Theiler Disease in Horses: Serum Hepatitis and Acute Liver Failure
- See your vet immediately if your horse has jaundice, sudden depression, poor appetite, head pressing, aimless wandering, or other neurologic signs.
- Theiler disease is an acute hepatitis syndrome in horses, often linked to equine-origin biologic products such as tetanus antitoxin, plasma, or serum given about 4-10 weeks earlier.
- Equine parvovirus-hepatitis is strongly associated with many cases, but some horses develop disease without a known biologic exposure.
- Diagnosis usually involves bloodwork showing severe liver injury, clotting assessment, and often liver ultrasound. Your vet may also discuss infectious disease testing and liver biopsy in selected cases.
- Treatment is supportive and can include hospitalization, IV fluids, nutritional support, lactulose, medications to reduce ammonia-related neurologic signs, and careful monitoring for bleeding.
- Prognosis is guarded to poor once clinical liver failure develops, especially if there is hepatic encephalopathy or hemorrhage. Early recognition improves the chance to stabilize the horse.
What Is Theiler Disease in Horses?
Theiler disease, also called serum hepatitis or post-serum hepatitis, is a severe form of acute liver injury in horses. It most often affects adult horses and can progress quickly from vague signs like lethargy and poor appetite to acute liver failure with jaundice, behavior changes, and neurologic signs.
Traditionally, this condition was recognized after a horse received an equine-origin biologic product such as tetanus antitoxin, equine plasma, or serum. Clinical signs often appear about 4 to 10 weeks later, although some reports describe a wider window. More recent research has linked many cases to equine parvovirus-hepatitis (EqPV-H), a virus that appears to play an important role in serum hepatitis.
Not every horse exposed to a biologic product becomes sick, and not every affected horse has a known history of serum or plasma administration. That can make this disease frustrating for pet parents and veterinarians alike. If one horse on a property is diagnosed, your vet may recommend monitoring other exposed horses for silent liver changes on bloodwork.
Symptoms of Theiler Disease in Horses
- Sudden lethargy or marked depression
- Poor appetite or complete anorexia
- Jaundice or yellow discoloration of the gums, eyes, or skin
- Neurologic signs such as aimless wandering, head pressing, ataxia, or behavior change
- Photosensitivity or sunburn-like skin irritation on unpigmented areas
- Colic-like discomfort
- Fever
- Bleeding tendency or bruising
Mild early signs can look nonspecific, but jaundice, worsening depression, stumbling, head pressing, or any sudden neurologic change should be treated as an emergency. Horses with acute liver failure can decline fast, and safety becomes a concern for both the horse and the people handling them. If your horse received tetanus antitoxin, plasma, or another equine blood product in the past several weeks, tell your vet right away.
What Causes Theiler Disease in Horses?
Theiler disease has long been associated with equine-origin biologic products, especially tetanus antitoxin, but also equine plasma and serum-based products. In classic cases, a horse appears normal after treatment and then develops signs of liver failure weeks later. Because of that delay, the connection is not always obvious unless your vet asks about recent injections, plasma transfusions, or serum products.
Current evidence strongly links many cases to equine parvovirus-hepatitis (EqPV-H). Cornell researchers identified this virus in association with equine serum hepatitis, and regulatory efforts have pushed for improved screening of equine blood products. Even so, not every infected horse becomes clinically ill. Many horses appear to have subclinical hepatitis, meaning bloodwork changes are present without obvious outward signs.
There are also cases that look like Theiler disease where no biologic exposure is known. Some may reflect natural viral transmission, while others may ultimately prove to be different causes of acute hepatitis. Your vet may also consider toxic plants, mycotoxins, cholangiohepatitis, hemolytic disease, and other liver disorders when working through the diagnosis.
How Is Theiler Disease in Horses Diagnosed?
