Equine viral hepatitis review sharpens focus on two key pathogens
Bottom line
Viral hepatitis in horses is getting clearer, but not simpler. In a review article for Veterinary Clinics of North America: Equine Practice, Joy Tomlinson outlines how two viral causes of equine hepatitis identified since 2012 have reshaped the field: equine parvovirus-hepatitis, now tied to acute hepatic necrosis previously called Theiler’s disease, and equine hepacivirus, which is increasingly recognized as a cause of chronic fibrosing hepatitis. The review emphasizes that major gaps remain around transmission, risk factors for clinical disease, diagnosis, and treatment, even as the evidence linking these viruses to liver injury has strengthened. (us.elsevierhealth.com)
Why it matters: For equine veterinarians, the shift is practical as much as academic. AAEP’s updated viral hepatitis guidance says equine parvovirus-hepatitis and equine hepacivirus should now be part of the differential for horses with unexplained hepatitis, with diagnosis centered on excluding other causes and documenting infection through serum or liver testing. That matters because equine parvovirus-hepatitis has been linked to fulminant acute disease, while equine hepacivirus appears more often to cause subclinical or resolving hepatitis but can, in some horses, progress to severe chronic liver disease. Research and clinical programs at Cornell and Penn Vet also suggest the field is moving toward better case definition, fibrosis assessment, and risk stratification rather than a one-size-fits-all approach. (aaep.org)
What to watch: Expect more data on transmission routes, persistence, and risk factors for severe disease, especially for equine hepacivirus, as current research programs and case-enrollment efforts mature. (vet.cornell.edu)
Key facts
- Topic
- Equine viral hepatitis
- Review author
- Joy Tomlinson
- Journal
- Veterinary Clinics of North America: Equine Practice
- Main viruses
- Equine parvovirus-hepatitis and equine hepacivirus
- Equine parvovirus-hepatitis association
- Acute hepatic necrosis, including Theiler’s disease
- Equine hepacivirus association
- Chronic fibrosing hepatitis
- AAEP guidance
- Both viruses should be part of the differential for unexplained hepatitis
- Diagnosis approach
- Exclude other causes and document infection with serum or liver testing
- USDA APHIS action
- Commercially licensed equine plasma or serum biologic products should test negative for equine parvovirus-hepatitis
Equine viral hepatitis has moved from an obscure, poorly defined problem to a more actionable clinical category, but the science is still catching up to the cases veterinarians see in practice. In her review for Veterinary Clinics of North America: Equine Practice, Joy Tomlinson summarizes the field’s central update: since 2012, two viruses have emerged as the main recognized infectious causes of hepatitis in horses, equine parvovirus-hepatitis and equine hepacivirus. The first is now associated with acute hepatic necrosis, including cases historically labeled Theiler’s disease, while the second is increasingly linked to chronic fibrosing hepatitis. (us.elsevierhealth.com)
That change reflects more than improved naming. Earlier equine hepatitis cases were often grouped by syndrome, exposure history, or pathology, especially when horses developed acute liver failure after administration of equine-origin biologic products such as serum or plasma. Over the last decade, equine parvovirus-hepatitis has become the leading suspect in many of those cases, and USDA-related policy has evolved alongside the evidence. AAEP’s 2024 viral hepatitis guidance notes that USDA APHIS’ Center for Veterinary Biologics has taken steps so commercially licensed equine plasma or serum biologic products test negative for equine parvovirus-hepatitis, underscoring how research findings have already altered product safety expectations. (aaep.org)
The core clinical picture now looks more defined, even if it’s far from complete. Reviews of equine parvovirus-hepatitis describe it as a hepatotropic virus associated with acute hepatitis and fulminant hepatic necrosis, but also note infections in horses without a history of biologic product administration and in-contact horses, suggesting transmission is not limited to contaminated products. Experimental and observational work has also shown that not every infected horse develops the same severity of liver injury. Meanwhile, equine hepacivirus appears to be relatively common, often causing resolving subclinical hepatitis, yet Cornell investigators have highlighted concern that persistent infection can, in some horses, lead to severe chronic hepatitis and liver failure. (pmc.ncbi.nlm.nih.gov)
Recent research has added nuance on both prevalence and pathology. A 2025 Veterinary Microbiology paper identified epidemiology and risk factors for equine parvovirus-hepatitis, equine hepacivirus, and related pegiviruses in horses from the southern United States, while a 2024 report detected equine parvovirus-hepatitis virus and equine hepacivirus in archived sera from horses in France and Australia, reinforcing that these are not narrowly local findings. Another 2024 pathology paper linked naturally acquired equine parvovirus-hepatitis with a wide range of hepatic lesions, broadening the disease picture beyond classic fulminant presentations. (sciencedirect.com)
Industry guidance has started to catch up. AAEP published equine viral hepatitis guidelines in late 2024 covering transmission, risk factors, clinical signs, and diagnostics for both viruses. The guidance frames diagnosis as a process of ruling out other causes of hepatitis while documenting infection with serum or liver testing, which aligns closely with the review article’s message. That’s an important signal for practitioners: viral hepatitis is no longer a fringe consideration reserved for referral centers or dramatic post-biologic cases. It’s becoming part of routine workups for unexplained hepatic disease. (aaep.org)
Expert commentary from the field also points to where the unanswered questions are. Cornell’s funded EqHV research program explicitly targets transmission, clinical characterization of chronic hepatitis, and risk factors for persistence and severe disease, including host immune response and viral genetics. At Penn Vet, Tomlinson’s laboratory is enrolling horses with liver disease and focusing on biomarkers, fibrosis assessment, viral characterization, and clinical translation. Taken together, that suggests the next phase of work will be less about whether these viruses matter and more about which infected horses are most likely to become clinically ill, how to identify them earlier, and what management changes could reduce spread. (vet.cornell.edu)
Why it matters: For equine veterinarians, this is a reminder to widen the differential diagnosis for hepatitis and to think separately about acute and chronic viral syndromes. Equine parvovirus-hepatitis may explain some acute necrotizing cases that once seemed idiopathic or were loosely attributed to serum hepatitis, while equine hepacivirus may be easier to miss because many infections are subclinical before a subset progresses. For practices advising pet parents, trainers, breeding operations, or performance barns, the practical implications include more targeted testing, more careful review of exposure history, and closer attention to chronic liver enzyme elevations that might previously have been dismissed as nonspecific. (aaep.org)
What to watch: Watch for new data on transmission routes beyond biologic products, validated risk factors for persistent or severe equine hepacivirus disease, and any further regulatory or industry guidance tied to biologic product screening, diagnostic standards, or biosecurity recommendations. (vet.cornell.edu)