presents problems in interpretation and requires evaluation of the clinical and epidemiological findings.
Several laboratory tests are available for detection of rotaviruses and coronaviruses in the feces and intestinal contents and tissues. The particular test used will depend on the facilities and equipment available.
Demonstration of the virus in feces using electron microscopy has been a standard diagnostic technique. It is easier to see the virus if it has been concentrated by ultracentrifugation or clumped by immune electron microscopy using specific antiserum. With electron microscopy, the virus can be detected for up to 6–10 d after the onset of diarrhea. Protein A-gold immunoelectron microscopy is a valuable test to detect bovine coronavirus in the feces and nasal secretions of infected calves. However, since the equipment and expertise necessary for electron microscopy are not available in many laboratories, alternative diagnostic techniques have been developed.
Several tests are based on immunofluorescence. These include immunofluorescent staining of fecal smears and cell culture immunofluorescence of fecal preparations. Immunofluorescent staining of a fecal smear is a more convenient test for diagnostic laboratories because a diagnosis can be made in a few hours and an electron microscope is not necessary. However, the immunofluorescence tests may not be as reliable as some other tests. The fluorescent antibody technique will only detect the virus within epithelial cells which are present in the feces for 4–6 h after the onset of diarrhea. In some studies the fluorescent antibody technique detects the virus in only 20% of samples while electron microscopy detected the virus in about 60% of the samples.
Immunodiffusion and electron microscopy.
This test is superior to the fluorescent antibody technique. Treatment of the feces with chymotrypsin improves the detection rate. Monoclonal antibodies to porcine Group C rotavirus can be used in an immunofluorescent test and may have wider applications in the study of Group C rotavirus diarrheas in swine, cattle, and potentially, other species.
Testing immunofluorescent sections of spiral colon is the diagnostic method of choice for the detection of coronavirus in calves; fecal samples are unreliable. Isolation of coronavirus in tracheal organ culture is the most sensitive in vitro culture technique. A hemadsorption–elution–hemagglutination assay test for the detection of coronavirus in the feces of calves is a simple and rapid procedure. A counterimmunoelectrophoresis test is available for the detection of coronavirus in calves. An immunohistochemical technique can be used to detect the virus of porcine epidemic diarrhea in the small intestine.55
These tests are more sensitive and simple than immunoelectro-osmophoresis, complement fixation, immunofluorescence on inoculated cell cultures or electron microscopy for the detection of rotavirus in calf feces. The ELISA is effective in detecting the presence of porcine rotavirus in feces and was confirmed in two-thirds of the samples tested using electron microscopy, immunofluorescence, and polyacrylamide gel electrophoresis (PAGE).56 A blocking ELISA using monoclonal antibodies can detect the porcine epidemic diarrhea virus in feces and serum antibodies in both naturally and experimentally infected piglets and earlier than an indirect immunofluorescence test.57
A competitive blocking ELISA (CB-ELISA) is considered most suitable for routine detection of porcine epidemic virus in the feces of pigs.58
The ELISA or electron microscopy of feces are equally reliable in detecting the rotavirus and coronavirus in the feces of experimentally infected calves. The agreement between the two tests was 95% for coronavirus and 84% for rotavirus. There will always be borderline samples containing antigen in quantities near the detection limit for each test. Some samples will be positive for one test and negative for the other, and vice versa. This problem can be minimized if several individual samples from a disease outbreak are examined. The morphological identification of rotavirus is usually straightforward but the pleomorphism of bovine coronavirus can present problems. The ELISA may also fail to detect viral antigen in feces which also contain antibody. The test can provide diagnostic results within 24 h after collection of the fecal samples.
Reverse passive hemagglutination (RPHA), ELISA and polyacrylamide gel electrophoresis (PAGE). Three techniques for the detection of rotavirus in fecal samples from diarrheic calves have been compared. The RPHA was at least as sensitive as the ELISA, and both were compared with the PAGE. The overall agreement between RPHA and PAGE was 96%; the ELISA was not as sensitive. The commercial ELISA has a slightly higher sensitivity than agglutination, PAGE, and concentrated PAGE, but the specificity of ELISA is lower. The latex agglutination test has a lower sensitivity than ELISA but its specificity is higher.59 The latex agglutination test is easy to perform, more sensitive than electron microscopy and more specific for detection of rotavirus. A dot hybridization assay can detect and differentiate two serotypes of porcine rotavirus.
