Hepatitis in Horses: Inflammation of the Liver Explained

Hepatitis is inflammation of the liver. The liver helps your horse process nutrients, clear toxins, make important proteins, and support digestion. When the liver becomes inflamed, those jobs become harder to do. Horses can have mild liver inflammation with vague signs, or severe disease that progresses to liver failure.

One challenge is that the liver has a large reserve capacity. That means a horse may not look very sick early on, even while bloodwork is already abnormal. Clinical signs often become more obvious only after significant liver damage has occurred.

Hepatitis is not one single disease. It is a description of what is happening in the liver, and the underlying cause can vary. Some horses develop hepatitis after exposure to infectious agents or equine-origin biologic products, while others are affected by toxic plants, metabolic stress, biliary problems, or less common bacterial and parasitic disease.

Because the causes and outcomes differ so much, the most helpful next step is not guessing at home. Your vet can help determine whether your horse has mild, reversible inflammation or a more serious liver problem that needs close monitoring or hospital-level care.

Hepatitis in horses has several possible causes. Viral causes include equine parvovirus-hepatitis and equine hepacivirus. Equine parvovirus-hepatitis is strongly linked with Theiler disease, also called serum hepatitis, a severe form of acute liver disease that has historically been associated with equine-origin biologic products such as plasma or tetanus antitoxin given weeks to months earlier.

Toxins are another important category. Horses can develop liver inflammation after eating hepatotoxic plants, especially pyrrolizidine alkaloid-containing plants such as ragwort and groundsel. Blue-green algae and some other pasture or feed contaminants may also injure the liver. In some horses, the problem is not infectious or toxic at all, but related to biliary obstruction, gallstones, liver lobe torsion, or parasite migration through the liver.

Bacterial hepatitis is less common in adult horses, but it can occur. In foals, Tyzzer disease caused by Clostridium piliforme is a serious liver infection that can progress quickly. Other bacterial causes are uncommon but possible, especially when there is systemic illness or tissue damage.

In real life, the cause is not always obvious from symptoms alone. A horse with jaundice and poor appetite could have viral hepatitis, toxic liver injury, cholangiohepatitis, or another liver disorder entirely. That is why a careful history matters, including recent medications, biologic products, pasture access, weeds, supplements, travel, and herd exposures.

Diagnosis usually begins with a physical exam and bloodwork. Your vet will often run a complete blood count and serum chemistry panel to look for liver enzyme changes, bilirubin elevation, glucose abnormalities, and signs of inflammation. In horses, enzymes such as GGT, GLDH, and AST can help show whether liver cells or bile ducts are affected, though results need to be interpreted in context.

Additional testing may include serum bile acids, clotting assessment, and abdominal ultrasound. Ultrasound can help evaluate liver size and texture, look for abscesses or biliary dilation, and guide sample collection. If your horse has neurologic signs, your vet may also assess for hepatic encephalopathy and other causes of abnormal behavior.

A liver biopsy is often the most definitive test for characterizing the type and severity of liver disease. It can help distinguish active hepatitis from chronic scarring, toxic injury, cholangiohepatitis, or other patterns of disease. In some cases, your vet may also recommend PCR or other infectious disease testing when viral hepatitis is suspected.

Because horses can have abnormal liver enzymes without full liver failure, diagnosis is about more than one lab value. Your vet will combine the exam, history, bloodwork trends, imaging, and sometimes biopsy findings to decide how serious the problem is and what level of care makes sense.

Not every case of hepatitis can be prevented, but risk can often be lowered. Good pasture and hay management matter. Walk fields regularly, remove toxic weeds when possible, and be cautious with new hay sources if plant contamination is a concern. Store feed properly to reduce mold and contamination, and avoid offering supplements or herbal products unless your vet feels they are appropriate.

Review your horse's medical history with your vet before giving equine-origin biologic products such as plasma or tetanus antitoxin. These products can be important in specific situations, but they should be used thoughtfully because of the historic association between serum products and Theiler disease. If your horse receives one, keep records of the date and product used.

Routine wellness care also helps. Periodic exams and bloodwork can catch liver abnormalities before severe signs appear, especially in horses with weight loss, poor performance, or repeated exposure risks. If one horse on the property develops liver disease, your vet may recommend evaluating pasture, feed, and possibly other horses depending on the suspected cause.

Prevention is really about reducing avoidable liver stress and acting early. If your horse develops jaundice, photosensitivity, appetite changes, or odd behavior, prompt veterinary attention gives you the best chance to identify the problem before it becomes more serious.