Equine Liver Disease: More Common Than We Think?

Decades ago, practitioners recognized that the practice of administering an equine-origin biologic product at vulnerable times—to avoid tetanus infection, for instance—could put equine patients at risk for developing Theiler’s disease, or equine serum hepatitis, which can lead to liver failure. For years they suspected viruses were likely at play, but little more. In recent years new information has come to light that explains why Theiler’s disease and the liver viruses that cause it result in consequential health issues. Additionally, researchers have been trying to determine how liver viruses are naturally transmitted between horses, and manufacturers of biologic products are taking additional steps to prevent inadvertent introduction of certain liver viruses.

Identifying New Hepatic Viruses

Since 2012 researchers have identified five new liver viruses in equids:

  1. Nonprimate equine hepacivirus (EqHV, aka nonprimate hepacivirus, or NPHV)
  2. Equine pegivirus-1 (EpgV-1)
  3. Equine pegivirus-2 (EpgV-2, previously known as Theiler’s-disease-associated virus, or TDAV)
  4. Equine parvovirus-hepatitis (EqPV-H)
  5. Equine hepatitis B (EqHBV, so far only identified in donkeys and zebras)

Testing for Liver Viruses

Joy Tomlinson, DVM, Dipl. ACVIM, research associate at Cornell University’s College of Veterinary Medicine, in Ithaca, New York, investigates viral liver diseases. “We don’t recommend testing for liver viruses unless there is already a diagnosis of liver disease,” she says. “This might be based on veterinary examination of a horse for lethargy, inappetence, icterus (yellow whites of the eyes and gums), and/or abnormal mentation such as persistent yawning, playing in the water bucket, ataxia (incoordination), head-pressing, compulsive walking. Bloodwork done during such an exam, or for any other reason, may also reveal abnormal liver function values.”

She emphasizes that many horses can test positive on polymerase chain reaction (PCR) tests for a liver virus, yet the pathogen isn’t actively causing disease, and the horse doesn’t show clinical signs. For this reason infections can go undetected in apparently healthy horses, and they might even be missed in horses with fever of unknown origin; equine hepacivirus and equine parvovirus, for instance, do not cause a fever, she says, so veterinarians wouldn’t test for either virus.

“There aren’t yet clear published criteria for histopathologic indicators of viral liver disease,” she adds. “This means that pathology reports don’t always pick up on possible viral disease yet.”

Hepacivirus (EqHV)

The equine hepacivirus is the closest known virus to human hepatitis C. This hepatotropic (showing an affinity for the liver) infection causes disease with a high viral load in the liver and serum. Tomlinson says EqHV causes both acute and chronic disease, but an acute infection typically doesn’t cause overt signs. Equine hepacivirus is not associated with Theiler’s disease (described below).

After a horse is exposed and becomes infected, viremia (virus in the bloodstream) occurs quickly, with the horse seroconverting (developing antibodies in the blood to the virus) six to eight weeks later—this is the point when antibodies to the virus attempt to clear the disease. At that time, a reaction within the inner tissues of the liver (lymphocytic and plasmocytic reaction in the portal area) leads to cellular necrosis.

“Disease is probably caused when the immune system kills infected cells or when chronic inflammation from persistent infection causes damage and tissue remodeling,” says

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