The fast and inexpensive ELISA combined with the highly specific and sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) is a practical approach epidemiological studies of bovine rotavirus.60
PCR assays are now available for the detection of bovine rotaviruses in feces.9
Non-radioactive PCR-derived cDNA probe assays can be used to detect rotavirus serotypes.61
A rapid ELISA using monoclonal antibodies can be used for the simultaneous detection of bovine coronavirus, rotavirus serogroup A and E. coli K99 antigen in the feces of calves.35 The specificity of all of the components was more than 90% specific and the sensitivity for bovine coronavirus, K99 E. coli, and rotavirus, 77%, 93% and 100%, respectively.35
This is a test for the detection of Group A rotavirus in the feces of calves, piglets and foals, has a sensitivity of 89% and specificity of 99% compared to the ELISA, and its reproducibility is 100%.62 It is a one-step procedure, simple to use, very rapid and can be carried out on the farm.
A field enzyme immunoassay test (Rotazyme test) is highly accurate and reliable for the detection of rotavirus in the feces of horses with and without diarrhea. The test is a simple, rapid and specific procedure which can take the place of a more expensive and slower procedure such as electron microscopy.
ImmunoCardSTAT Rotavirus [ICS-RV] is a human group A assay can be used as an on-site diagnostic test for bovine rotavirus with a sensitivity and specificity of 87.0% and 93.6%, respectively.63 The assay is a 10-min one-step test with all the necessary reagents included in the kit and with no need for any laboratory equipment.
Several serological tests are available for the measurement of rotaviral antibody in serum and lacteal secretions. An ELISA is used to detect porcine epidemic diarrhea coronavirus antibodies in swine sera.40 The radioimmunoassay is the most sensitive test compared to the agar gel immunodiffusion, complement fixation, hemagglutination and hemagglutination inhibition tests.
The pathology of experimentally induced rotavirus and coronavirus diarrhea in colostrum-deprived and gnotobiotic calves, lambs, and piglets has been described. Grossly, the changes are unremarkable and consist of dehydration, fluid-filled intestinal tract and distension of the abomasum. The microscopic changes consist of shortening of the length of the villi and replacement of the tall columnar villous epithelial cells by cuboidal and squamous cells. Segments of the small intestine may reveal villous fusion, rounded absorptive cells, villous atrophy and exposure of lamina propria. Crypt hyperplasia occurs in response to the loss of columnar epithelial cells from the villi. Histological lesions due to previous rotavirus infection may be present in the upper small intestine of clinically normal calves. The rate at which enterocytes are affected in older disease-resistant calves is due to the slowing of the virus from entering the cells.64
In coronavirus enteritis in calves, there is commonly villous atrophy of both the small and large intestines and destruction of the crypt epithelium; destruction does not occur in rotavirus enteritis. The changes are more severe in field cases of acute diarrhea in calves in which both viruses and enteropathogenic E. coli can be isolated. Concurrent infection with BVDV has also been demonstrated to be synergistic in bovine rotaviral diarrhea.51
The histological appearance of the intestinal lesions of experimental infection of calves with Breda virus, calicivirus-like agent, and parvovirus have been described. In general, the lesions are similar to those associated with rotavirus and coronavirus infection.
The wide array of diagnostic tests available to confirm the presence of enteric viruses has already been discussed. Owing to the frequency of subclinical infection with these agents, it is important to histologically confirm concurrent atrophy of villi.
• Histology – duodenum, jejunum, ileum, colon (LM, IHC)
• Virology – colonic content (EM, ELISA, LATEX AGGLUTINATION); colon, ileum, jejunum (FAT, ISO).
The cause of acute diarrhea in newborn farm animals cannot be determined clinically. All of the common bacterial and viral enteropathogens can cause an acute profuse fluid diarrhea with progressive dehydration and death in a few days.
When outbreaks of diarrhea are encountered, a detailed examination of the possible risk factors should be made and the appropriate fecal samples and tissues from affected animals should be submitted to the laboratory. The most reliable specimens include fecal samples obtained from animals within a few hours after the onset of diarrhea, and untreated affected animals which are submitted for necropsy and microbiological examination within a few hours after the onset of diarrhea.
The clinical and epidemiological characteristics of the common acute diarrheas of neonatal farm animals are as follows:
Enteric colibacillosis occurs primarily in calves under 4 d of age and is characterized clinically by an acute, profuse liquid diarrhea. Recovery following treatment usually occurs in 2 d. Outbreaks occur in beef and dairy calves. Rotavirus and coronavirus diarrhea usually occur in calves over 5–10 d of age and up to 3 weeks of age. Explosive outbreaks occur, characterized by an acute profuse liquid diarrhea with recovery in 2–4 d. Recovery is assisted by oral fluid therapy.
