Diseases associated with Leptospira spp.

Nomenclature of leptospires

For efficiency, in this text, the specific leptospire serovars will be abbreviated, for example, L. pomona for Leptospira interrogans serovar pomona, or the serovar only will be used, e.g., pomona.

Animal risk factors

Serovars and species susceptibility. The epidemiology of leptospirosis is most easily understood by classifying the disease into two broad categories: host-adapted and non-host-adapted leptospirosis. An animal infected with a host-adapted serovar of the organism, is a ‘maintenance’ or ‘reservoir’ host. Exposure of susceptible animals to non-host-adapted serovars results in accidental or incidental disease. Each serovar is adapted to a particular maintenance host, although they may cause disease in any mammalian species.

A serovar behaves differently within its maintenance host species than it does in other, incidental or accidental hosts. A maintenance host is characterized by:

Examples of this relationship are serovar bratislava in swine, and serovar hardjo type hardjo-bovis in cattle. In contrast, an incidental host is characterized by:

An example of this relationship is serovar pomona (kennewicki) infection in cattle.

Some common leptospiral serovars and their maintenance hosts are as follows:

Some common leptospiral serovars and their accidental hosts are as follows:

Calves and lambs are highly susceptible to infection and septicemia is likely to occur.

Occurrence and prevalence of infection

Most leptospiral infections are subclinical³ and infection is more common than clinical disease. L. pomona is the commonest infection in all farm animals but its international distribution is unpredictable; it had not been present in the United Kingdom until recent years and then only sporadically. The number of serovars of concern in domestic animals has been increasing and serovars and their antibodies have been detected which were thought previously to be exotic.

L. canicola infection has been recorded in cattle and in pigs and specific antibodies have been detected in horses. L. icterohaemorrhagiae is a rare isolation in large animals but has been reported in cattle and pigs, and serological evidence of infection has been found in the horse. L. hyos (L. mitis) has been isolated from cattle and pigs, L. grippotyphosa from cattle and goats, and positive serological tests have been obtained in horses. L. sejroe, L. hebdomadis and L. australis infection have been observed in cattle. L. szwajizak is thought to be the predominant serovar in Israel.

Serological surveys of cattle in the African continent reveal evidence of antibodies against numerous leptospiral serovars⁴ and some previously not described strains of serovars. In West Africa, serosurveys of dairy herds revealed 45% of cattle were positive to one or more serovars, which probably represented natural infection because vaccination had not been practiced.⁴

Leptospirosis is common in farm animals in Portugal.⁵ Outbreaks of clinical disease have been recorded in cattle and pigs, and also in sheep and goats, and in horses to a lesser extent. In Italy, serological surveys indicate that sheep, horses, pigs, and dogs have the highest number of positive responses.⁶

Cattle

Leptospira interrogans is divided into seven genospecies of leptospires, two of which are L. interrogans and L. borgpetersenii.⁷ L. borgpetersenii serovar genotype hardjo-bovis formerly L. interrogans serovar hardjo-bovis occurs worldwide³ and in many areas outranks L. pomona in cattle. It is now the commonest serovar in cattle in some parts of Australia, New Zealand, the United States, and Canada. L. interrogans serovar genotype hardjo-prajitno so far has been recovered only in the United Kingdom, Nigeria, India, Malaysia, Brazil, Mexico, and the United States. L. hardjo is a common cause of abortion in dairy cattle in Brazil.⁸ New serovars occur occasionally, as for example, serovar ngavi, in the serogroup Tarassovi isolated from oxen in Zimbabwe.⁹L. interrogans serovar hardjo (type hardjoprajitno) and L. borgopetersenii (type hardjo-bovis) are serologically indistinguishable but genetically distinct.¹⁰ The former has been isolated primarily from cattle in the UK, while the latter is common in cattle populations throughout the world.

L. hardjo-prajitno has not yet been identified as a pathogen of North American cattle.¹¹ Because this genotype, instead of hardjo-bovis is used in vaccines it may explain the lack of complete protection of animals against hardjo-bovis and the difficulties in laboratory diagnosis. In addition, there are at least two variations of the hardjo-prajitno genotype. These differences may be associated with differences in pathogenicity of hardjo strains, and although the cross-agglutination absorption test cannot distinguish these strains as separate serotypes, the degree of cross-protection conferred by these heterologous genotypes in vaccines is not known.

Cattle are the maintenance host for L. hardjo and are the only reservoir. L. hardjo is an important cause of bovine abortion³ and is the commonest leptospiral infection in man. It is a common infection in Australian sheep and may affect up to 40% of the population.

Seroprevalence surveys found 34–65% of sera obtained from cows at slaughter in parts of the United States had antibodies to leptospira serovars.³ Approximately 30% of all sera were positive when tested for L. hardjo, the host-adapted serovar of cattle. This seroprevalence is similar to reports from Europe, Australasia, and South America, in which 25–65% of all cows tested for L. hardjo were positive. The morbidity rate for clinical disease may vary from 10–30%, depending on the clinical manifestation of infection, and the case–fatality rate is usually low at about 5%. The case–fatality rate in calves is much higher than in adult cattle. A high rate of abortions (up to 30%) and loss of milk production are the major causes of loss but deaths in calves may also be significant.

Serovar hardjo-bovis is the most common serovar of cattle in the UK,³ Australia, New Zealand, and North America. Abattoir surveys of sera and kidneys of beef cattle in Quebec found a high prevalence of infection and nephritis associated with hardjo-bovis in contrast to pomona, which occurs more frequently in dairy cattle in that area. It is also possible that hardjo-bovis infection may be more common in feedlot cattle because they originated from Alberta, where the prevalence of infection with that serovar is prominent.

In serological survey of dairy cows in herds with suboptimal reproductive efficiency in a region in Spain. L. bratislava and L. grippotyphosa were the most prevalent serovars.¹² The risk of seroconversion against L. grippotyphosa was higher during the spring season while L. bratislava did not differ among seasons. The prevalence of L. hardjo was low which indicates that the reproductive inefficiency was unassociated with hardjo. In surveys of dairy and beef cattle in Spain, L. bratislava is the most frequently detected serovar while hardjo is at a relatively low seroprevalence compared with similar studies in western European countries.¹³ In Spain, serovars grippotyphosa, tarassovi and copenhagi are more frequent in dairy herds, probably related to management practices and geographical location of these herds which facilitate the contact with maintenance hosts for these serovars.

A serological sampling of adult dairy cows in Brazil, not vaccinated for leptospirosis, and from herds with lowered fertility, 47% were positive for L. hardjo.¹⁴ The major risk factor associated with seropositivity was co-grazing with other species, mainly pigs.

L. pomona and L. hardjo-bovis are responsible for most bovine leptospirosis in Australia.¹¹ Serovar pomona is a pig pathogen for which cattle are an accidental host and is a cause of bovine abortion, and fatal hemolytic disease in calves. Serovar hardjo-bovis is adapted to cattle as a maintenance host, is maintained in the bovine population but has a relatively low pathogenicity.¹¹ It is responsible for epidemics of agalactia, the milk drop syndrome, and a major cause of infertility. However, diagnostic surveys in Australia suggest that hardjo-bovis is not responsible for any substantial proportion of bovine abortion in contrast to the situation in Northern Ireland, where the genotype hardjo-prajitno is present. In beef cattle in Queensland, Australia, the major serovars in order of decreasing crude seroprevalence were hardjo (15.8%), tarassovi (13.9%), pomona (4.0%), and szwajizak (2%).¹⁵ Vaccinates were not included in the hardjo and pomona seroprevalence; and the seroprevalence for hardjo and pomona tended to increase with age of the animals. The data indicate that serovars other than hardjo, pomona, and tarassovi, are unlikely to have a significant role in bovine infertility, and cattle are unlikely to be a source of human infection in central Queensland.

Seroprevalence surveys in Ontario, found hardjo-bovis was most commonly found in beef cattle, whereas pomona was most commonly found in dairy cattle. In Prince Edward Island, 14% of dairy cows were serologically positive for serovar hardjo. Serological surveys of cattle farms in Alberta found infection with hardjo-bovis was widespread across the province and the prevalence has increased. In contrast, pomona reactors were found usually on single premises within a locality as compared to the clustering of hardjo reactor herds. This is an expression of the difference between host–parasite relationships in which cattle are well-adapted reservoir hosts for hardjo, generally inducing a weak agglutinin response to natural infection and remaining capable of transmitting infection for months or years.

