Equine Viral Hepatitis in Horses: Theiler Disease and Serum Hepatitis
- See your vet immediately if your horse has jaundice, sudden depression, behavior changes, head pressing, stumbling, or stops eating. These can be signs of acute liver failure.
- Theiler disease, also called equine serum hepatitis, is now strongly linked to equine parvovirus-hepatitis (EqPV-H). Cases often appear 4 to 13 weeks after equine-origin biologic products such as plasma or antitoxin, but some horses get infected without any known biologic exposure.
- Many infected horses never look sick. Others develop severe hepatitis with hepatic encephalopathy, and reported mortality is high once clinical liver failure develops.
- There is no specific antiviral treatment. Care focuses on supportive treatment, monitoring liver values, managing neurologic signs, and reducing stress while your vet tracks whether the liver is recovering.
What Is Equine Viral Hepatitis in Horses?
Equine viral hepatitis is inflammation and injury of the liver caused by certain viruses that infect horses. In everyday equine practice, the best-known form is Theiler disease, also called equine serum hepatitis. This condition can range from mild, temporary liver enzyme changes to sudden, life-threatening liver failure.
Current evidence strongly links most classic Theiler disease cases to equine parvovirus-hepatitis (EqPV-H). Another virus, equine hepacivirus (EqHV), can also infect the liver, but it is more often associated with mild or chronic hepatitis than with the dramatic acute liver failure seen in classic serum hepatitis. That means a horse may test positive for a liver virus without having the most severe form of disease.
One challenging part for pet parents is that many horses with EqPV-H never develop obvious illness. Others become critically ill very quickly once enough liver tissue is damaged. Because the liver helps process toxins, make proteins, and support normal brain function, severe hepatitis can lead to hepatic encephalopathy with behavior changes, weakness, and neurologic signs.
This is why equine viral hepatitis is treated as an emergency when clinical signs appear. Early bloodwork, close monitoring, and a realistic discussion with your vet about treatment options can make a meaningful difference.
Symptoms of Equine Viral Hepatitis in Horses
- Loss of appetite or suddenly going off feed
- Lethargy, depression, or unusual quietness
- Jaundice or yellow discoloration of the gums, eyes, or skin
- Behavior changes, aimless wandering, or seeming mentally dull
- Ataxia, weakness, stumbling, or poor coordination
- Head pressing, apparent blindness, aggression, or other neurologic signs
- Dark urine
- Fever
- Photosensitivity or sun-sensitive skin lesions
- Ventral edema or swelling under the belly
Some horses have only vague early signs, like poor appetite and low energy. Others deteriorate fast and show jaundice or neurologic changes once liver function drops. That is the point where the condition becomes especially dangerous.
See your vet immediately if your horse has yellow gums or eyes, sudden behavior changes, staggering, head pressing, apparent blindness, or marked weakness. These signs can mean acute liver failure and hepatic encephalopathy, which need urgent veterinary care.
What Causes Equine Viral Hepatitis in Horses?
The classic cause of Theiler disease is now considered equine parvovirus-hepatitis (EqPV-H). Historically, the disease was noticed after horses received equine-origin biologic products, which is why it became known as serum hepatitis. Reported products associated with transmission have included tetanus antitoxin, botulinum antitoxin, Streptococcus equi antiserum, equine plasma products, pregnant mare serum, and some allogeneic stem cell preparations.
When infection follows a biologic product, signs of liver disease usually appear about 4 to 13 weeks later. That delay can make the connection easy to miss. Your vet may ask about any plasma, antitoxin, serum, or other equine-derived product your horse received in the past two to three months.
Not every case is tied to a biologic product. Horses can also develop EqPV-H infection without known serum exposure. Herd cases have been reported, and ongoing research suggests non-biologic spread may occur, with late summer and early fall patterns raising concern for possible insect involvement. Needle sharing between horses is also considered a risk.
A second liver virus, equine hepacivirus (EqHV), is now recognized as a cause of hepatitis in horses as well. EqHV appears much more common than severe Theiler disease and often causes no obvious illness, mild enzyme elevations, or more chronic liver inflammation rather than sudden fulminant liver failure.
