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What is Theiler's Disease?

Theiler’s disease is an acute form of hepatitis and liver failure in horses. Theiler’s disease in horses is closely associated with the use of tetanus antitoxin. Whether your horse receives it directly or comes into contact with a horse that has received it, your horse is at risk of developing Theiler’s disease. Symptoms can range from lethargy to central nervous system related symptoms to death. There is no cure for Theiler’s disease; all you can do is offer your horse supportive therapies in response to the symptoms he is experiencing. Some horses are able to recover and have a good prognosis of long term health. Other horses are not so lucky. If symptoms develop and progress quickly, the prognosis is guarded to poor.

Theiler’s disease is a serious condition causing hepatic damage that could possibly lead to death if left untreated. If your horse is experiencing any type of symptoms of illness, especially icteric mucous membranes, contact your veterinarian immediately.

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Symptoms of Theiler's Disease in Horses

Symptoms can vary in each case of Theiler’s disease and can onset suddenly. Symptoms may include:

  • Lethargy
  • Anorexia
  • Weight loss
  • Diarrhea 
  • Discolored urine
  • Fever
  • Ataxia 
  • Icteric mucous membranes
  • Photosensitization
  • Respiratory distress 
  • Clotting abnormalities
  • Neurologic signs (aggression, maniacal behavior, central blindness)

Types

Theiler’s disease is also known as serum hepatitis, postvaccinal hepatitis, and more commonly idiopathic acute hepatic disease (IAHD). It mainly affects adult horses and is the most common cause of acute hepatitis in horses.

Causes of Theiler's Disease in Horses

The exact cause of Theiler’s disease is unknown. In many cases, horses that come down with this condition contract it soon after receiving an equine origin biologic agent, for example tetanus antitoxin. It is also possible for horses that have not received the tetanus antitoxin to contract Theiler’s disease just by coming into contact with a horse that has received it. 

There are other indications the disease may be caused by a virus. It has been known to occur more frequently in certain seasons versus the other times of the year. There have been indications of the disease occurring naturally or after receiving some sort of serum product that contains commercial equine plasma. 

There is not enough evidence to support either possible cause fully. Therefore, the actual cause of Theiler’s disease is unknown.

Diagnosis of Theiler's Disease in Horses

The veterinarian will begin by performing a complete physical exam. It will give her the opportunity to examine your horse entirely and note all of his symptoms. She will also collect a history from you by asking questions such as when his symptoms began, if they have been getting progressively worse, if he has been in any new areas recently, and other similar questions. She will continue by performing blood work. A complete blood count (CBC) and chemistry panel would be ideal tests to run. The results can give needed information on the kidney and liver values, coagulation time, and other organ function. 

Observation of your horse’s symptoms will also help with diagnosis. Icteric mucous membranes are uniquely associated with liver function, or lack of. His symptoms plus the blood work results will help the veterinarian come to a probable diagnosis.

Treatment of Theiler's Disease in Horses

Unfortunately, there is no exact treatment protocol for horses diagnosed with Theiler’s disease. Supportive care is the best thing you can offer him. 

If your horse is anorexic, the veterinarian may need to tube feed him in order to get him the food items he needs to keep his system functioning properly. Fluid therapy may also need to begin in order to keep him from becoming dehydrated. The fluids will also help his kidneys and liver to continue to function and filter. 

Stressful situations can also exacerbate the impact the disease has on your horse. You should try to keep him in a calm environment, do not move him if unnecessary, do not bring anything new or scary into the area, and only use sedatives if needed for his safety or yours.

Recovery of Theiler's Disease in Horses

Recovery will all depend on how severely the liver is affected. Horses that continue to eat through their illness and are determined to be stable for multiple days have a higher prognosis of recovery than those who do not. If your horse does contract the disease but recover, long term prognosis is also good. For horses that are not stable during their illness have a much lower prognosis of recovery. If your horse’s symptoms appear suddenly and worsen quickly, he has a poor prognosis. If there is any type of blood clotting issue, the prognosis is even less favorable.