With pomona infection, cattle tend to develop high agglutinin titers with or without clinical disease, and not remain long-term carriers. Thus, pomona infections can remain limited to a single herd unless cattle are dispersed to other herds at the height of infection. In addition, reservoir hosts such as skunks may become infected and contaminate other premises, or a water supply common to a number of premises becomes contaminated when the water is near neutral pH (about 15–25°C) and of appropriate volume to deliver infectious numbers of organisms to cattle downstream.

As a measure of the loss associated with L. hardjo-bovis in beef cattle, the percentage of cows which are serologically positive has been related to the proportion of the herd affected with lactation failure, and there is a greater wastage in reactor cows.

Seroprevalence surveys of mature cattle in the United States found antibodies of hardjo-bovis were most common, followed by pomona and copenhagi.¹⁶ This confirmed earlier observations that the nationwide seroprevalence of hardjo-bovis was greater than pomona. The isolation rates were also higher in beef cattle than in dairy cattle and higher in bulls than in cows. Combined culture and immunofluorescence results found that 2% of mature cattle were renal carriers of leptospires.¹⁶

In Greece, the seroprevalence in pigs, goats, sheep, cattle, and dogs is 17.8%, 16.2%, 5.7%, 12.6%, and 11.4%, respectively.¹⁷ The overall prevalence is high, and against multiple serovars, indicating a considerable level of infection in each animal species, and that the zoonotic risk is considerable. In Turkey, L. hardjo is the dominant serovar identified in serological surveys of cattle, but L. grippotyphosa is the dominant serovar causing clinical disease in cattle while the disease in sheep is uncommon.¹⁸

The distribution of isolates and prevalence of serovars varies according to regions of the country.¹⁶ Both isolation rate and seroprevalence are higher in the south eastern, south central, and Pacific coast regions than for other regions of the United States.¹⁶ The isolation rate is related more to regional temperature than to amount of precipitation. This suggests that high levels of precipitation are not required for transmission of leptospirosis; oases in arid lands and deserts may become well-defined endemic zones by introduction of carrier animals.

Farmed deer

Leptospirosis is a well-established clinical disease in farmed deer in New Zealand.¹⁹ Slaughterhouse surveys of farmed deer in New Zealand found serological evidence of serovar hardjo in 73.6%, pomona in 41.5%, copenhagi in 11.3%, and tarassovi in 15.1% of farms. Antibodies to serovars australis, ballum, and balonica were present in three, one, and four of six herds studied, respectively. The titer prevalence to hardjo was higher than that of pomona and other serovars within farms. Renal lesions were characteristic of subclinical leptospirosis, and spirochetes were present in renal tubules. Cultures were positive in 10 stags from six lines with similar prevalence across age groups. On-farm surveys found a 10 to 30% within-herd prevalence of pomona and hardjo titers in 56% of 3-month-old deer herds; by 11 months of age, 100% of herds were titer-positive with high prevalences to one or both serovars.

Sheep and goats

The disease in sheep has been reported in many countries and in goats in Israel. Leptospirosis in sheep can cause lamb losses due to congenital infections, starvation of lambs because of the acute agalactia of hardjo infection of the ewes, and fatal infection of feedlot lambs infected with grippotyphosa. Deaths of animals and loss of condition in mildly affected animals are the main causes of loss.

Although few outbreaks are recorded, an infection rate of 75% is not uncommon in sheep and case–fatality rates usually average about 20% in this species and up to 45% in goats. L. pomona is the common infection and the cause of most clinical leptospirosis in sheep. Infection with L. hardjo occurs but is unlikely to be a source of infection for cattle herds. Sheep are not natural maintenance hosts for pomona or hardjo and are likely to have infections of relatively short duration, producing severe pathological effects. However, persistent leptospiruria due to hardjo in sheep where no contact with cattle has occurred suggests that sheep may be a maintenance host for this serovar.²⁰ This could complicate control of hardjo infection in cattle which are free of this serovar, and infected sheep are a potential zoonotic risk to abattoir workers, sheep farmers, and shearers which previously had not been considered. Infection with serovar hardjo is widespread in Merino stud rams in South Australia.²¹

Pigs

In infected herds the prevalence of positive serological reactors is high, and in large infected pig populations is about 20%. Economic losses are about equally divided between abortions and deaths of weak and unthrifty newborn pigs. In Iowa, 38% of sera from National Animal Health Monitoring System herds were positive to one or more of 12 serovars. Infection of pigs at slaughter is associated with multifocal interstitial nephritis, which results in condemnation of kidneys.² L. pomona (genotype kennewicki) has been the predominant infection in pigs but with the widespread use of bacterins against it, other infections are assuming importance. L. tarassovi, copenhageni, ballum, bratislava, muenchen, and hardjo are now isolated more frequently.

Swine are affected by several leptospiral serovars and the clinical signs often associated with these infections include poor reproductive performance.^22-24 Seropositive sows have a greater risk of weak newborn pigs and having more weak newborn piglets per litter.²³ In some areas suboptimal reproductive performance was associated with certain serovars such a grippotyphosa and not others such as autumnalis, bratislava, pomona, and icterohemorrhagiae.²⁴

The most common serovar antibodies found in pigs in Prince Edward Island swine herds were L. icterohemorrhagiae, L. bratislava, L. autumnalis, and L. pomona.²⁵ Only herds with a higher prevalence of L. bratislava had more infertility as measured by non-productive sow days per parity.

Pigs in intensive housing present a different problem from those in more conventional housing or at pasture. In large pig units the possibility for cross-infection is high because of high population density. The movement of pigs from pen to pen, and access to effluent from other pens are the critical means of spread in these circumstances. The spread of infection within piggeries is encouraged by mixing infected with uninfected pigs, which results in epidemics within the pens. Transmission from infected to susceptible grower pigs occurs continuously in grower houses, with a constant proportion of pigs becoming infected each week. Introduction onto a farm may be via an imported boar who frequently is found to harbor leptospires in the genital tract. Lepospira were found commonly in the kidneys of slaughter fattening pigs in Vietnam but are not considered to be the cause of the white spotted kidneys of pigs.²⁶

The Australis serogroup of leptospires is now important because of an increasing awareness that antibodies to bratislava are widespread in the pig populations of many countries, the recovery of lora, muenchen, and bratislava from pigs, and the involvement of bratislava and muenchen in reproductive problems of swine herds. All of the pig isolates of the Australis serogroup have been identified as either bratislava or muenchen, and there are also differences at the subserovar level which may be important in understanding the epidemiology of the Australis serogroup, the development of efficacious vaccines, and the pathogenesis of disease. Certain genotypes are associated with abortion and stillbirths in pigs, while another genotype may be responsible for meningitis in piglets.

House mice on swine farms may be serologically positive to L. bratislava, which suggests a possible source of the organism.

Horses

The risk factors associated with the likelihood of seropositivity to L. pomona, L. autumnalis, and L. bratislava in horses in New York State have been quantified.³¹

Rodent exposure was associated with risk of exposure to all serovars. Management was associated positively with the risk of exposure to serovars pomona and bratislava, but not with risk of exposure to autumnalis. Soil and water had a positive association with risk of exposure to pomona and autumnalis but not to bratislava. The wildlife index value and the population density of horses turned out together were associated with risk of exposure to autumnalis.

A bacteriological survey of kidneys from abattoir horses in Portugal found serogroups L. australis and L. pomona, which were identified as L. bratislava, and L. kirschneri serovar Tsaratsovo, respectively.³² Leptospiral antibodies were more than 1:10 in 37% of the horses.

Methods of transmission

The source of infection is an infected animal which contaminates pasture, drinking water, and feed by infective urine, aborted fetuses, and uterine discharges. All of the leptospiral types are transmitted within and between species in this way. A viable infected neonate can harbor the infection for several weeks after birth. The semen of an infected bull may contain leptospirae and transmission by natural breeding or artificial insemination can occur but is uncommon. In rams, the semen is likely to be infective for only a few days during the period of leptospiremia; in boars there is no evidence of coital transmission. L. interrogans serovar hardjo is excreted from the genital tract of aborting cows for as long as 8 days after abortion or calving and is detectable in the oviducts and uterus for up to 90 d after experimental infection and in naturally infected cows. It may also be present in the genital tract of bulls and venereal spread of the infection is possible. Young pigs may act as carriers for 1 year and adult sows for 2 months. Because of the high intensity and long duration of the infection in pigs, they play an important role in the epidemiology of leptospirosis.

