Bit & Bridle

Equine Theiler’s Disease

Other Names

  • Idiopathic Acute Hepatitis Disease (IAHD)
  • Serum-associated hepatitis
  • Serum sickness
  • Postvaccinal hepatitis
  • Equine serum hepatitis

What is Theiler’s Disease?

Theiler’s disease is a severe and often fatal acute hepatic necrosis (liver failure) in adult horses. It was first described by Sir Arnold Theiler in South Africa in the early twentieth century, originally in association with the administration of equine-origin biological products such as antitoxins, immunoglobulins, and vaccines made from horse serum. Today, cases occur both in horses that have received these products and in horses with no known exposure — the latter referred to as idiopathic cases.

The disease is characterised by massive destruction of liver tissue over a very short period. Unlike chronic liver disease, which develops gradually and may remain subclinical for a long time, Theiler’s disease causes rapid deterioration. Affected horses can progress from apparently normal to severe hepatic encephalopathy and death within days.

The underlying cause is now believed to be a viral infection. Research published in 2018 identified equine parvovirus-hepatitis (EqPV-H) as the most likely causative agent in many cases, and the virus is thought to be transmitted via contaminated equine-origin biologicals. However, not all infected horses develop clinical disease, suggesting other factors — such as immune status or concurrent stressors — play a role.

Causes

  • Equine parvovirus-hepatitis (EqPV-H): The primary suspected causative agent. The virus is present in horse plasma and is transmitted via equine-origin biological products. Sexual transmission has also been proposed.
  • Administration of equine-origin biologicals: Tetanus antitoxin, hyperimmune plasma, and certain vaccines prepared from equine serum are the most commonly implicated products. Cases typically appear 4–10 weeks after administration.
  • Idiopathic: A significant proportion of cases have no identifiable exposure to biological products. These cases may represent natural viral transmission through other routes.

Symptoms and Signs

Clinical signs reflect acute liver failure and can develop very rapidly:

  • Jaundice (icterus): Yellowing of the mucous membranes, conjunctivae, and skin — often the first sign owners notice
  • Hepatic encephalopathy: As toxins accumulate in the bloodstream due to failing liver function, neurological signs develop — these include disorientation, head pressing, circling, apparent blindness, and aggressive or unusual behaviour
  • Anorexia and depression: Loss of appetite and profound lethargy
  • Photosensitisation: Some horses develop skin lesions, particularly on white or lightly pigmented areas, due to accumulation of photodynamic compounds the liver can no longer process
  • Colic: Abdominal discomfort may be present
  • Dark or discoloured urine
  • Rapid deterioration: In severe cases, horses may progress to recumbency, seizures, and death within 24–72 hours of onset

Diagnosis

Diagnosis is based on clinical findings combined with laboratory evidence of severe hepatic damage:

  • Blood biochemistry: Markedly elevated serum hepatic enzymes including gamma-glutamyl transferase (GGT), alkaline phosphatase (ALP), and aspartate aminotransferase (AST). Elevated serum bilirubin accounts for the jaundice. Hypoglycaemia (low blood glucose) reflects impaired gluconeogenesis.
  • Serum bile acids: Significantly elevated, confirming hepatic dysfunction rather than simple hepatocellular damage
  • Clotting profile: The liver produces clotting factors; horses with Theiler’s disease may develop coagulopathy
  • Ultrasound: Hepatic ultrasonography may show abnormal echogenicity or reduced liver size in peracute cases
  • Liver biopsy: Confirms the diagnosis histologically, showing coagulative necrosis. Carries some risk in horses with coagulopathy and requires careful case selection.
  • History of biological product administration: A history of antitoxin or hyperimmune plasma given 4–10 weeks prior strongly supports the diagnosis

Treatment

There is no specific antiviral treatment for Theiler’s disease. Management is entirely supportive and aims to keep the horse alive while the liver attempts to regenerate:

  • Intravenous fluids: Corrects dehydration, supports blood pressure, and helps maintain blood glucose
  • Glucose supplementation: Essential if hypoglycaemia is present
  • Hepatic encephalopathy management: Reducing ammonia production through dietary modification (reducing protein intake), oral lactulose, and neomycin or metronidazole to reduce ammonia-producing gut bacteria
  • Vitamin supplementation: B vitamins, vitamin C, and vitamin E are commonly administered to support liver function
  • SAMe (S-adenosyl methionine) and milk thistle: Used as hepatoprotectants in some protocols, though evidence is largely empirical
  • Sedation: Horses with severe encephalopathy and associated danger to themselves or handlers may require carefully chosen sedation — diazepam is preferred over acepromazine and detomidine, which rely on hepatic metabolism
  • NSAIDs: Used cautiously — many rely on hepatic metabolism and can worsen gastrointestinal complications

Horses that survive the acute crisis may require weeks of supportive care and dietary management. Those with significant hepatic necrosis face a guarded to poor prognosis even if they survive initial treatment.

Prevention

  • Avoid the use of equine-origin biological products unless absolutely necessary — tetanus toxoid vaccination is far preferable to tetanus antitoxin for routine prevention
  • When tetanus antitoxin must be used, inform the owner of the risk and monitor the horse closely for 10 weeks post-administration
  • Vaccinate against tetanus using the toxoid formulation to remove the need for antitoxin in vaccinated horses
  • Research into EqPV-H serology as a screening tool for donor horses is ongoing

Prognosis

The prognosis for Theiler’s disease is guarded to grave. Mortality rates in published case series range from 50% to over 90% in severe cases. Horses that develop hepatic encephalopathy and coagulopathy have the worst outcomes. Those identified very early, before significant encephalopathy develops, and treated aggressively have the best chance of recovery. Liver regeneration can occur if enough viable hepatic parenchyma remains, and some horses do make full functional recoveries.

When to Call the Vet

  • Any horse showing jaundice (yellow gums, yellow whites of the eyes) — this is always a veterinary emergency
  • Neurological signs in a horse — head pressing, circling, apparent blindness, or abnormal behaviour
  • Any horse showing depression, anorexia, and jaundice within 10 weeks of receiving equine-origin biological products
  • Suspected photosensitisation with liver involvement

Frequently Asked Questions

Is Theiler’s disease contagious between horses?

The evidence suggests EqPV-H, the virus associated with Theiler’s disease, can be transmitted between horses, though clinical disease appears to require additional factors. Horses that share contaminated needles, receive products from infected donor horses, or — potentially — through close contact may be at risk. Biosecurity precautions are sensible if a case occurs on a yard.

Can a horse that survives Theiler’s disease return to full work?

Some horses that recover from acute Theiler’s disease return to normal liver function and resume full work, particularly if a large proportion of liver tissue was preserved. Serial liver function tests and ultrasound examinations are used to monitor recovery over several months. Those with extensive necrosis may have permanently reduced hepatic reserve.

Should I use tetanus antitoxin to protect an unvaccinated horse?

This is a risk-benefit decision. Tetanus antitoxin provides rapid passive immunity in an unvaccinated horse at immediate risk of tetanus infection. However, it carries a real risk of Theiler’s disease. For unvaccinated horses with a wound, many vets now discuss the risk openly with owners. The best long-term solution is ensuring all horses are kept up to date with tetanus toxoid vaccination — see the British Equine Veterinary Association (BEVA) vaccination guidelines.

How quickly does Theiler’s disease progress?

It can progress extremely rapidly. Some horses are found dead without prior clinical signs, while others deteriorate from apparently normal to moribund over 24–72 hours. Any horse that has recently received equine-origin biological products and develops jaundice or neurological signs should be assessed by a vet immediately — this is not a wait-and-see situation.