Cryptosporidiosis occurs in calves from 5–15 d of age and is characterized by a persistent diarrhea which may last for several days. The cryptosporidia may be detected by Giemsa stain of fecal smears or by fecal flotation.
BVD. Whether or not the BVDV causes clinically significant diarrhea with lesions of the small intestine of calves 3–6 weeks of age is unknown. Diagnostic laboratories report the presence of intestinal lesions such as villous atrophy and crypt cell destruction in calves 3–6 weeks of age which have been affected with an intractable diarrhea and from which the BVD virus was isolated from the feces. However, to date there is no evidence of a cause and effect relationship.
TGE occurs most commonly in piglets under 1 week of age and explosive outbreaks are common. There is acute profuse diarrhea and vomiting. Affected piglets may continue to nurse for several hours after the onset of the diarrhea. The case–fatality rate is high in piglets under 7 d of age; older pigs commonly survive.
Porcine epidemic diarrhea type I affects piglets under 4–5 weeks of age and is characterized by profuse watery diarrhea, high morbidity and low mortality.
Porcine epidemic diarrhea type II causes a profuse fluid diarrhea in pigs of all ages, including nursing piglets. Explosive outbreaks may occur and the morbidity may reach 100%. Mortality is usually restricted to piglets under 3 weeks of age.
Enteric colibacillosis usually occurs in piglets under 3 d of age. There is acute diarrhea, dehydration and rapid death. Pigs with coliform septicemia may die without obvious diarrhea and usually appear cyanotic. Entire litters may be affected and the case–fatality rate may be 100%. Early treatment with antibiotics and SC fluids will result in recovery. Coccidiosis occurs in piglets from 5–10 d of age and is characterized by an acute diarrhea in which the feces are foul-smelling and vary in consistency from cottage cheese-like to liquid, and gray or yellow and frothy. The diarrhea is persistent for several days and non-responsive to antibiotics. Some pigs recover spontaneously, others die in 2–4 d. Coccidial oocysts can be detected in the feces. The morbidity rate varies from 50–75% and the case–fatality rate from 10–20%.
Hemorrhagic enterotoxemia due to Clostridium perfringens Type C affects entire litters of pigs under 1 week of age, is characterized clinically by severe toxemia, dysentery and rapid death, and at necropsy there is a hemorrhagic enteritis.
Enteric colibacillosis occurs in lambs most commonly under 1 week of age and is characterized by dullness, failure to suck and acute diarrhea which responds to antibiotic and fluid therapy.
Coliform septicemia affects lambs under a few days of age and usually causes sudden deaths. Lamb dysentery occurs most commonly in lambs under 10 d of age and there may be sudden death or acute toxemia, tucked-up abdomen and a severe diarrhea and dysentery. At necropsy the characteristic finding is hemorrhagic enteritis.
Rotaviral diarrhea occurs in foals from 5–35 d of age, but most commonly in foals under 2 weeks of age. There is acute profuse watery diarrhea, failure to suck, recumbency, dehydration. Recovery is common within 1 week. A mild fever is common.
Foal heat diarrhea occurs in foals 6–10 days of age whose dams are in estrus 7–10 d after foaling.
Salmonellosis, C. perfringens Type B and dietary diarrhea from excessive consumption of milk are less common causes of diarrhea in newborn foals.
The treatment of viral diarrheas in newborn farm animals is essentially the same as described for acute undifferentiated diarrhea of newborn calves. There is no specific therapy for viral diarrhea, but antimicrobial agents may be used both orally and parentally for the poss-ible occurrence of secondary enteric and systemic bacterial infections. In the absence of complications, recovery from viral enteritis usually occurs without specific treatment in 2–5 d, which parallels the replacement of the villous epithelial cells whose complete replacement and maturation requires several days after the cessation of diarrhea.
The withholding of milk for 24–48 h is beneficial, but often not possible or practical with nursing beef calves or litters of pigs. Milk can be easily withheld from hand-fed calves and replaced with oral fluids and electrolytes.
Oral and parenteral fluid therapy as indicated is essential and details are described in the sections on colibacillosis and fluid and electrolyte therapy). Affected foals may require fluid and electrolyte therapy for up to 72 h. A glucose–glycine electrolyte formulation is an effective fluid therapy for pigs affected with experimental rotaviral diarrhea. The formula is: glucose 67.53%, sodium chloride 14.34%, glycine 10.3%, citric acid 0.8%, potassium citrate 0.2% and potassium dihydrogen phosphate 6.8%. A weight of 64 g of the above is dissolved in 2 l water to produce an isotonic solution.
