Scientific detective work has linked two viruses to hepatitis in horses, but much remains to be learned about the disease.
Theiler’s Disease, or equine serum hepatitis, is an infrequent but sometimes life-threatening liver disease in adult horses.
Sir Arnold Theiler first described it in the early 1900s as a form of highly fatal acute liver failure that occurred 4 to 12 weeks after horses were given an equine antiserum used as part of a vaccination strategy for African Horse Sickness.
Since then, many additional cases of serum hepatitis have been reported, and a variety of blood products of equine origin have been associated with the disease.
Researchers Thomas Divers, Joy Tomlinson and Gerlinde Van de Walle, from the College of Veterinary Medicine at New York’s Cornell University, discussed serum hepatitis in horses in the latest issue of Equine Disease Quarterly.
In North America, most recent cases have been associated with the administration of tetanus antitoxin, although commercial plasma products also have been incriminated, they report.
The number of horses that become ill after receiving a specific lot of incriminated blood product is estimated at 1 to 2 percent, although more horses might have sub-clinical disease.
“An infectious agent, such as a virus, has long been suspected to cause this condition, and a new equine parvovirus was recently discovered in a horse with a fatal case of serum hepatitis,” they wrote.
“The parvovirus was present in the diseased horse and in the biologic product that it had received nine weeks earlier. Inoculation of experimental horses with the commercial product resulted in transmission of this newly discovered virus and in liver disease.”
Variations in individual immune responses to the virus could explain the low percentage of clinically affected horses.
An identical disease, both in terms of clinical signs and pathologic findings, is sporadically seen in adult horses without recent administration of an equine-sourced biological product. These cases tend to mostly occur between June and November and can occur in small outbreaks that span a few weeks.
Such cases seem to occur most commonly on broodmare farms.
Their seasonal incidence suggests the possibility of insect transmission of a parvovirus, they suggest.
Clinically affected horses with Theiler’s disease frequently have both neurologic signs (hepatic encephalopathy: head pressing, stumbling, blindness) and jaundice (yellow gums and eyes).
Once the neurologic signs are noted, there is a rapid progression to death in around 70 percent of cases.
Horses that receive supportive therapy and survive for five days after the sudden onset of disease generally recover and have no long-term effects.
Another recently discovered virus that causes liver disease in horses is non-primate hepacivirus (NPHV).
NPHV is the most closely related virus to the human hepatitis C virus (HCV) discovered to date.
Experimental horse infections consistently result in evidence of hepatitis, but the disease is mild and clinical signs are either absent or very mild in recently infected horses.
NPHV and equine parvovirus are present in some healthy horses, indicating that horses can become healthy carriers of these viruses.
The US Department of Agriculture’s Center for Veterinary Biologics has issued a notice that all licensed equine blood products have to test free of equine parvovirus and NPHV.
“This should improve horse health by eliminating most of the blood product-associated cases of hepatitis,” the researchers suggest.
Cases that have no link to equine-sourced biological products will likely continue to occur until the natural means of virus transmission are determined and necessary control methods implemented, they added.
Equine Disease Quarterly is funded by Underwriters at Lloyd’s, London.