Diagnosis starts with a careful history and physical exam. Your vet will want to know whether your horse received tetanus antitoxin, plasma, serum, stem-cell products, or other equine-origin biologics in the previous 4 to 12 weeks. Bloodwork is central. A complete blood count and chemistry panel often show marked liver abnormalities, including increased liver enzymes and bilirubin. Your vet may also run bile acids, ammonia, fibrinogen or serum amyloid A, and clotting tests to understand how severe the liver failure is and whether bleeding risk is present.
Ultrasound can help assess liver size and appearance, although some horses with Theiler disease may have a liver that is difficult to visualize. In selected cases, your vet may recommend EqPV-H testing, additional infectious disease testing, or a liver biopsy if the horse is stable enough and clotting status allows it. Biopsy is not always possible in a critically ill horse.
Because there is no single bedside test that confirms every case, diagnosis is often based on the pattern: acute liver failure, compatible history, and exclusion of other causes. Horses with neurologic signs need prompt stabilization while the diagnostic workup is underway.
Treatment Options for Theiler Disease in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Urgent farm call or clinic exam
- Basic bloodwork to confirm severe liver involvement
- Removal of unnecessary medications or supplements that could stress the liver
- Quiet stall rest with careful handling
- Diet changes your vet recommends, often with controlled protein intake if neurologic signs are present
- Targeted medications for hepatic encephalopathy when appropriate, such as lactulose
- Monitoring for worsening jaundice, depression, or neurologic signs
Recommended Standard Treatment
- Hospitalization for serial exams and repeat bloodwork
- IV fluids and electrolyte support as indicated
- Frequent monitoring of liver values, bilirubin, and coagulation status
- Medications to reduce ammonia-related neurologic effects, often including lactulose and sometimes antimicrobials selected by your vet
- Nutritional support and careful feed management
- Ultrasound and additional diagnostics to rule out other liver diseases
- Protection from injury if the horse becomes disoriented or ataxic
Advanced / Critical Care
- Referral hospital or ICU-level monitoring
- Aggressive management of hepatic encephalopathy and severe dehydration
- Expanded coagulation testing and blood product support when indicated
- Advanced imaging and specialized infectious disease testing
- Liver biopsy or additional procedures in carefully selected stable cases
- Continuous nursing care and safety management for recumbent or neurologic horses
- Consultation with internal medicine specialists
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Theiler Disease in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- You can ask your vet whether my horse’s recent tetanus antitoxin, plasma, or serum exposure could fit the timing for Theiler disease.
- You can ask your vet which bloodwork changes suggest acute liver failure versus another liver problem.
- You can ask your vet whether my horse is showing signs of hepatic encephalopathy and what safety steps we should take right now.
- You can ask your vet whether EqPV-H testing would change diagnosis, monitoring, or management in this case.
- You can ask your vet what the clotting results mean and whether there is a bleeding risk.
- You can ask your vet which feeding plan is safest while the liver is recovering.
- You can ask your vet what signs would mean my horse needs hospitalization or referral immediately.
- You can ask your vet whether other horses on the property should have screening bloodwork.
How to Prevent Theiler Disease in Horses
Prevention centers on careful use of equine-origin biologic products. Ask your vet whether a product is truly needed, what alternatives exist, and whether the source material has been screened for relevant infectious agents. This matters most for products such as tetanus antitoxin, plasma, serum, and some regenerative or blood-derived therapies.
Good vaccine planning can also help reduce the need for tetanus antitoxin in the first place. Horses that stay current on tetanus vaccination are less likely to need passive antibody products after a wound. Your vet can help you decide when vaccination, boosters, or antitoxin are appropriate based on your horse’s history and risk.
If one horse on the property develops Theiler disease, your vet may recommend monitoring stablemates, especially any that received the same biologic lot or had close contact with an affected horse. Follow-up bloodwork can sometimes identify subclinical hepatitis before severe signs appear. Prevention is not always possible, but thoughtful product selection, record-keeping, and early monitoring can lower risk.