When possible, affected animals, particularly calves, should be isolated from calving grounds and other newborn calves, which are susceptible up to 3 weeks of age. When outbreaks of the disease occur in any species, the principles of good sanitation and hygiene should be emphasized to minimize the spread of infection.
The principles of control of viral diarrhea are similar to those described for acute undifferentiated diarrhea of newborn calves:
• Insure adequate colostral intake
• Vaccinate the dam to induce specific immunity in the colostrum (passive immunization).
The management of pregnant animals at the time of parturition must insure that the degree of exposure of the newborn to infectious agents is minimized. Control of population density to avoid overcrowding, and attention to sanitation and hygiene are important. Because infected neonates excrete large numbers of viral particles for several weeks, effective control is dependent on management of the environment of the newborn species with particular emphasis on hygiene. The ingestion of adequate quantities of good quality colostrum as soon after birth as possible is also important.
Two major approaches have been used to provide specific immunity for the control of rotavirus and coronavirus diarrhea in calves:
1. Stimulation of active immunity by vaccinating the newborn calf with an oral vaccine containing the modified live viruses (MLVs)
2. Enhancement of lactogenic immunity by vaccinating the dam during pregnancy (passive immunization).
A MLV rotavirus vaccine for oral administration to calves immediately after birth has been available commercially for many years. Initially, good results were claimed but vaccine field trials did not include contemporary controls and the efficacy of the vaccine was uncertain. The incidence of diarrhea in herds not vaccinated in the previous year was compared with the incidence during the year of vaccination, which is inadequate to assess the efficacy of the vaccine.
Field trials using the oral vaccine indicate a failure of protection of calves against rotavirus infection and rotavirus–coronavirus infection. Effective oral vaccination of calves may be hindered by the presence of specific antibodies in the colostrum – the colostrum barrier – and may explain the failure of the vaccine under field conditions. The intestinal antibody response of young calves to an enteric viral infection is associated with the production of IgM and IgA antibodies locally in the intestine. This response is absent or diminished in calves which have ingested colostrum with specific antibodies to the viruses. Most of the efficacy trials with the vaccine were carried out on colostrum-deprived gnotobiotic calves which were vaccinated orally at birth and experimentally challenged a few days after birth. It is probably futile to vaccinate calves orally immediately after birth, particularly in herds where the disease is endemic because the colostrum will contain high levels of specific antibodies.
Fecal shedding of oral vaccine rotavirus seldom occurs after oral inoculation of gnotobiotic calves with a commercial modified live bovine rotavirus-bovine coronavirus vaccine.65 Because of low shedding of virus in gnotobiotic calves which do not have the interfering effects of colostral antibodies, it seems unlikely that vaccine rotavirus will be shed in quantities from orally vaccinated conventional calves which have ingested colostrum containing antibody. Thus detection of the virus by negative stain electron microscopy in feces from orally vaccinated calves is most likely to be virulent field virus rather than vaccine virus.
Vaccination of the pregnant dam to enhance specific colostral immunity can provide passive protection against enteric viral infection of newborn farm animals. The success of this method depends on the continuous presence of a sufficient amount of specific antibody to the rotavirus and coronavirus in the intestinal lumen.35 Normally, the colostral levels of antibody are high in the first few milkings after parturition. However, there is a rapid decline in colostral antibodies to below protective levels within 24–48 h following parturition. Most cases of rotavirus and coronavirus diarrhea occur from 5–14 d after birth when the antibody levels in the post-colostral milk are too low to be protective.
The parenteral vaccination of the pregnant dam before parturition with a rotavirus and coronavirus vaccine will usually increase the level and duration of specific antibody in the colostrums.38 There is a need for a vaccine which when given to pregnant cattle will initially result in protective levels of specific antibody to the rotavirus and coronavirus in the colostrum, and then in the milk for a sufficient period such as 10 d to 3 weeks, the period in which animals are most susceptible to these viral diarrheas. The use of a modified live rotavirus–coronavirus vaccine stimulated a small but insignificant increase in colostral and milk antibodies. However, by 3 d after parturition, the rotavirus and coronavirus antibody titers in the milk of vaccinated heifers had declined to low or undetectable levels.
