Two liver viruses can cause hepatitis in horses, and they’re not the ones scientists originally thought they were.

Acute hepatic necrosis—often called Theiler’s disease—results from an equine parvovirus-hepatitis (EqPV-H) infection, whereas chronic hepatitis can arise after infection with equine hepacivirus (EqHV), which is very similar to the virus that causes hepatitis C in humans, said Joy Tomlinson, DVM, Dipl. ACVIM, PhD, research associate and lecturer at the Cornell University College of Veterinary Medicine, in Ithaca, New York.

Both viruses can be transmitted through biological products coming from horses, including tetanus antitoxin, but they can also pass naturally from horse to horse, Tomlinson said during the 2021 American Association of Equine Practitioners’ (AAEP) convention, held Dec. 4-8 in Nashville, Tennessee. Her presentation earned the 2021 AAEP Research Award.

The two previously suspected culprits—pegivirus E and D (still referred to as Theiler’s-disease-associated virus in studies published more than a couple of years ago)—are not associated with disease, according to her recent research. “The important thing to remember about these two viruses is to forget them,” she said, adding that they have now being removed from hepatitis testing panels at Cornell.

“They still offer the (pegivirus) test, but don’t go looking for it, and don’t use it,” said Tomlinson.

A variety of conditions, including bacterial infections, poor feed quality, and toxicity, can cause acute and chronic hepatitis. But if veterinarians can’t find the cause of the horse’s liver disease, they can run diagnostic testing for parvovirus and/or hepacivirus.

Tomlinson’s series of studies over the past four years has revealed that parvovirus and hepacivirus are common in horses, even healthy ones. But the viruses can apparently cause disease in certain horses in certain situations—although the factors are currently unclear, she said. Disease can range from subclinical to fatal, she said.

“Horses are sometimes found dead in the field having been normal the last check, or they present often already with overt hepatic encephalopathy … demonstrated by head-pressing,” she said.

Elevated temperature is, surprisingly, not a clinical sign associated with these diseases, Tomlinson added. “Even though it is a viral disease, it is not associated with fever,” she said.

If veterinarians rule out all other more common hepatitis causes, they can run a series of polymerase chain reaction (PCR) tests on serum samples to check for the presence of the viruses, Tomlinson said.

A positive result does not mean the horse’s hepatitis is caused by that virus, however. Rather, the series of PCR tests should show telltale curves of changing parameters over time—with viremia decreasing gradually over several weeks for acute hepatitis (parvovirus infection) and persistently high liver enzymes, either stable or worsening, along with persistent infection over at least six months for chronic hepatitis (hepacivirus infection).

Other supportive diagnostics include histopathology to look for lymphocytic infiltrate and/or individual hepatocyte necrosis, with or without biliary ductular reaction, in parvovirus cases, she said. Histopathology in hepacivirus infections is more variable but usually shows mild to severe fibrosis, and liver ultrasound sometimes reveals a nodular pattern.

“The important thing here is that you do need to rule out other causes carefully,” Tomlinson explained. “If the horse has bacterial causes of hepatitis and is also hepacivirus-positive, and you don’t treat the bacteria, you’re not doing that horse any favors,” she said. “So you really can’t just rely on PCR to rule out other causes.”

Horses shed EqPV-H through their nose, mouth, and feces, before and after they develop hepatitis, she said. As for EqHV, shedding outlets remain unclear. In both cases, research on insect involvement virus spread is underway.

No treatment or prevention strategy currently is in place for these infections, she said. Horses can remain persistently infected for months or even years, well after their clinical signs have subsided. This results in high prevalence of infection in horses and makes biosecurity or isolation protocols largely impractical, she says.

Better screening of equine-derived biological agents can help prevent EqPV-H and EqHV spread, Tomlinson said. The USDA added a regulation that commercial equine biologic products must be tested negative for EqPV-H, and many companies are also testing for EqHV, though it isn’t required. Freezing the products is not an effective solution, she added. Both EqHV and EqPV-H survive the freezing-thawing process.