Environmental and management risk factors

Survival of the organism in the environment depends largely upon variations in soil and water conditions in the contaminated area. The organism is susceptible to drying, and a pH lower than 6 or greater than 8 is inhibitory. Low urinary pH in cattle fed with brewer’s grains may inactivate leptospires in animals with leptospiruria.³⁶ Ambient temperatures lower than 7–10°C (44.6–50°F) or higher than 34–36°C (93–96°F) are detrimental to its survival.

Ground surface moisture and water is the most important factor governing the persistence of the organism in bedding or soil; it can persist for as long as 183 days in water-saturated soil but survives for only 30 min when the soil is air-dried. In soil under average conditions, survival is likely to be at least 42 d for L. pomona. It survives in free, surface water for long periods; the survival period is longer in stagnant than in flowing water although persistence in the latter for as long as 15 d has been recorded. Contamination of the environment and capacity of the organism to survive for long periods under favorable conditions of dampness may result in a high incidence of the disease on heavily irrigated pastures, in areas with high rainfall and temperate climate, in fields with drinking water supplies in the form of easily contaminated surface ponds, and in marshy fields and muddy paddocks or feedlots. Because of the importance of water as a means of spreading infection, new cases are most likely to occur in wet seasons and low lying areas, especially when contamination and susceptibility are high. A differential distribution has been observed in the prevalence of seropositives in cattle in Australia. L. hardjo antibodies have a high prevalence through all rainfall areas, but L. pomona is much more common in low rainfall areas.

Certain management factors have been identified which pose risks of L. hardjo infection being introduced into dairy herds:³⁷

Economic importance

Leptospirosis is a major cause of economic loss in farm animals. The majority of leptospiral infections are subclinical and associated with fetal infections causing abortions, stillbirths, and the birth of weak neonates with a high death rate in cattle, sheep, horses, and pigs. In cattle, epidemics of abortions, infertility, and increased culling rate cause major economic losses. Epidemics of agalactia in dairy herds, the milk drop syndrome, are associated with infection with L. hardjo.

Zoonotic implications

In the past decade, leptospirosis has emerged as a globally important infectious disease in human medicine.^38,39 It is uncommon in developed countries but the incidence is increasing in travelers returning from endemic countries. The epidemiology has undergone major changes, with a shift away from the traditional occupational disease in developed countries, to a disease associated with recreational exposures.³⁹ It is now recognized as an emerging, potentially epidemic disease associated with excess rainfall in tropical settings, representing a significant public health hazard. Mortality in humans with leptospirosis remains significant because of delays in diagnosis due to lack of diagnostic infrastructure and adequate clinical suspicion when patients are presented for medical diagnosis and care. The differential diagnosis of febrile illness in returning travelers should include leptospirosis. Pulmonary hemorrhage is increasingly being recognized as a major, often lethal, manifestation of leptospirosis in humans, the pathogenesis of which is unclear. Treatment consists of tetracyclines and beta-lactam/cephalosporins. No vaccine is available for the prevention of leptospirosis in humans.

Leptospirosis is an important zoonosis and is an occupational hazard to butchers, farmers, and veterinarians.⁴⁰ The high prevalence of leptospiral infection (serovars pomona and hardjo) in Texas cattle represents potential threats to human health and agricultural economics.⁴¹ The incidence of positive agglutination tests in humans in contact with infected cattle is surprisingly low and clinical cases in humans in which the infection is acquired from animals are not common. Human infection is most likely to occur by contamination with infected urine or uterine contents. Veterinarians may become infected by handling the tissues and urine of sows which have aborted from pomona infection. Although leptospirae may be present in cow’s milk for a few days at the peak of fever in acute cases, the bacteria do not survive for long in the milk and are destroyed by pasteurization. However, farm workers who milk cows are highly susceptible to L. interrogans serovar hardjo infection and one New Zealand survey found 34% of milkers were seropositive, mostly to L. interrogans serovar hardjo, but a high proportion also to L. interrogans serovar pomona. This has aroused alarm and leptospirosis became known as ‘New Zealand’s No. 1 dairy occupational disease’. A campaign of vaccination of dairy cattle across the country resulted in a marked decrease in the incidence of the disease in humans. In most situations, dogs, cats, and horses are unlikely to contribute to human infection.⁴⁰

Leptospirosis is New Zealand’s most common occupationally acquired infectious disease, and the incidence of the disease is high compared with other temperate developed countries.⁴² The epidemiology of leptospirosis in New Zealand has been changing. The annual incidence of human leptospirosis in New Zealand from 1990–98 was 4.4 per 100 000. Incidence was highest among meat processing workers (163/100 000), livestock farm workers (91/100 000) and forestry-related workers (24/100 00). The most commonly detected serovars were L. borgpetersenii sv. ballum (11.9%). The annual incidence of leptospirosis declined from 5.7/100 000 in 1990–92 to 2.9/100 000 in 1995–98. The incidence of L. borgpetersenii sv. hardjo and L. interrogans sv. pomona infection declined, while the incidence of L. borgpetersenii sv. ballum infection increased. The increasing incidence of L. borgpetersenii sv. ballum suggests changing transmission patterns via direct or indirect exposure to contaminated water. The risk of transmission of leptospirosis from dairy cattle infected with L. hardjo to dairy workers in Israel was low.⁴³

Leptospirosis is a risk factor for swine producers and slaughterhouse workers. An epidemiological investigation of people exposed to infected pigs from a university-owned swine herd found 8% of the workers were confirmed to have leptospirosis.⁴⁴ Risk factors included smoking and drinking beverages while working with infected pigs. Washing hands after work was protective. Leptospirosis is infrequently diagnosed by human physicians in the United States and veterinarians have often informed physicians of the potential of leptospirosis in humans. The disease can be prevented through appropriate hygiene, sanitation, and animal husbandry.⁴⁴ It is essential to educate people working with animals or animal tissues about measures for reducing the risk of exposure to such zoonotic pathogens as leptospira.

Veterinary students may be exposed to leptospirosis by taking courses in food inspection and technology, on-farm clinical work experiences, contact with pets especially carnivores, and contact with animal traders.⁴⁵ In a one-year period, the seroprevalence of leptospirosis in veterinary students in a veterinary school in Spain increased from 8.1% to 11.4%. The incidence of the disease during the study was 0.0394.

Acute form

After penetration of the skin or mucosa, the organisms multiply in the liver and migrate to, and can be isolated from, the peripheral blood for several days until the accompanying fever subsides. At this time serum antibodies begin to appear and organisms can be found in the urine.

Septicemia, capillary damage, hemolysis, and interstitial nephritis

During the early period of septicemia, sufficient hemolysin may be produced to cause overt hemoglobinuria as a result of extensive intravascular hemolysis. This is an unlikely event in adult cattle, but is common in young calves. If the animal survives this phase of the disease, localization of the infection may occur in the kidney. Hemolysis depends on the presence of a serovar which produces hemolysin. Capillary damage is common to all serovars and during the septicemic phase, petechial hemorrhages in mucosae are common. Vascular injury also occurs in the kidney and if the hemolysis is severe, anemic anoxia and hemoglobinuric nephrosis may occur. There is some evidence that leptospires produce a lipopolysaccharide endotoxin which may exacerbate the vascular lesions. The infection localizes in the renal parenchyma, causing an interstitial nephritis and persistence of the leptospirae in these lesions results in prolonged leptospiruria. The renal lesion develops because the infection persists there long after it has been cleared from other tissue sites. In the acute phase of the disease, the animal may die from septicemia or hemolytic anemia or both. Subsequently, the animal may die of uremia caused by interstitial nephritis.