How Is Equine Viral Hepatitis in Horses Diagnosed?
Diagnosis starts with an urgent exam and bloodwork. Your vet will usually run a chemistry panel to look at liver enzymes such as GGT, AST, SDH, and GLDH, along with bilirubin, bile acids, glucose, electrolytes, and sometimes ammonia. Horses with clinical hepatitis often have marked liver enzyme elevations and evidence that the liver is no longer functioning normally.
History matters. Your vet will want to know whether your horse received plasma, tetanus antitoxin, botulinum antitoxin, or another equine-origin biologic product in the previous 4 to 13 weeks. They will also consider other causes of acute liver disease, including plant or drug toxicosis, cholangiohepatitis, mycotoxins, and other systemic illness.
Further testing may include abdominal ultrasound to assess liver size and appearance, plus PCR testing on serum for EqPV-H and sometimes EqHV. A positive viral PCR can support the diagnosis, but it does not always prove the virus is the cause of the current illness. Serial testing and interpretation alongside liver values are often needed.
A liver biopsy may be recommended when the horse is stable enough, especially if the diagnosis is unclear or your vet needs to distinguish viral hepatitis from toxic, inflammatory, or chronic liver disease. In the sickest horses, biopsy may not be safe right away, so your vet may prioritize stabilization first.
Treatment Options for Equine Viral Hepatitis 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
- Baseline bloodwork with liver enzymes and bilirubin
- Targeted supportive medications chosen by your vet
- Strict rest, low-stress handling, and close at-home monitoring
- Repeat bloodwork over days to weeks if the horse remains stable
Recommended Standard Treatment
- Full exam and serial chemistry panels
- Abdominal ultrasound
- EqPV-H PCR and other targeted infectious or toxicology testing as indicated
- IV or enteral fluids, electrolyte support, and nutritional support
- Medications to manage hepatic encephalopathy, inflammation, and GI protection as directed by your vet
- Short hospitalization or intensive outpatient rechecks
Advanced / Critical Care
- Referral hospital admission with 24-hour monitoring
- Aggressive IV fluid therapy and correction of glucose or electrolyte abnormalities
- Frequent blood gas and chemistry monitoring
- Management of severe hepatic encephalopathy and recumbency risk
- Liver biopsy when safe and clinically useful
- Isolation or biosecurity precautions and intensive nursing care
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Equine Viral Hepatitis in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Do my horse’s bloodwork changes fit acute liver failure, mild hepatitis, or something in between?
- Should we test for EqPV-H, EqHV, toxins, or other liver diseases at the same time?
- Has my horse received any plasma, antitoxin, or other equine biologic in the last 4 to 13 weeks that could be relevant?
- Does my horse need hospitalization now, or is careful at-home monitoring still reasonable?
- What neurologic signs would mean I should call immediately or transport my horse right away?
- Would ultrasound or liver biopsy change treatment decisions in this case?
- How often should we repeat liver values, bile acids, or ammonia to track recovery?
- Should other horses on the property be monitored or tested, especially if they shared needles or received the same biologic product?
How to Prevent Equine Viral Hepatitis in Horses
There is no vaccine currently available for EqPV-H. Prevention focuses on reducing exposure risk and catching problems early. If your horse needs plasma, antitoxin, or another equine-origin biologic product, ask your vet whether the product source has been PCR tested for EqPV-H or otherwise screened according to current practice guidance.
Good injection hygiene matters. Do not share needles or syringes between horses. If a horse on the property is diagnosed with Theiler disease or another form of viral hepatitis, your vet may recommend monitoring stablemates with exams and bloodwork, because some infected horses stay outwardly normal while still showing liver enzyme changes.
Because non-biologic spread is still being studied, practical biosecurity is reasonable even when the exact route is unclear. That includes careful handling of blood-contaminated equipment, reducing unnecessary injections, and keeping records of any biologic products each horse receives.
If your horse received an equine biologic product, stay alert for appetite changes, jaundice, or behavior changes over the next 4 to 13 weeks. Prompt testing during that window can help your vet identify liver injury before the horse becomes critically ill.