Theiler’s Disease (Equine Serum Hepatitis) in Horses

Key takeaways

Theiler’s disease is a form of liver disease affecting adult horses. Also known as equine serum hepatitis, it is a rare but potentially life-threatening condition. 

  • The cause of Theiler’s disease is not well understood but there is some connection between the condition and administration of equine-origin blood products
  • A viral origin is also suspected as there is some evidence of contagion
  • Symptoms include jaundice, altered mental state, blindness, colic, and lethargy
  • Theiler’s disease might also lead to hepatic encephalopathy which presents with behavioral changes, depression, and other neurological signs such as seizures and coma
  • Diagnostics include a complete physical evaluation, bloodwork, and liver ultrasound
  • Typical treatment consists of supportive therapy and may include antibiotics, anti-inflammatories, intravenous fluids, nutritional support, and liver supplements
  • Prognosis varies depending on severity and timing of and response to treatment
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A closer look: Theiler’s Disease (Equine Serum Hepatitis) in Horses

Theiler’s disease is a rare condition but might have fatal consequences. It might also lead to hepatic encephalopathy and long-term complications. As a life-threatening disease, signs of Theiler’s disease must be considered an emergency. Horses that continue to eat on their own have a good prognosis compared to those that develop neurologic signs consistent with hepatic encephalopathy (altered mentation, uncoordinated gait, head pressing, blindness), which has a guarded prognosis.

Risk factors

Since the causes of Thelier’s disease are not well understood, it is unclear which horses are at higher risk. There is some association between the condition and equine-origin blood products. Therefore, horses receiving treatment protocols using these products may be at higher risk. There is also some suggestion that the condition may have a viral cause, so horses in contact with diagnosed individuals may also be at increased risk.

If neurological signs present, the prognosis becomes poorer with a rapid progression to death in around 70% of cases.

Horses with Theiler’s disease can develop hepatic encephalopathy.

Horses may also develop photosensitization secondary to the ongoing liver disease. This is caused by molecules that travel from the compromised liver to the skin, and react with sunlight.

Possible causes

The cause of Theiler’s disease is still the subject of ongoing research. It is often associated with administration of equine-origin blood products. In recent times, most cases followed the administration of tetanus antitoxin and commercial plasma products.

A viral infectious agent is also among the suspected causes of Theiler’s as there are cases of horses falling ill after contact with horses who received incriminated blood products.

Main symptoms

Testing and diagnosis

The diagnostic process involves a complete physical examination, including blood work to evaluate liver enzymes and metabolites. Other tests to determine a definitive diagnosis include diagnostic imaging of the liver such as ultrasound.

Steps to Recovery

Treatment focuses mainly on supportive care to prevent further damage to the liver. Fluid therapy, antibiotics, anti-inflammatories, nutritional support, and liver-supporting supplements are often part of therapy. Some horses that develop hepatic encephalopathy might need sedation to reduce the symptoms.

Horses that react well to treatment, especially in the first 5 days, have a fairer prognosis and do not normally suffer from long-term complications. Horses that develop hepatic encephalopathy have a poorer prognosis and in most cases the condition is fatal.


As the cause of the condition is still an object of speculation, prevention is difficult. To reduce the chances of developing Theiler’s disease, horses should not come in contact with incriminated equine-origin blood products. Also, as the cause might have a viral component, horses that received such products must be kept separate from other horses to prevent any chance of contagion.

Is Theiler’s Disease (Equine Serum Hepatitis) in Horses common?

Theiler’s is a rare disease in horses with about 1 or 2% of horses receiving equine blood products developing the disease.

Typical Treatment

  • Antibiotics
  • Anti-inflammatories
  • Nutritional support
  • Liver-supporting supplements
  • Intravenous fluids

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