Focal chronic interstitial nephritis, also called ‘white spotted kidney’ is a common finding in clinically healthy cattle at slaughter and has frequently been assumed to be related to current of prior infection with Leptospira spp. However, studies of ‘white spotted kidney’ in cattle at the abattoir indicate that neither Leptospira spp. nor active infection by other bacteria are associated with the lesions.⁴⁶

Abortion

Following systemic invasion, abortion may occur due to fetal death, with or without placental degeneration. Abortion usually occurs several weeks after septicemia because of the time required to produce the changes in the fetus, which is usually autolyzed at birth. Abortion occurs most commonly in the second half of pregnancy, due probably to the greater ease of invasion of the placenta at this stage, but may occur at any time from 4 months on. Although abortion occurs commonly in both cattle and horses after either the acute or the subacute form of the disease, abortion without prior clinical illness is also common. This is particularly the case in sows and occurs to a less extent in cows and mares; this may be due to degenerative changes in the placental epithelium. Leptospirae are rarely present in the aborted fetuses, however if the aborted fetus has survived the infection long enough to produce antibodies, these may be detectable.

Experimental infection of serologically negative pregnant cattle with a north-Queensland strain of L. borgpetersenii serovar hardjo resulted in seroconversion and shedding of the organism in the urine.⁴⁷ Elective cesarean sections were done 6 weeks after challenge. No evidence of L. hardjo infection of the fetuses occurred. Some of the fetuses had histopathological lesions consistent with Neospora sp. infection.

Encephalitis

Localization of leptospirae in nervous tissue is common in sheep and goats and may result in the appearance of signs of encephalitis.

Subacute and occult forms

In the subacute form, the pathogenesis is similar to that of the acute septicemic form, except that the reaction is less severe. It occurs in all species, but is the common form in adult cattle and horses. Occult cases, with no clinical illness but with rising antibody titers, are common in all animals. These are difficult to explain but may be associated with strains of varying pathogenicity. But with leptospirosis, characteristically, differences between groups may be associated with prior immune status, environmental conditions, or number of carriers in relation to severity of exposure.

Periodic ophthalmia (recurrent uveitis) in the horse

There is some evidence of a causal relationship between leptospiral infection and periodic ophthalmia in the horse.⁴⁸ The incidence of serologically positive reactors is higher in groups of horses affected with periodic ophthalmia than in normal animals. Agglutinins are present in the aqueous humor in greater concentration than in the serum. Serological surveys indicate that leptospira infection is not a major factor in the etiology of equine anterior uveitis in the United Kingdom, but serological evidence of pomona is associated with uveitis in horses in the United States. The opacity in both cornea and lens is a consequence of the antigenic relationship between leptospires and components of the ocular tissues and does not require the presence of living bacteria. A 52-kDa protein appears to be involved in the antigenic relationship between the leptospires and equine ocular tissues and is located inside the bacterium. The uveitis alters the composition of the aqueous humor and impedes the nutrition of the ocular structures, leaving sequelae such as iris atrophy, synechiae, and corneal opacity.

Retinal immunopathology in horses with uveitis has been described and may be a primary immunological event in equine uveitis, providing evidence that leptospira-associated uveitis may be a distinct subset of equine uveitides.⁴⁸

Immune mechanisms

Following infection, specific antibodies are induced which opsonize leptospires, facilitating their elimination from most parts of the body. However, leptospires which reach the proximal renal tubules, genital tract and mammary gland appear to be protected from circulating antibodies. They persist and multiply in these sites, and may be excreted and transmitted to susceptible, in-contact animals, primarily by urine. Furthermore, and of major importance, the level of serum antibody commonly declines to undetectable levels in animals which are persistently infected.

The first serological response with L. hardjo infection is the production of immunoglobulin M (IgM) antibodies. These rise rapidly but commonly decline to undetectable concentrations by 4 weeks after infection. Within 1–2 weeks of infection, IgG1 antibodies appear, and at 3 months they represent 80% of antibodies detected in the microscopic agglutination test (MAT). The MAT titer peaks 11–21 d after infection but may vary from 3200 to an undetectable concentration. The MAT titer declines gradually over 11 months but the persistence is variable. Vaccination induces antibodies that are mainly of the IgG class with levels peaking at 2 weeks after a two-dose vaccination but decreasing rapidly to levels lower than those after natural infection. Approximately 95% of vaccinated heifers do not have MAT antibodies 20 weeks after the second of two vaccinations given 4 weeks apart and the absence of titers is not necessarily an indication that protection has waned. Vaccinated animals are protected from natural challenge for many months after their MAT titers become undetectable. The serological response of calves vaccinated at 3 months of age is lower than those vaccinated at 6 months of age because of the presence of maternal antibody. Transfer of passive immunity antibodies to newborn calves occurs via the colostrum and the antibodies persist in the calves for 2–6 months.

While antibodies against leptospiral lipopolysaccharides (LPS) give passive protection in some animal models, cattle vaccinated against serovar hardjo with pentavalent vaccines are vulnerable to infection with serovar hardjo despite the presence of high titers of anti-LPS antibody. It is now known that peripheral blood mononuclear cells (PBMC) from cattle vaccinated with an L. interrogans serovar hardjo vaccine which protects against serovar hardjo proliferated in vitro in response to hardjo antigens. Thus a cell-mediated immune response to serovar hardjo may be necessary for protection.¹⁰ A protective killed vaccine against serovar hardjo induces a strong antigen-specific proliferative response by PBMC from vaccinated cattle 2 months after the first dose of vaccine. This response was absent from unvaccinated cattle. The mean response peaked by 2 months after completion of the two-dose vaccination regimen, and substantial proliferation was measurable in vitro cultures throughout 7 months of the study period. Up to one-third of the PBMC from vaccinated animals produced gamma interferon (IFN-γ after 7 days in culture with antigen. One-third of the interferon gamma producing cells were (gamma delta lymphocytes, with the remainder cells being CD4⁺ T-cells.¹⁰ Thus a very potent Th1-type immune response was induced and sustained following vaccination with a killed bacterial vaccine adjuvanted with aluminum hydroxide and the involvement of gamma delta T-cells in the response. The induction of this Th1-type cellular immune response is associated with the protection afforded by the bovine leptospiral vaccine against L. borgpetersenii serovar hardjo.¹⁰

The immune response of naïve and vaccinated cattle following challenge with a virulent strain of L. borgpetersenii serovar hardjo has been examined.⁴⁹ Beginning at 2 weeks after challenge, gamma interferon was measured in antigen-stimulated PBMC cultures from nonvaccinated animals, although the amount produced was always less than that in cultures of PBMC from vaccinated animals. IFN-γ⁺ cells were also evident in antigen-stimulated cultures of PBMC from vaccinated but not from nonvaccinated animals throughout the post-challenge period. Naïve and vaccinated animals had similar levels of antigen-specific immunoglobulin G1 (IgG1) following challenge; vaccinated animals had two-fold more IgG2. It is evident that while infection may induce a type 1 response, it is too weak to prevent establishment of chronic infection.

Cattle

Leptospirosis in cattle may be acute, subacute or chronic and is usually associated with pomona or hardjo.

Pigs

Pomona is the common infection, tarassovi is the other common infection, and chronic leptospirosis is the commonest form of the disease in pigs. It is characterized by abortion and a high incidence of stillbirths. Failure to conceive is not usually observed in leptospirosis but has been reported in infections with canicola. In an infected herd the rearing rate may fall as low as 10–30%. An abortion ‘storm’ may occur when the disease first appears in a herd but abortions diminish as herd immunity develops. Most abortions occur 2–4 weeks before term. Piglets produced at term may be dead or weak and die soon after birth. Hardjo may be a sporadic cause of reproductive disease and muenchen and bratislava are occasional isolations during investigations of porcine abortion and stillbirths. In a survey of swine farms in Canada, those herds with a high serological prevalence of serovars bratislava and pomona had more infertility as measured by non-productive sow days per parity.²⁵ There was no association between infertility and antibodies to serovars autumnalis and icterohemorrhagiae.²⁵

Rarely the acute form as it occurs in calves also occurs in piglets in both natural field outbreaks and in experimentally produced cases. Icterohaemorrhagiae infection causes septicemic leptospirosis with a high mortality rate.

Sheep and goats

The disease is rare in these species so that good descriptions of the naturally occurring disease in them are lacking; most affected animals are found dead, apparently from septicemia. Affected animals are febrile, dyspneic, snuffle, and hang their heads down. Some have hemoglobinuria, pallor of mucosae and jaundice, and die within 12 h. Lambs, especially those in poor condition, are most susceptible. The chronic form may occur and is manifested by loss of bodily condition, but abortion seems to be almost entirely a manifestation of the acute form when the infection is pomona. With hardjo, abortion has been recorded as the only clinical sign, and oligolactia and agalactia, similar to the bovine milk drop syndrome, have been observed in lactating ewes.

