Tyzzer’s disease in foals: Retrospective studies from 1969 to 2010

Abstract

Reports of 148 cases of Tyzzer’s disease in foals in central Kentucky were analyzed to identify features of the disease and factors associated with it. The records indicate that Tyzzer’s disease is a rapidly progressive, highly fatal hepatitis caused by Clostridium piliforme. Common clinical findings are lethargy, fever, anorexia, and icterus. Seizures, coma, and death may rapidly ensue. Laboratory findings are leukopenia, metabolic acidosis, hypoglycemia, and increased activity of hepatic enzymes. Diagnosis is primarily based on clinical signs and postmortem findings but a polymerase chain reaction (PCR) is now available to detect C. piliforme DNA in organs and feces. Disease occurred most frequently in foals between 9 and 30 days of age that were born in April to May and was associated with heavy rainfall in the spring and high protein and nitrogenous diets fed to nursing mares. The findings are consistent with the ingestion of C. piliforme in the feces of adult horses and overgrowth in the intestine of foals with a high level of nutrients in their intestine.

Introduction

Tyzzer’s disease was first described in mice in 1917 (1) and in laboratory rabbits in 1965 (2). As early as 1964 a fatal hepatitis consistent with Tyzzer’s disease occurred on a Thoroughbred horse farm in central Kentucky. These cases and others on the same farm were first reported as Tyzzer’s disease by Swerczek et al in 1973 (3). Other cases have subsequently been reported in foals elsewhere in the USA (4–15), England (16), Canada (17,18), New Zealand (19), Australia (20,21), South Africa (22), Japan (23), and Germany (24,25).

Tyzzer’s disease frequently occurs in rodents, lagomorphs, and foals, and has been reported sporadically in other domestic and wild animals, including muskrats (26), cottontail rabbits (27), coyotes (28), dogs (29), snow leopards (30), lesser panda (31), gray fox (32), raccoon (33), cattle (34), marsupials (35), and white-tailed deer (36).

Tyzzer’s disease is caused by a Gram-negative, pleomorphic, motile, endospore-bearing rod-shaped and flagellated obligate intracellular bacterium previously called Bacillus piliformis. Tyzzer originally named the unique organism Bacillus piliformis based on its morphology, but a new name, Clostridium piliforme, was proposed because the ribosomal nucleic acid sequences of this organism are closely related to clostridia (37). Clostridium piliforme may be common in the environment and, since it is difficult to culture, little knowledge has accumulated on epidemiology, pathogenesis and immunity.

The purpose of this study was to investigate predisposing factors for Tyzzer’s disease in foals, and diagnosis and treatment of affected foals. These factors include age of foals, time of the year when they were born, rainfall, and diets fed to nursing mares.

Materials and methods

Complete case reports and epidemiological investigations on affected farms performed by the author from 1969 to 1986 were used to identify risk factors for Tyzzer’s disease in foals. Horses from 76 cases of Tyzzer’s disease from 1969 to 1986 were submitted to the Department of Veterinary Science, University of Kentucky for necropsy. Archives of cases that were submitted to the University of Kentucky during 1986 to 1993 were incomplete and these cases were not investigated. An additional 72 cases submitted to the Livestock Disease Diagnostic Laboratory, University of Kentucky from 1993 to 2010 were also investigated. Published reports on Tyzzer’s disease in foals involving clinical pathologic, serologic, and molecular tests, and treatments for the disease were reviewed and included in this study. Data on the average number of Thoroughbred foals born each month were provided by the Thoroughbred Jockey Club, Lexington, Kentucky.

Results

From 1969 to 1986 Tyzzer’s disease occurred on 44 horse farms, 4 of which experienced more than 1 case. One farm (A) had 24 cases, another (B) had 6 cases, a third farm (C) had 4 cases, and the fourth farm (D) had 2 cases. In 1970 no cases were reported, but from 1971 to 1985, 1 to 13 cases occurred each year. From 1993 to 2010, 72 cases occurred with an average of 4 cases reported per year.

Age of affected foals at the day of death ranged from 5 to 30 d. Most foals died between 9 and 30 d, with the average age being 20 days. Younger affected foals were often found dead, while older foals showed clinical signs 2 to 48 h before death. The gender of foals was equally distributed. Breeds affected were Thoroughbreds, Standardbreds, American Saddle horses, Tennessee Walking horses, and crossbred foals. The majority of foals were Thoroughbreds, but this reflected the horse population in the central Kentucky area.

Diets fed to nursing mares were commercial 14% or higher protein mixed grain ration in a textured grain mixture or complete pellet. Forage rations were primarily legume-grass mixtures, clover, and alfalfa hay. Pasture grasses were a mixture of orchard, timothy, fescue, bluegrass, and white and red clover. Farms with multiple cases of Tyzzer’s disease fed their mares a higher protein diet than farms without any cases of the disease.

