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Also Known As

Chronic active hepatitus, Serum hepatitis, Theiler's disease


Hepatitis is a life-threatening liver disease that can be caused by a number of toxins, antiserums, anti-toxins, plants, chemicals, and drugs. Chronic active hepatitis is an inflammatory liver disease without a known cause. It differs from hepatitis in humans and is less severe than other forms of liver disease.

The liver has many vital functions, including synthesis of enzymes, sugars, and proteins, and the removal of ammonia and other wastes from the bloodstream. The liver also functions to detoxify drugs and poisons that are ingested.

When liver function is compromised by toxins, antiserums, anti-toxins, ingestion of toxic plants, chemicals or drugs, it fails to remove ammonia and other wastes from the blood stream resulting in damage to the horse's major systems.This combined with a decreased ability to synthesize enzymes, sugars and proteins may lead to further damage.  By the time the symptoms of hepatitis are displayed, sometimes the damage is too severe to start treatment

Liver functions may also be compromised by other diseases.


  • Jaundice, noted in the yellow appearance of eyes and mucous membranes
  • General malaise
  • Loss of appetite
  • Loss of weight
  • Mental depression
  • Staggering and aimless wandering
  • Obliviousness to surroundings
  • Photosensitivity


Tetanus antitoxin is thought to be the cause of most cases of serum hepatitis, although seasonal cases of hepatitis in summer and fall reflect an infectious cause not fully understood. The cause of chronic active hepatitis is also unknown.


Prevention of hepatitis includes avoiding use of tetanus antitoxin as an antiserum because of its implication in nearly all cases of serum hepatitis. Since the cause of chronic active hepatitis is unknown, the best prevention is good horse health management, with plenty of fresh water, clean hay, and well-ventilated stable areas to keep horses healthy enough to withstand whatever might cause hepatitis.


Treatment of hepatitis or acute liver failure is directed at supporting the horse in every way possible. Corticosteroids have been successfully used in some cases and the veterinarian will determine how and when to use them.

Keeping the horse calm, stabled, and giving intravenous fluids recommended by a veterinarian may allow the horse to overcome the effects of hepatitis. Serum ammonia levels may need to be reduced, depending on liver tests or biopsies. Mixed-grain rations, containing beet pulp, molasses, or other sorghums may be offered in four to six small feedings a day. Vitamins and folic acid supplements may be prescribed by a veterinarian.

As the appetite returns, good quality oat hay may be introduced to the diet. At best, the prognosis for recovery is guarded, but not hopeless. A veterinarian who is familiar with the horse and the circumstances is the best option for a satisfactory outcome.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.