Horses

General considerations

Laboratory procedures used in the diagnosis include culture or detection of leptospires in blood or body fluids, and detection and measurement of antibody in blood and body fluids such as urine, cerebrospinal fluid and cervico-vaginal mucus.⁶⁰ Culture of leptospires is laborious and can take up to 2 months. Serological and microbiological detection of chronically infected animals is difficult, as is the confirmation of leptospirosis as a direct cause of reproductive losses in a herd. A positive diagnosis of leptospirosis in individual animals is often difficult because of the variation in the nature of the disease, the rapidity with which the organism dies in specimens once they are collected and their transient appearance in various tissues. During the septicemic stage, leptospirae are present only in the blood and there may be laboratory evidence of acute hemolytic anemia and increased erythrocyte fragility and often hemoglobinuria. A leukopenia has been observed in cattle while in other species there is a mild leukocytosis. However, the only positive diagnostic measure at this stage of the disease is culture of the blood. If abortion occurs, the kidney, lung and pleural fluid of the aborted fetuses should be examined for the presence of the organism. Serological testing at the time of abortion is often unreliable because the acute titers have already peaked and are declining. In the stage immediately after the subsidence of the fever, antibodies begin to develop and the leptospirae disappear from the blood and appear in the urine. The leptospiruria is accompanied by albuminuria of varying degrees and persists for varying lengths of time in the different species.

The diagnosis of leptospirosis is much easier on a herd basis than in a single animal because in an infected herd, some animals are certain to have high titers and the chances of demonstrating or isolating the organism in urine or milk are increased with samples being taken from many animals. On the other hand, in a single animal, depending on when the infection occurred, the titer may have declined to a low level and be difficult to interpret. This becomes particularly important for the clinician confronted with a diagnosis of abortion due to leptospirosis in which the infection may have occurred several weeks previously and the serum may be negative or the titers too low for an accurate interpretation. Examination of the urine may be useful in these cases.

Serological and related tests

Acute and convalescent sera taken 7–10 d apart should be submitted from each clinically affected animal, or from those with a history of abortion, and sera should also be taken from 15–25% of apparently normal animals. Ten blood samples should be taken from each of the yearlings, the first-calf dams, the second-calf dams, and the mature age group in order to determine the infection status across the herd. If possible, wildlife or rodents which are known to inhabit the farm and use nearby water supplies should be captured and laboratory examinations of their tissues and blood carried out and the results compared with those obtained in the farm animals.

The Microscopic Agglutination Test (MAT) is the most commonly used serological test for the diagnosis of leptospirosis. In animals which survive infection, acute leptospirosis can readily be diagnosed on the basis of demonstrating a rising antibody titer in acute and convalescent sera.⁶⁰ Although paired sera are normally considered necessary so that a rise in titer can be detected, in cases of bovine abortion or stillbirth, infection may have occurred 1–4 weeks before the abortion by which time the MAT titers may be declining. Following infection, the IgM class of antibodies are first to appear followed by IgG antibodies, which persist for longer than IgM antibodies. The MAT detects both IgM and IgG antibodies. The MAT is particularly useful in diagnosis of disease associated with incidental, non-host-adapted serovars or acute disease associated with host-adapted serovars. It is less useful in diagnosis of chronic disease in maintenance hosts since antibody response to infection may be negligible in chronic infections or may persist from subclinical infections. In pigs, MAT has an adequate sensitivity for some serovars, such as pomona, but is insensitive to infection associated with bratislava. The herd serological response to infection is often more helpful than the individual’s response in chronic infections in maintenance hosts. Because agglutinating antibodies wane, the sensitivity of the MAT in detecting animals infected for more than 2 years is low, probably less than 50%. A major concern is the failure of the MAT to differentiate between titers after vaccination and those after natural infection since the titers may be of similar magnitude; however, titers after infection are in general, higher and persist longer than vaccination titers. Vaccinated cattle which subsequently become infected, may not mount an agglutinating antibody response.

A MAT titer of ≥100 is considered positive but there are several considerations in evaluating the MAT response. The MAT is a serogroup-specific test and serovars representative of all suspected serogroups should be tested. The test is a more sensitive detector of IgM antibody than IgG. It has a low sensitivity in chronic leptospirosis for detecting maintenance hosts. The test is inadequate for the detection of the carrier state in maintenance hosts, because titers ≥100 against host-adapted serovars have a low sensitivity but high specificity. The MAT is not a measure of immunity to infection because vaccination results primarily in an IgG response, with low (100–400) and transient (1–4 months) titers but immunity commonly persists in vaccinated animals long after MAT titers are negative. In cattle, titers of ≥100 are considered significant and a four-fold rise in titer on a paired sample taken 2 weeks apart is diagnostic. In abortion associated with incidental serovars, MAT titers against pomona and other incidental serovars are high, often ≥3000.

Paired sera are of limited value in chronic infections because abortion occurs after infection and titers are static or declining. In chronic hardjo infections, a recently aborting cow with a titer of ≥300 has about a 60%, of ≥1000 an 80%, and ≥3000 a 90% chance of fetal infection. If several aborting cows have high titers (≥300), this is evidence for the diagnosis of leptospirosis in unvaccinated herds. A semiautomated complement fixation test is available and is comparable in efficiency with the MAT.

The ELISA test is much more accurate than the others and has many advantages from the point of view of laboratory practice. It can be specific for IgM antibodies or IgG antibodies. A positive IgM-specific ELISA result can therefore indicate that infection occurred within the previous month. It has excellent diagnostic specificity and sensitivity, convenient technical features including automation, and can be used efficiently as a screening test for large numbers of serum samples. Some difficulty is encountered in interpreting the significance of titers of antibody in serum. For a diagnosis of leptospiral abortion in cattle, a reciprocal titer of 3000 is proposed as the threshold for pomona but no similar critical figure is available for hardjo. For a herd diagnosis of leptospirosis due to hardjo 10 animals from each of the yearling, first-calf heifer, second-calf heifer and adult cow groups should be tested.

An indirect ELISA has been developed for the detection of bovine antibodies to multiple Leptospira serovars including canicola, copenhagi, grippotyphosa, hardjo-bovis pomona, and sejroe.⁶¹

An antibody capture ELISA is available to detect antibodies to a protective lipopolysaccharide fraction of Leptospira borgpetersenii serovar hardjo in cattle.⁶²

A commercially available ELISA and the Immunocomb Leptospirosis Kit which detect L. hardjo antibodies have been compared with the MAT.⁶³ The Immunocomb and ELISA tests both exceeded the positive results obtained with the MAT.⁶³ The Immunocomb is very simple, and quick, requiring no sophisticated equipment.

Aqueous humor antibody. Measurement of aqueous humor antibody titers against leptospires in horses offers a more accurate means of establishing a diagnosis of leptospiral-associated uveitis than serology alone.

Serology in pigs. A comparison of diagnostic procedures for the diagnosis of porcine leptospirosis reveals that the apparent (maximum) sensitivities of diagnostic procedures for detecting infection were as follows: MAT (at a titer of 64 or 1024) 95%; IgM enzyme immunoassay 82%; culture of kidneys 61%; presence of white spots 55%; immunogold staining 52%; presence of large white spots 30%; and, Warthin–Starry silver staining 20%. An axial filament ELISA is a sensitive and specific test for the detection of antibodies against L. interrogans on a species rather than serovar level and has advantages over the MAT.

Demonstration or culture of organism or antibody

A number of tests are available to detect leptospires or leptospiral DNA in tissues or body fluids.⁶⁴

Culture of urine. Of all the laboratory diagnostic tests for leptospirosis, the examination of urine samples for the organism probably offers the best opportunity to demonstrate the presence of infection. Following natural infection with L. hardjo, cattle may shed leptospires in the urine for between 28 and 40 weeks;⁶⁵ following experimental infection, shedding occurs for about 26–32 weeks. After the initial infection, large numbers of leptospires are shed in the urine for several weeks and thereafter there is a progressive decline in the numbers shed, which may be associated with a sharp increase in urinary anti-leptospiral IgG and IgA antibody levels.⁶⁶ Urine samples should be obtained from a cross-section of affected and non-affected (in-contact) animals. Furosemide can be given IV at 0.5–0.8 mg/kg BW and the second voiding of urine collected for culture. For maximum efficiency, one-half of each urine sample should be submitted with added formalin (1 drop to 20–30 mL of urine) and the other half submitted in the fresh state. The formalin prevents bacterial overgrowth and the fresh urine sample may be used for culture. Examination of urine using dark-field microscopy or fluorescent antibody test are useful tests. The fluorescent antibody test is more sensitive than dark field microscopy, detects degenerated as well as intact leptospires and may be serovar specific. Standard techniques for culture of leptospires have been described.⁶⁵

Samples for confirmation of diagnosis

The zoonotic potential of this organism should be noted when handling carcasses and submitting specimens.