The average percentages of foals contracting Tyzzer’s disease each month were as follows: February, 5%; March, 18%; April, 31%; May, 30%; and June, 16%. The average percentages of the North American registered Thoroughbred foal crop born each month are as follows: January, 7.8%; February, 17.8%; March, 27.7%; April, 29.5%; May, 15.5%; June, 1.4%; July and later, 0.2%. On average, 54% of foals are born in January, February, and March, yet only 23% of the cases of Tyzzer’s disease occurred during these months. Also, 46% of foals were born in April, May and June, while 77% of Tyzzer’s disease cases occurred during these months.

Clinical signs

Clinical signs were acute, and typically, previously healthy, robust, fast-growing foals between 1 and 4 wk of age had acute onset of clinical signs. Often, younger foals were found dead without having shown signs of disease. Generally, foals died 2 to 48 h after the onset of illness. Typical signs included weakness, lethargy, anorexia, dehydration, pyrexia, tachycardia, and icterus. Some foals had a yellowish semi-liquid diarrhea. Often, seizures, coma, and death rapidly ensued. A provisional diagnosis of Tyzzer’s disease was made based on the clinical signs and age of the foal.

Clinicopathologic findings

The hematological and serum biochemical values were similar in all affected foals. Leukopenia, hypoglycemia, and elevated hepatic enzymes were common findings. When foals first showed clinical signs, the majority had a severe leukopenia with values between 2000 to 4000 white blood cells (wbc)/μL. Foals had marked hypoglycemia, metabolic acidosis, elevated serum fibrinogen, and elevated bilirubin. Aspartate aminotransferase (AST), alanine aminotransferase (SGPT), lactate dehydrogenase (LDH), and alkaline phosphatase were elevated. Similar hematologic and hepatic enzyme values in foals with Tyzzer’s disease were reported by others (9,12).

Treatment of foals with Tyzzer’s disease

On farms with confirmed cases of Tyzzer’s disease, clinicians reported successful treatment with IV dextrose and broad spectrum antibiotics in a few cases. In the majority of cases, foals responded temporarily to the IV dextrose, but then rapidly deteriorated and succumbed to the disease. A presumptive case of Tyzzer’s disease in a foal was successfully treated by intensive administration IV of dextrose, sodium bicarbonate, potassium chloride, penicillin, and sulphamethazole-trimethoprim (13). Successful treatment has also been reported with IV fluids, antimicrobial and anti-inflammatory drugs, and parenteral nutrition (15).

Necropsy findings

Postmortem examination of foals notably revealed a pronounced hepatomegaly with multifocal light-colored areas in the liver capsule and parenchyma. The hepatic lymph nodes, and often the mesenteric lymph nodes as well, were hyperplastic and edematous. Contents of the gastrointestinal tract were liquid or semi-solid and yellowish or normal. Ecchymotic and diffuse hemorrhages occurred on the serosal surface of the heart, diaphragm, and the small and large intestine.

Histopathologic lesions in the liver of affected foals consisted of numerous widespread multifocal areas of necrosis and hepatitis and were pathognomonic. Often necrotic foci coalesced. Hepatocytes in the center of the necrotic foci were destroyed and replaced by mixed mononuclear cells, neutrophils, and red blood cells. Hepatocytes at the periphery of the necrotic foci contained intracellular filamentous bacilli that were faintly stained with hematoxylin and eosin. Silver and Giema stains demonstrated more clearly the intracellular interlacing bundles of filamentous bacilli. To a lesser extent, microscopic inflammatory changes and bacilli were present in the hepatic lymph nodes, in the mucosal cells of the small and large intestine and in focal inflammatory areas in myocardial cells.

Diagnosis and diagnostic tests

The diagnosis of Tyzzer’s disease was made in all cases by observing the characteristic gross and microscopic lesions in the liver of foals at necropsy. In the early cases, when the disease was first confirmed as Tyzzer’s disease (3), the bacterium was isolated in the yolk sac of developing chicken embryos. However, the causative agent, C. piliforme, is very difficult to culture from clinical or postmortem specimens. The agent can be cultivated in tissue culture (38), but this technique has not been attempted for the cultivation of the bacterium from foals.

A recently developed real-time polymerase chain reaction (RT-PCR) assay detects DNA from C. piliforme and is a feasible test for routine diagnosis. The PCR targeting 16S ribosomal ribonucleic acid (rRNA) genomic sequences can detect the organism in the feces of foals showing clinical signs and provides for early diagnosis and possible treatment. There is also a real-time TaqMan assay that detects C. piliforme gene sequences in liver tissue from affected foals (15).

Serologic tests can detect exposure to C. piliforme in horses (39). In laboratory animals serologic tests are used to detect C. piliforme antibodies, but are insufficient for a definitive diagnosis of an active C. piliforme infection (40,41). Serologic tests have been evaluated for the diagnosis of Tyzzer’s disease in multiple studies in laboratory animals (42–44).

Serological tests in rodents and rabbits showed that seroprevalence to C. piliforme isolates was more widespread in rodents than was clinical disease (45–47). This suggests that asymptomatic infections in rodents and lagomorphs are widespread, and unless animals are compromised by stress factors or immunologic dysfunction, clinical signs of the disease may be absent. Immunosuppressive treatment with corticosteroids (48) and challenge without steroids (49) often result in asymptomatic infection in rodents.