Inactivated rotavirus vaccines given to pregnant cows in the last trimester will significantly increase antirotavirus antibody in colostrum and milk from vaccinated dams compared to controls, but the severity of diarrhea may be the same in calves from both groups. The increased milk antibody delays the establishment of infection but does not reduce the severity of clinical disease which was experimentally induced. An inactivated oil-adjuvanted rotavirus vaccine given 1 month before calving to beef cattle significantly reduced the morbidity and mortality in 16 of 17 herds with a total of 4066 cows.66
The use of an adjuvanted rotavirus vaccine given simultaneously intramuscularly and by the intramammary route significantly enhanced serum, colostrum and milk rotavirus antibody titers, whereas intramuscular vaccination with a commercial modified live rotavirus–coronavirus vaccine did not.67 Colostrum supplements, from the cows vaccinated by the intramammary and intramuscular routes, fed to rotavirus-challenged calves at a rate of 1% of the total daily intake of milk, provided protection against both diarrhea and shedding. The 30-day milk antibody titers from these experimental cows were also considered to be protective for calves by which time the calves should have developed a high degree of age-specific resistance to rotavirus infection. The use of an inactivated rotavirus vaccine in an oil adjuvant given to pregnant cows 60–90 d before calving and repeated on the day of calving resulted in a significant increase and prolongation of colostral antibodies up to 28 d after calving. Diarrhea in calves from vaccinated cows was less common and less serious. Similar results were obtained with a combined inactivated adjuvanted rotavirus and E. coli vaccine. Similar results have been achieved by vaccination of pregnant ewes. Vaccination of ewes can result in an elevation of specific colostral antibody and prolong the period over which the antibody is present in the lumen of the intestines of the lambs. The vaccination of cows with a monovalent vaccine results in a heterotypic response to all serotypes of rotavirus to which the animals have been previously exposed, which suggests that single serotype vaccination may be sufficient.
The lactogenic antibody responses in pregnant cows vaccinated with recombinant bovine rotavirus-like (VLPS) of two serotypes or inactivated bovine rotavirus vaccines have been evaluated.68 Bovine rotavirus antibody titers in serum, colostrum and milk were significantly enhanced by the use of triple-layered VLPs and inactivated vaccines but higher antibody responses occurred in VLP vaccinated cows.68
An oil adjuvanted vaccine containing bovine coronavirus antigen to enhance lactogenic immunity in the calf by vaccinating pregnant cows and heifers between 2 and 12 weeks before calving increased serum antibody in the dams which was reflected in a similar increase in the titer and duration of specific antibody in colostrum and milk for up to 28 days after calving.69 The overall response was dependent on an adequate antigen payload being incorporated within the single dose vaccine.
The original rotavirus and coronavirus vaccines for use in pregnant cows to provide passive immunization were not sufficiently efficacious because of the rapid decline in specific colostral antibodies, which renders the calves susceptible to the viral diarrhea several days after birth. The relative success of the enterotoxigenic K99+ E. coli bacterins has resulted in a shift of the epidemic curve for acute diarrhea in calves under 30 d of age from a few days of age to 2–3 weeks of age.
More recently developed vaccines are efficacious. An inactivated combined vaccine against rotavirus, coronavirus and E. coli F5 (K99+) administered 31 days before the first expected calving date has been evaluated and compared to controls.70 There was a significant increase in serum antibody against all three antigens in vaccinated animals, which was accompanied by increased levels of protective antibodies to rotavirus, coronavirus, and E. coli in their colostrum and milk for at least 28 days. The levels of specific rotavirus and coronavirus antibodies in the milk of vaccinated cows were greater than a four-fold higher than in the control cows for at least 28 days after calving.
The primary vaccination of pregnant cows with a trivalent commercial vaccine containing live attenuated bovine rota- and coronavirus and E. coli F5 followed by an annual booster at 6 and 3 weeks before calving, or using the same protocol with an inactivated trivalent vaccine resulted in significant increase in the serum antibody of all vaccinated animals compared to controls.71 The antibody titers were higher in cows receiving the live vaccine compared to those receiving the inactivated vaccine. The colostral antibodies against all three antigens increased in all live vaccinated groups whereas inactivated vaccinated animals had only significant increases in F5 titers. The colostrum of live vaccinated cows contained much higher specific antibody titers. Thus the modified-live virus vaccine can significantly enhance the specific response to rota-and coronavirus and E. coli F5 after a primary vaccination followed by a booster annually.