Biosecurity and biocontainment

On an individual farm, leptospirosis can be eradicated or controlled by vaccination. With the diagnostic tests available, the success of antimicrobial therapy in eliminating the carrier state, and the vaccines, it is now reasonable to attempt eradication of the disease from individual herds, and possibly from areas. The major risk is the introduction of carrier animals of any species, or by reintroduction of the infection by rodents or other wildlife. It is because of this risk that most programs aim at containment rather than eradication. In these circumstances where only sporadic cases occur, it might be more profitable to attempt to dispose of reactors or treat them to insure that they no longer act as carriers.

The first step in control is to identify the source of the original source of infection and to interrupt transmission.² Sources of infection include clinically affected animals, aborted fetuses, placentas, carrier animals, wildlife, dogs and cats, and environmental sources such as water supplies. Education about leptospirosis is an effective method for reducing its incidence and its effects. Intensive well directed education and publicity campaigns in New Zealand, used in conjunction with a campaign for immunization of cattle, reduced the incidence of leptospirosis.² Groups to which educational efforts should be directed include professionals in human and veterinary medicine and public health, primary human and animal health care practitioners, wildlife and conservation scientists, water and sewage engineers and planners, health administrators and educators, and not least, the public at risk.

Risk-assessment and computer simulation models of the possible costs of infections due to L. hardjo have been developed to explore the risks and likely consequences to producers of the disease in dairy herds in the United Kingdom. Three main considerations are important when assessing the risks and likely financial implications of the disease to dairy producers:

The probability of infection of cattle by L. hardjo is increased by four factors:

Assessment of the risks facing different types of herds suffering losses from L. hardjo can then be used to help support decisions concerning control of the disease.

The major consequences of L. hardjo infection are milk loss and abortion in the dairy herd, and the risk of illness in man. The financial losses associated with disease in a herd according to the presence of different risk factors can be estimated using a static risk model.³⁷ The cost of various control strategies can then be considered in the light of these expected costs which result from doing nothing to control the disease. A dynamic simulation model can be used to consider how the disease might develop in a dairy herd following initial infection and over the next few years.

Producers with one or more of the main risk factors should consider strategies which (i) directly remove or diminish those risk factors or (ii) indirectly diminish their importance for the herd, for example, by vaccination. Strategies which successfully diminish one or more of the risk factors, but leave one other will yield little benefit due to the importance of each of the identified risk factors.

Vaccination is one strategy which can diminish all of the risk factors and provide some degree of assurance against potentially high and costly disease losses. Producers with high-risk herds are likely to choose vaccination. The dynamic simulation model estimates that the cost of endemic disease can be relatively high and that some form of vaccination strategy would be cost-effective. If a herd continues to have any of the risk factors, then whole-herd vaccination is likely to be the preferred option, since the disease could otherwise easily be reintroduced. Decision tree analysis of leptospirosis vaccination in beef cattle in Australia indicates that the beneficial economic effects of vaccination depend on the value of the calf and the probability of calf loss due to leptospirosis.

Eradication

Occupational hygiene

Occupational hygiene is important for prevention of leptospirosis wherever the disease is known to occur predominantly in certain occupational groups. Exposure to risk is unavoidable in people whose livelihoods depend on rice planting or farming, sugar cane cutting, work in tropical forests, keeping peridomestic pigs, building or maintaining drains or sewers, milking cows, and slaughtering or herding pigs or cattle. Occupational hygiene is concerned with means of minimizing the risks, by employing measures to reduce direct or indirect contact with animals which might be infected.

Control of the source of the organism is achieved by appropriate hygienic strategies. If the environmental sources of infection are identifiable, in the form of yards, marshes and damp calf pens, every attempt must be made to avoid animal contact with these infective surroundings. In dairy farming areas where human leptospirosis comes from cattle, milkers and transporters should be protected. The main measures available are immunization of cattle and pigs, education, and modification of work practices, including protective clothing. It is important to avoid urine splash. Cows should be handled calmly to minimize urination in the milking sheds. Sheds and yards should be hosed out so as to avoid aerosol and splash from urine on the floors. Herdspeople and milkers must be made aware of the risks of infection from mud contaminated with urine. Wet areas should be drained or fenced and pens disinfected after use by infected animals. The possibility that rats and other wild animals may act as a source of infection suggests that contact between them and farm animals should be controlled.

Meat industry workers are at high risk during the examination and dressing of carcasses. The dangerous parts of carcasses are the bladder, urine and kidneys. Handling bovine or porcine kidneys with bare hands is especially risky and rubber gloves should be worn. Wearing protective clothing in abattoirs is not well accepted because speed is paramount and gloves are an impediment. Nevertheless it offers protection.

Control of clinical disease by immunization

A control program to limit the occurrence of clinical disease is achieved by vaccination to maintain a high level of immunity in herd.

Occurrence and prevalence of infection

The disease has been described in cattle,² horses,³ sheep,⁴ dogs,⁵ and is now considered one of the most common vector-borne infections in humans in the United States.¹ Lyme disease has been recognized in Canada,⁶ Europe,⁷ Asia, Australia, and in the countries of the former USSR.¹ In the United States, the infection is primarily found in three regions: the northeast (from Massachusetts to Maryland); the midwest (in Wisconsin and Minnesota); and the northwest (in California and Oregon). The disease occurs most commonly in areas with an appropriate density of the insect vector, intermediate hosts, and the environmental conditions favoring transmission. Much of the available information on borreliosis is based on serological studies which must be interpreted carefully because clinical disease is not common.

Methods of transmission

The principal hosts for B. burgdorferi are rodents such as the white-footed mouse, Peromyscus leucopus. The cottontail rabbit in the eastern United States and the jack rabbit in California can also serve as hosts. It is suggested that migrating birds acting as carriers may account for the widespread nature of the infection in a country.

The spirochete is transmitted by Ixodes ticks including Ixodes scapularis, the deer tick, in the northeastern and midwestern United States, Ixodes pacificus, the western black-legged tick, in the western United States,²² Ixodes ricinus the sheep tick in Europe, and Ixodes persulcatus in Asia.^1,6 The lifecycle of the deer tick is 2 years and includes the stages of egg, larvae, nymph and adult. The white-footed mouse is the primary rodent which infects the tick. During the larval stage, the tick will feed on an infected mouse. Both immature stages of the tick feed on the white-footed mouse which makes the life cycle of the organism dependent on horizontal transmission from infected nymphs to mice in the early summer and from infected mice to the larvae in late summer. The white-footed mice are susceptible to oral infection and transmit the infection to each other by direct contact.²⁶ The white-footed mouse is thus linked to the transmission and maintenance of the organism. Infection with the spirochete does not cause clinical or pathological changes or alter the biological features of the mouse.²⁵ These combined factors indicate a longstanding relationship between the mouse and the spirochete. Furthermore, the mouse is an excellent reservoir host but an unsatisfactory laboratory model for the study of Lyme disease. When the tick becomes infected it is ready to feed on animals and humans. A nymphal tick will feed on small animals such as rodents, squirrels, birds, dogs and cattle. The adult tick feeds primarily on larger animals such as deer, horses, cattle, and dogs. All three stages of the tick will feed on humans.

The white-tailed deer is the preferred host for the adult stage of the tick and often harbors large numbers of adult ticks. Adult ticks are likely to be responsible for transmission of infection to horses and cattle.

The ticks, Dermacentor variabilis and Amblyomma americanum, tabanid flies and mosquitoes have also been shown to carry the organism.²

The organism can be found in the urine of infected animals and it is possible that transmission may occur through close contact without the bite of a tick. Infected cattle purchased from an endemic area could shed the organisms in the urine and transmit them to animals in a different herd. The resistance of the organism to heat is generally less at 50°C, and greater at 70°C than that of other non-spore-forming pathogens.²⁷ Heat treatments similar to that of high temperature, short time pasteurization of milk are expected to decrease the population of the organism but not eliminate it. When present in meats in large numbers, it is likely that the organism can survive heat treatments sometimes used to process these products.