Discussion

Tyzzer’s disease is characterized primarily by severe hepatic lesions in young foals, with lesser involvement of the intestinal tract and rarely the myocardium. In contrast, gerbils infected with C. piliforme often exhibit diarrhea, and, although focal necrosis may be present in the liver and myocardium of rodents and lagomorphs, lesions also occur in the intestinal tract (50).

Clostridium piliforme is difficult to culture from animals with clinical signs or from tissues at necropsy. Other than the early work that first reported the discovery of Tyzzer’s disease in foals (51,52), the bacterium has not been cultured from any of the subsequent reported cases in foals. The bacterium may be common in the environment, but since it is difficult to culture, little knowledge has accumulated on its epidemiology, pathogenesis, and immunity.

Carriage and excretion of C. piliforme are difficult to measure because it is not possible to grow the organism in cell-free media. The PCR can detect the presence of C. piliforme DNA, but not the quantity of the organisms in feces (15,53). Adult horses may be carrying C. piliforme in the gastrointestinal tract and passing it to susceptible young foals through feces. Although no information is available on the survival of the endospores on open pastures, endospores are resistant to heating up to 60°C for 30 min or to exposure to 70% ethanol, 3% cresol, 4% chlorhexidine, or 0.037% formaldehyde; they are sensitive to heating at 80°C for 15 min or exposure to 0.4% peracetic acid, 0.015% sodium hypochlorite, 1% iodophol, or 5% phenol (54).

Tyzzer’s disease often occurs sporadically, but multiple cases occurred on some farms. The source of C. piliforme in the environment and the factors predisposing foals to Tyzzer’s disease are open to speculation. Diarrhea and fecal contamination of bedding are involved in transmitting the organism in laboratory rodents, and coprophagous oral transmission of C. piliforme occurs in foals (51).

Adult horses are resistant to disease caused by C. piliforme, but the organism appears to proliferate in the gut, especially if adults are fed a nutrient-rich diet. Foals normally consume freshly deposited feces from their dams most often between the second and fifth week of age, and rarely after the fifth week of age (55). Interestingly, foals most often succumb to Tyzzer’s disease between the second and fourth week of age, and Tyzzer’s disease rarely occurs in foals after the fifth week of age. It is likely that the feces of nursing mares are a primary source of C. piliforme infection for young foals.

Nutrient-rich diets fed to nursing mares and coprophagia were suspected to be the primary predisposing factors for developing Tyzzer’s disease in young foals. Since C. piliforme is commonly found in rodents and lagomorphs, it was suspected that nursing mares may be infected after being exposed to fetal contamination from wildlife. On horse farms where Tyzzer’s disease was most common, nursing mares often grazed pastures where the waterways routinely flooded lowland pastures after heavy rainfall. The streams contained numerous muskrats, which are known to be carriers and infected with C. piliforme(26).

There have not been any studies in foals involving stress factors and immunosuppressive steroids, but they likely play a role in the pathogenesis of the disease. Stress-related factors and dietary factors, including high-protein diets fed to nursing mares seemingly are involved in predisposing suckling foals to Tyzzer’s disease (56). Similarly, it has been shown that high-protein diets predispose laboratory animals to Tyzzer’s disease (57).

The sporadic nature of Tyzzer’s disease in foals reported in the literature has not revealed any predisposing factors which may increase the susceptibility of foals to the disease. However, numerous cases seen on Kentucky horse farms and a case-control study (14) designed to determine variables associated with increased risk of C. piliforme infection in foals on a California Thoroughbred breeding farm have identified some predisposing factors. In the California study, foals born between March 13 and April 13 were 7.2 times more likely to develop C. piliforme infections than foals born at other times of the year. A similarly high incidence of the disease occurred in foals born in the spring on Kentucky horse farms where 58% of the foals were affected in April and May. Also, foals from nonresident mares were 3.4 times more likely to develop C. piliforme infection. And, foals of mares less than 6 years of age were 2.9 times more likely to develop C. piliforme-associated illness (14).

Amount of rainfall during the early spring dramatically affects growth of cool-season grasses. Furthermore, a spike in the protein and non-protein nitrogenous compounds, including nitrate, commonly occurs in the early spring pasture forages (58). If there is a sudden increase in nitrogenous compounds in the diet, there may be a change in the distribution of the normal bacterial gut flora which often leads to increased nitrate and ammonia levels in the hindgut (59). Farms with multiple cases of Tyzzer’s disease also had foals with congenital musculoskeletal lesions. The cause of skeletal abnormalities is unknown, but Canadian researchers suggest that there may be an association with nitrate toxicity and hyperthyroidism (60,61).

The increase in availability of nutrients may encourage the overgrowth of C. piliforme in the gut of the nursing mare and foal. Other similar peracute clostridial diseases in foals (62), and clostridial infections in herbivores in general, also often affect apparently healthy fast-growing young animals when the intestinal environment is altered by sudden changes in the diet that cause the overgrowth of clostridial organisms in the gut of affected animals (63). Since Tyzzer’s disease is a peracute, highly fatal clostridial disease of foals, avoiding sudden changes in the diet of nursing mares may be helpful in controlling the disease in foals.