Feeding 2 liters of colostrum within 12 hours after birth, from cows vaccinated 8 weeks before calving with an inactivated vaccine containing rotavirus, coronavirus and E. coli K99+/F41 antigens, is efficient in raising the serum antibody titer of calves to a high level, and, thus, protecting them against rotavirus infections which occur in the first few weeks of life.72
The high levels of viral antibody in the colostrum of the first two milkings of cows can be used to advantage in hand-fed calves. The daily feeding of stored colostrum from the first few milkings of cows from the affected herd will reduce the incidence of clinical disease in the calves. The colostrum must be fed daily because rotavirus antibody is not retained in the intestinal lumen for more than 2–3 d. In affected herds the specific antiviral antibody in the stored colostrum may be sufficient to prevent the disease if colostrum is fed daily for up to 20–30 d. If a large number of cows are calving over a short period of time, the colostrum from immunized cows can be pooled and fed to the calves daily. Even small amounts of colostrum from immunized cows are efficacious if mixed with cows’ whole milk or milk replacer.38 This supplemental feeding of colostrum may be required for only 3–4 weeks, since older calves generally possess a high degree of age-specific resistance to rotavirus infections.
For many years it was uncertain if circulating colostral antibody in calves was transferred back into the intestinal tract. Evidence shows that passive immunity to calf rotavirus diarrhea can be achieved by adequate calf serum colostral antibody titers.73 Calves fed colostrum on the first day of life had significant rotavirus-neutralizing antibody titers in their small intestinal lumina for 5 d and 10 d later. The intestinal antibody titers correlated with the serum antibody titers derived from colostrum and were predominantly of the IgG1 isotype. Intestinal antibody titers were approximately equivalent in 5- and 10-day-old calves, suggesting that antibody transfer to the intestinal tract is a continuing process for up to 10 days after birth. Additional evidence that transfer of passive immunity occurs is that calves can be protected from rotavirus challenge by the administration of colostral immunoglobulins by parenteral injection. This protection was not due to lactogenic antibody, since the calves received no source of dietary antibody. The transfer of circulating antibody into the intestinal tract may be the mechanism which results in the decreased morbidity and case fatality due to diarrhea in calves with high concentrations of passive serum immunoglobulins.73
Oral porcine rotaviral vaccines have been unsuccessful. Maternal rotavirus vaccines used to induce passive immunity have been examined. In pigs, as in ruminants, IgG antibodies to rotavirus are predominant in colostrum and decline 8–32-fold in the transition to milk. However, secretory IgA is the primary isotype of rotavirus antibody in the milk of pigs. Increased levels of sIgA and IgG antibodies to rotavirus occur in the milk of sows after natural rotavirus infection of nursing piglets or following parenteral inoculation of pregnant or lactating sows with live attenuated rotaviruses.38 But titers decline by the end of lactation, suggesting that repeated natural rotavirus infection of sows or parenteral revaccination may be necessary to maintain high sIgA antibody to rotavirus in milk. This observation may account for the higher prevalence of rotavirus infection during the first week of life in pigs born to gilts (38%) than in those born to sows (3%). There are few studies of maternal rotavirus vaccines for use in swine.
An inactivated equine rotavirus vaccine given to mares at 8, 9, and 10 months of gestation was safe and immunogenic, and provided reasonable protection under field conditions.74 Antibody titers were significantly increased at the time of foaling and 35 d after foaling in vaccinated, compared with control mares and for 90 d after birth in foals born to vaccinated, compared with foals born to control mares. The incidence of rotaviral diarrhea was lower in foals born to vaccinated, compared with foals born to control mares but the difference was not significant. Parenteral vaccination of mares with inactivated rotaviral vaccine stimulates production of high levels of specific IgG, and not IgA, in colostrum and milk.75
Subunit rotaviral vaccines consisting of virus-like particles given parenterally can enhance bovine rotavirus antibody titers in serum, colostrum and milk.76 These vaccines offer advantages over conventional modified-live or inactivated vaccines including:
• Exclusion of adventitious agents associated with live vaccines
• Consistent production of outer capsid proteins
• Genetic engineering to allow updating of efficacious vaccines for boosting lactogenic immunity. Field studies to evaluate the vaccine under naturally occurring conditions have not yet been carried out.
Preliminary studies on the passive protection of anti-rotavirus chicken egg yolk immunoglobulins given orally to calves against experimental rotavirus infections in neonatal calves have been reported.77 Treated calves had increased body weights and the number of calves shedding high titers of rotavirus in the feces was decreased compared to control calves.
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