Transplacental transmission of the organism from infected dams to their fetuses also occurs through in utero infection and can be a cause of mortality in foals²⁸ and calves.³

Horses

In horses, chronic weight loss, sporadic lameness, laminitis, persistent mild fever, swollen joints, muscle stiffness, anterior uveitis and neurological signs such as depression, behavioral changes, dysphagia, head tilting and encephalitis have been reported.^1,3 Polyarthritis and swelling of tendon sheaths in horses of all ages are commonly reported.^3,32 A confirmed case in a 20-year-old horse had a history of exposure to ticks, was depressed, had a temperature of 38.2°C, urticarial plaques, tendon sheath swelling and hindlimb lameness. This was followed by periodic episodes of fever, lameness with hindlimb stiffness, effusion of joints and tendon sheaths, and conjunctivitis. Acute bilateral blepharospasm, photophobia and excessive lacrimation are reported³ followed by neurological signs of depression, compulsive walking, stupor, holding the head against a wall and quadripedal ataxia and eventual recumbency.³ Infection of pregnant mares can result in abortion, and the birth of weak foals which die soon after birth.²⁸ An unexplained increase in early embryonic loss or failure of conception in mares has been associated with Lyme disease antibodies but not confirmed.³³

Cattle

In cattle, polyarthritis, lameness, chronic weight loss, and a persistent mild fever have been reported.^2,34 In acute Lyme disease, fever, stiffness, swollen joints and decreased milk production are common.¹ Erythema of the udder or the skin between the digits has also been described.^1,22 Edematous lesions on the hairless skin of the udder, poor bodily condition, inappetence, and decreased milk production, stiff gait, and swollen joints have been described in cows with B. burgdorferi infection.³⁵

Sheep

In sheep, lameness, swollen joints, unthriftiness, and a persistent fever occur.²⁹

Detection of organism

The organism is difficult to isolate because B. burgdorferi is found in low numbers in blood or tissues. Aseptically collected blood, cerebrospinal fluid, urine and colostrum can be examined under dark-field microscopy or in a culture. Cultures are difficult to maintain and require several weeks to grow.

Serology

Serological testing is the most practical method of making a diagnosis of B. burgdorferi infection.¹ Serum and synovial fluid samples may contain antibodies to the organism in horses.²⁰ The indirect immunofluorescent antibody (IFA) test has been used with reliable results in horses and cattle.¹ The ELISA is ideal for high-volume testing, the results are quantitative, and can detect total immunoglobulins or class-specific IgM and IgM antibodies to the organism.³⁶ An ELISA and immunoblots using certain antigens of the spirochete are more specific for the diagnosis of Lyme borreliosis in horses.³⁷ An ELISA using a purified protein of the organism detects antibody in cattle and is useful as a screening method for borreliosis in cattle.³⁸ Western blotting techniques and the ELISA have been used for serological surveys and for examination of synovial fluids of horses in the United Kingdom where the incidence of infection is common in some areas.³⁹ The positive results in horses are not due to cross-reactions with Leptospira which has been suspected.

In cattle, the infection has been diagnosed by detection of B. burgdorferi sensu strictu DNA in samples of synovial fluid and milk of affected cows.⁴⁰

A polyvalent ELISA, incorporating highly specific recombinant antigens of B. burgdorferi has been used to determine seropositivity to B. burgdorferi and Anaplasma phagocytophilum infection.⁴¹

Subclinical infections are common in domestic animals and the interpretation of serological results must be done in conjunction with the clinical findings. Positive antibody results are an aid to diagnosis but are not conclusive evidence of current infection or clinical disease. False-positive results may be due to infection with other Borrelia species.

Samples for confirmation of diagnosis

Occurrence

Swine dysentery occurs in most major pig-producing countries and was an important disease of pigs in North America (11% in USA), Australia, and the United Kingdom.⁶

A postal survey suggested that 10.5% of herds were infected.⁷ In Denmark 14% had BH.⁸ However, in recent years with the increase in minimal disease herds, the incidence of the disease has decreased. It is most common in the 7- to 16-week-old age group but may affect older pigs to 6 months. Adult pigs are seldom affected, and rarely suckling piglets. The overall occurrence is probably around 10% with a considerable control through drugs, particularly growth-promoting antibiotics, in countries where these are allowed. A Swedish study,⁹ showed that Brachyspirae species were isolated from 58.5% of all samples. Of these 25.4% were B. hyodysenteriae, 16.4% were B. pilosicoli, and 58.2% were intermedia, innocens or murdochii.

Morbidity and case–fatality rate

Morbidity within a group of pigs can range from 10–75% and if untreated the case–fatality rate can be as high as 50%.

Risk factors

Methods of transmission

The organism is present in the feces of affected pigs. Infection is by ingestion and transmission is enhanced by conditions leading to fecal–oral cycling. Spread of infection within a group is slow, taking up to 7–14 days, and may spread to other pens of pigs over a 2–3-week period. Pigs which have recovered from clinical disease with or without treatment may become carriers and still have the ability to shed the organism and infect in-contact animals for 50–90 days. Clinical disease may initially be precipitated by stress but infection subsequently spreads by direct contract. The frequency of shedding varies with time and only a small proportion of a convalescent population may be expected to be carriers.

The organism has been isolated from a dog on a swine farm where swine dysentery was present. Experimentally, mice are susceptible to the infection and may be a potential source of infection in a piggery. Direct evidence of transmission has been shown from mice⁴² when eight farm mice were trapped and three were found which had BH which was of the same PFGE pattern as that of the pigs. This suggests that farm mice are a confirmed reservoir of infection. They are capable of carrying the organism for up to 180 days after inoculation.

Economic importance

The disease can cause heavy mortality in growing pigs but it is equally important for its effect on the efficiency of production. The economic losses from decreased feed efficiency are estimated at four times the cost of medication. The infection tends to be persistent within a herd and may have a cyclic occurrence, which is a problem in intensive pig-rearing enterprises and frequently control can be achieved only by costly continuous prophylactic medication.

Detection and culture of organism

The organism may be detected in the feces of affected pigs by dark-field microscopy as highly motile organisms with a characteristic serpentine motility or in dried smears with Giemsa or Victoria blue 4R staining. The best diagnosis is achieved by taking samples from the upper colon. Fecal samples submitted for laboratory examination should be diluted (1:10) in phosphate-buffered saline or rectal swabs placed in Amies medium to avoid death of the organisms which will occur when the samples are stored at room temperature or sent in the mail. Microagglutination tests (MATs), slide agglutination tests, and indirect and direct fluorescent antibody tests are also used to detect the organisms.⁵

The organism can be cultured on Trypticase Soy agar containing 5% defibrinated bovine blood under specific atmospheric conditions.⁶

Fluorescent antibody staining aids considerably in its demonstration, but may not distinguish non-pathogenic strains and false-positive and false-negative results are common.⁵³ The presumptive diagnosis from the fluorescent antibody test (FAT) can be supplemented with a variety of laboratory tests which serve to identify the spirochetes as pathogenic. The slide agglutination test is a useful and specific means of identifying an organism but requires an appreciable amount of growth of spirochetes on the surface of the agar to carry out the test.⁵⁴ The microscopic agglutination test is a rapid test for the definitive laboratory identification of B. hyodysenteriae but it cannot distinguish the avirulent strains of the organism.⁵⁴

A major diagnostic problem has been the identification of carrier pigs which are infected with the organism and are a potential source of infection to other pigs. Indirect and direct FATs used to examine feces and colonic material from pigs for B. hyodysenteriae have not been sensitive or specific enough to identify individual infected pigs.

Any diagnostic test must be able to distinguish between the different Brachyspira spp.⁵⁵ Some are harmless commensals, while others are potentially pathogenic. Brachyspira innocens, a non-pathogenic inhabitant of the porcine large intestine, is very similar to B. hyodysenteriae in both morphology and growth characteristics and shares many of the same surface antigens. Numerous serological tests with sera from pigs that have recovered from B. hyodysenteriae infection have demonstrated the presence of cross-reactive antibodies between B. hyodysenteriae and B. innocens, which makes differentiation difficult.

