Equine hepatitis viruses are a group of viruses that can affect horses’ liver health. While some infections may go unnoticed, others can lead to significant health concerns. The two viruses currently identified as liver-specific pathogens are equine hepacivirus (EqHV) and equine parvovirus-hepatitis (EqPV-H). 

What Are EqHV and EqPV-H?

EqHV is a virus that primarily causes chronic hepatitis in horses. Most horses infected with EqHV clear the virus within 20 weeks without showing any clinical signs. However, about 20% of cases may develop chronic hepatitis. This can persist for more than six months. This chronic condition may present as subclinical or with signs of liver disease, such as jaundice, lethargy, or poor appetite. 

EqPV-H, on the other hand, is associated with acute hepatitis. This is a more sudden form of liver inflammation that can frequently present with severe clinical signs. This virus has been linked to Theiler’s Disease, a condition that can cause rapid liver failure in horses. 

EqHV is ubiquitous. Between 20% to 40% of horses have circulating antibodies against the virus without signs of liver disease. Only a small number of horses develop clinical disease. This suggests that exposure to the virus is frequent but in most cases the horse’s immune system can successfully combat the virus without causing visible signs of disease.

Similarly, approximately one-third of horses have been exposed to EqPV-H. While these infections often go unnoticed and do not cause clinical signs, a small percentage of cases can result in severe, potentially life-threatening liver disease. This can sometimes manifest in several horses simultaneously as an outbreak. 

Horses can contract these hepatitis viruses through contaminated biologics, such as plasma or stem cells. They also can be exposed through natural horizontal transmission. For example, the parvovirus can pass through nose-on-nose contact. Natural transmission routes still have to be explained further.

How Are These Viruses Diagnosed?

Diagnosing these hepatitis viruses involves a combination of blood testing and liver biopsies. For EqHV, veterinarians may use serial biochemistry and PCR tests to monitor viral load and liver enzyme activity.

Chronic cases are confirmed when viremia (presence of the virus in the blood) and hepatitis persist for more than six months. For EqPV-H, liver biopsies are commonly used to confirm the diagnosis. 

Managing Infected Horses

Managing horses infected with EqHV or EqPV-H requires supportive care and careful attention to biosecurity. 

For EqPV-H, infected horses should be isolated for four to eight weeks after the onset of hepatitis. Horses that test positive for the virus can be housed together, but they must be kept separate from uninfected horses until the viral load decreases sufficiently to make transmission unlikely. Regular screening of all horses in the herd is recommended to identify new infections early.

Typically, horses infected with EqHV do not require isolation due to the widespread distribution of the virus among non-clinically infected horses. However, monitoring the horse’s health and liver function is essential.

With both viruses, severely ill horses may need to be referred to a veterinary care facility for specialized treatment. Routine monitoring of serum biochemistry and signs of disease progression are important. 

Prevention Tips for Horse Owners

While no specific antiviral treatments or vaccines are currently available for these hepatitis viruses, prevention efforts emphasize the importance of quality control in equine biologic products.

It is crucial to use biologics that have been tested and confirmed as free from these viruses to minimize the risk of transmission.

Research and Future Directions

Research on equine hepatitis viruses is moving forward quickly to help us better understand how these viruses cause disease and to explore potential treatment options.

Another important research focus is investigating how environmental factors and potential reservoirs/carriers might spread these viruses. Although small amounts of viral genetic material have been found in stable flies (Stomoxys calcitrans), it is still unclear if this represents an infectious dose of virus or if stable flies can actually transmit the disease. Clarifying these transmission pathways could lead to improved prevention strategies. 

For prevention, early pilot studies on new vaccines for EqHV are already in progress, paving the way for more effective ways to protect horses in the future. 

Conclusion

Equine hepatitis viruses can pose a serious risk to a horse’s health. However, with proper management and preventive measures, the impact can be minimized.

Awareness of these viruses, their modes of transmission, and the importance of early diagnosis can help horse owners take proactive steps to protect their animals. If you suspect your horse may be affected, consult your veterinarian promptly for diagnosis and guidance.

(Editor’s note: For more information you may contact author Jessika-M. Cavalleri, Dr.med.vet., DECEIM, Head of the Clinical Department for Small Animals and Horses, University of Veterinary Medicine, Vienna. The Equine Disease Quarterly is a publication by the University of Kentucky Department of Veterinary Science, Maxwell H. Gluck Equine Research Center, and is funded by EQUUS/Standardbred Station, Inc. and M&J Insurance.)

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