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Tyzzer’s disease is a rare, highly fatal liver condition that can affect any foal, even those who are well fed and in good health. The disease is caused by infection with C. piliforme bacteria, which is passed to foals through the feces of infected horses.

The presence of bacteria in a contaminated environment can lead to outbreaks of multiple cases on the same farm. For example, foals may become infected by consuming their dam’s feces.

Affected foals tend not to show any prior symptoms, with most cases only being diagnosed after rapid and sudden death. When symptoms are observed, they include lethargy, reduced appetite, dehydration, fever, seizures, weakness, jaundice, abdominal pain, and diarrhea.

Unfortunately, there are currently no treatment options available and the vast majority of affected horses die suddenly or within a day and a half from the onset of symptoms.

Prevention is complex and no vaccine is available. General preventive strategies include maintaining a clean environment, making sure foals receive adequate colostrum within 24 hours of birth, and minimizing stress.

Tyzzer’s Disease in Foals

Tyzzer’s disease, caused by the bacterium Clostridium piliforme, is a severe and highly fatal liver condition that can affect any foal, even those who are otherwise healthy and well-nourished.

C. piliforme is an anaerobic gram-positive bacteria. Various species from the Clostridium genus are responsible for serious health conditions in both humans and animals, including botulism and tetanus.  The majority of affected foals die suddenly without prior symptoms. [1]

This condition is named after the American parasitologist Ernest E. Tyzzer who first identified the disease and named the pathogen Bacillus piliformis in 1917. Tyzzer’s disease was described in the literature as infection with B. piliformis until 1994 when the bacteria was reclassified as Clostridium piliforme, due to its closer relation to the Clostridium genus than to the Bacillus genus. [2]

In horses, Tyzzer’s disease primarily affects foals between seven and 42 days of age. The condition can occur sporadically or can affect multiple foals on the same farm simultaneously. [3][4] Typically foals are exposed to the bacteria in the environment, primarily through fecal matter. [1]

Transmission and Pathogenesis

Not much is currently known about the mechanism of disease progression for C. piliforme infection in foals. To date, the prevailing theory suggests that transmission occurs through the fecal-oral route. Foals are believed to ingest bacterial spores present in contaminated feed, feces, water, or bedding. [4]

These spores enter the environment through the feces of infected animals, including: [4]

  • Rodents
  • Cats
  • Adult horses

Clostridium piliforme is highly resilient and can survive in harsh conditions for over a year. It is also resistant to most disinfectants and heat. [4]

Once ingested, the bacteria infect the horse’s gastrointestinal (GI) tract and, subsequently, the liver, where it causes hepatic necrosis – irreversible tissue death in the liver.  Foals die rapidly as the result of acute liver failure. [4]

The incubation period of Tyzzer’s disease is between four to seven days following oral exposure to bacterial spores. Many adult horses are thought to be asymptomatic carriers of C. piliforme, and foals likely become infected by bacteria shed in the feces of carriers. [4]

Young nursing mares that are newly exposed to the bacteria may play a significant role in spreading the infection. This is because foals naturally practice coprophagy in the early weeks after their birth, and mares without a history of exposure may not pass relevant antibodies to their foals through colostrum. [4]

As foals grow, their GI tract matures and their immune system develops. Mature horses are presumed resistant to Clostridium piliforme bacteria. [4]

Effects of Liver Disease

It is unclear how C. pilliforme injures the liver, but the characteristic rapidly progressing fatal symptoms show severe liver tissue death in juvenile horses. The liver is one of the largest internal organs in horses, and is responsible for critical functions including: [5]

  • Detoxifying substances from the blood
  • Storing vitamins, minerals and glucose (stored in the form of glycogen)
  • Secreting bile, a digestive fluid involved in breaking down fats and proteins
  • Synthesizing amino acids and coagulation factors used in blood clotting
  • Metabolizing drugs
  • Influencing immune responses

Young foals do not have a fully developed immune system and are reliant on antibodies from their dam’s milk to protect them in the early weeks of life. [6][7]

Foals with C. piliforme infection do not have sufficient immune system defenses to fight off the bacteria, which allows them to infiltrate the liver.

Once the bacteria has taken hold, the young foal’s underdeveloped liver is susceptible to catastrophic, irreversible damage. Since the liver is involved in so many vital functions and the foal’s entire physiology is under-developed, symptoms progress rapidly as the liver stops functioning.

Coprophagia in Foals

Coprophagia, commonly known as eating feces or manure, is normal behavior in foals and not a sign of malnutrition. On the contrary, there is some evidence young horses may benefit from eating feces.

The theory is coprophagia may provide essential nutrients and help establish robust bacterial flora in the foal’s gut, thereby strengthening their immune system and digestive health. [8][9]

Coprophagy is commonly observed in foals from one week of age up to one and a half months old. Foals tend to prefer the manure of their mothers, which may suggest the presence of a maternal pheromone. [8][9]

Symptoms

Clostridium piliforme causes inflammation of the liver, resulting in symptoms that are common to other hepatic diseases. Affected foals are frequently found dead with no prior symptoms. When symptoms are observed, they appear suddenly and progress rapidly. [2][4][10]

Possible signs may include: [2][4][10]

Risk Factors

While the pathophysiology of Tyzzer’s disease is not fully understood, several environmental and nutritional factors are thought to increase the risk of infection, including: [2][4]

  • High protein/carbohydrate diet: when fed to breeding mares is considered a potential predisposing factor; such feed regimens can alter the mare’s intestinal microbiome and potentially allow overgrowth of C. piliforme
  • Nurse mares: equine surrogate nurse mares used for orphan foals may play a role in the occurrence of Tyzzer’s disease in new farms
  • Hygiene: foals living in unclean environments are more likely to ingest material infested with spores of C. piliforme
  • Mare maturity: foals born to young mares, under the age of six, appear to be more likely to develop the disease
  • Seasonal spore activity: foals born in March and April are at higher risk of developing the disease, which is more prevalent during spring. This season exposes nursing mares to lush, high-protein pastures, increasing nutrient availability from forages. This may promote the proliferation of C. piliforme bacteria in the mare’s gastrointestinal tract.

Diagnosis

Diagnosing Tyzzer’s disease before death is unusual and complex. In the rare case of investigation in a living foal, veterinarians may use a combination of the following diagnostic tests to confirm Tyzzer’s disease: [4]

  • Physical examination
  • Analysis of fecal samples
  • Bloodwork
  • Diagnostic imaging
  • Liver biopsy