Samples for confirmation of diagnosis

Antimicrobial therapy

Antimicrobials are usually administered by mass medication to all pigs within the affected group. Treatment by water medication rather than feed medication is preferable because it is generally easier and quicker to put into place and affected pigs usually continue to drink (but perhaps not in the same quantities as when unaffected) while they are anorexic. Pigs with severe hemorrhagic diarrhea and toxemia may not drink sufficient medicated water and must be treated initially by parenteral injection.

Medication of feed is most suitable for subsequent prophylaxis. When outbreaks occur, all severely affected pigs should be treated individually, and the drinking water medicated for several days at therapeutic levels, followed by possible medication of the feed for up to 3 weeks or longer at prophylactic levels.

Failure to respond to therapy

The major problems with the treatment of swine dysentery are the failure of some outbreaks of the disease to respond favorably to treatment, and relapses or new cases which may occur following withdrawal of medication of the feed or water. Several drug-related problems have been postulated to explain these problems.

Drug-delayed swine dysentery occurs several days after withdrawal of medicated feed. It may be due to either an ineffective drug or inadequate dosage of an effective drug and failure to eliminate the causative organism from the colon. However, reinfection from other swine must also be considered. The nitroimidazoles at high levels will apparently prevent the delay or recurrence of dysentery.

In experimentally induced swine dysentery using colon from affected pigs as the oral inoculums, tiamulin in the drinking water at 45 or 60 mg/L for 5 days was also effective in treating clinical disease. However, diarrhea commonly recurred 2–10 days after withdrawal of the drug and repeated medication of the water with tiamulin was necessary to reduce the severity of diarrhea and prevent deaths. After one to three retreatments, the pigs were immune to experimental exposure and there was a significant increase in their serum anti B. hyodysenteriae antibodies. This supports the observation that when certain antimicrobial agents such as dimetridazole, which are highly effective in preventing the development of diarrhea, are withdrawn, the affected pigs do not become immune.

Drug-diminished swine dysentery occurs when suboptimal levels of the drug are used. The severity of the diarrhea is reduced; deaths do not occur, but the disease is not eliminated. However, severe disease may follow withdrawal of medication.

The feeding of ronidazole at 60 ppm for 10 weeks, or carbadox at 55 ppm or lincomycin at 110 ppm for 6 weeks eliminated an experimental infection, and swine dysentery did not recur during a 9-week period after withdrawal of the medication. The feeding of sodium arsanilate at a level of 220 ppm for 3 weeks to pigs which had been fed ronidazole for only 6 weeks did cause the development of swine dysentery.

In both drug-delayed and drug-diminished swine dysentery, there are chronic lesions in the colon. In drug-resistant swine dysentery, medication of the feed is not effective and diarrhea and deaths occur. Certain outbreaks of the disease may be resistant to both tylosin and sodium arsanilate. Selection of an effective drug is necessary. The sensitivity of B. hyodysenteriae to dimetridazole has not decreased significantly following use of the drug over several years.

Drug-augmented swine dysentery is a more severe form of the drug-resistant disease in which affected pigs are more severely affected than non-medicated controls. The cause is unknown. The disease occurs in a severe form several days or weeks following withdrawal of successful medication for a previous outbreak of the disease. This form appears to occur most commonly in pigs which did not have clinical disease during an earlier outbreak, but received medication. The concentration of the drug administered was sufficient to prevent diarrhea, but not sufficient to eliminate the spirochetes from the colon. During the delay of the initial diarrhea by the drug, there may have been intraglandular recolonization of spirochetes throughout the colon. After withdrawal of medication, rapid intraglandular multiplication of the large spirochetes may occur and result in clinical disease. Drug-delayed augmented dysentery usually occurs only in those pigs that have been infected but did not develop clinical disease, which usually results in immunity. The occurrence of diarrhea is necessary for its development which occurs 4–13 weeks after infection. Treatment of swine dysentery with the more efficacious drugs has been shown to inhibit the development of this immunity and serum antibody to B. hyodysenteriae. However, the clinical significance of this is undermined and at present it is suggested that outbreaks of swine dysentery be treated vigorously.

It should be possible to minimize these drug-related problems of swine dysentery by the use of therapeutic levels of effective drugs in the drinking water for short periods followed by prophylactic levels in the feed for 3 weeks or more. This must be combined with proper management techniques and waste disposal systems which minimize or prevent re-exposure.

Regardless of the drug used, many pigs are reinfected following withdrawal of medication because of the continual presence of the organism in the environment. The sources of the organism include in-contact carrier pigs which are shedding the organism and survival of the organism in waste materials, which was presented under the heading epidemiology.

Cleaning and disinfection

After the institution of treatment, thorough cleansing of the contaminated pens is necessary to prevent reinfection or the transmission of infection to new groups of pigs. This is usually done after 3–6 days when all diarrhea has ceased. The decision to continue with prophylactic medication depends upon the hygiene and a knowledge of past patterns of the disease on the farm. It is generally recommended to continue prophylaxis for at least 2 weeks. Swine housing units with open gutter-flush systems in which swine dysentery-infected pigs have been maintained should remain idle for a longer period than 5 or 6 days to eliminate B. hyodysenteriae.⁸⁰

Control of infection/limitation of reinfection

Control of the clinical disease can be achieved by early treatment with adequate levels of antimicrobials for a sufficient length of time. This must be combined with adequate removal of fecal wastes to prevent reinfection. Pigs destined for market should be moved out as a group and their pens cleaned, disinfected and allowed to dry for a few days before pigs are restocked. Where possible the purchase of feeder pigs should be restricted to private sales from herds with no history of the disease. Communal trucks should not be used for transport. Where this is not possible pigs should be placed in isolation pens for 3 weeks and provided with medicated feed or water to eliminate the carrier state in infected pigs. Every effort should be made to avoid potential fecal– oral cycles and contamination by feces between pens. Preventing the build-up of fecal wastes is also of paramount importance. Pigs from different source farms should not be grouped in the same pen. It is also necessary to reduce the stress of transportation and overcrowding on the pigs.

In farrowing-to-market enterprises where the disease is always a threat, routine prophylactic medication may also be necessary. This is commonly carried out following weaning and during the early growing phase. In countries where withdrawal periods are in force, the use of certain microbials is precluded for this purpose.

The feeding of tiamulin at a dose of 20 mg/kg BW to pregnant sows beginning 10 days before farrowing and continuing until 5 days after farrowing when the piglets are weaned and transferred to an isolation unit has been successful in the prevention of infection of newborn piglets. This is known as the ‘barrier method’ which can be an efficient method of eradicating endemic infections. To reduce the risk of postnatal infection of the progeny, the piglets should stay with the latently infected sows for the shortest time possible. Furthermore, early weaning is necessary and strict isolation is an important condition to success. B. hyodysenteriae is spread primarily by carrier pigs, and contact between infected and uninfected pigs must be avoided.

The administration of tiamulin at 10 mg/kg BW IM daily for 5 days to all animals in a large herd, combined with cleaning, disinfection and rodent control was effective in controlling the disease and no further clinical signs occurred in the subsequent 2.5 years.

Eradication of infection

Vaccines

Pigs which have recovered from clinical swine dysentery may be protected against subsequent challenge, but attempts to immunize pigs with B. hyodysenteriae have been proven to provide incomplete protection and involve complex procedures which may have limited practical value. The development of effective vaccines will require attention to serospecificity of the organisms used to formulate the vaccines.

Effective vaccines are not widely available as yet. A commercial vaccine using a protein digested bacterin has shown efficacy in the reduction of disease due to B. hyodysenteriae.^83,84 It produced both a systemic and mucosal immunity. Both IFN gamma and lymphocyte blastogenesis responses were stimulated. A recombinant outer membrane lipoprotein has also been shown to be a hopeful vaccine.⁸⁵

An inactivated, adjuvanted, whole-cell vaccine against B. hyodysenteriae has been tested experimentally.⁸⁶ The vaccine provided significant protection in two small trials but some of the vaccinated and unvaccinated pigs developed late-onset swine dysentery, which is unexplainable. Field trials to test the vaccine are required. An experimentally inactivated B. hyodysenteriae vaccine adjuvanted with mineral oil resulted in exacerbation of the clinical disease following challenge; a majority of the vaccinated pigs developed the disease earlier and to a more severe degree than the unvaccinated pigs.⁸⁰