Cause of Equine Hepatitis Found

Theiler’s disease is a type of equine hepatitis that is associated with the administration of equine blood products. It has been known for almost 100 years, but its cause was never identified.

Researchers from the Novartis Institutes for BioMedical Research in California and the College of Veterinary Medicine at Cornell University in New York have collaborated in finding the virus that is believed to cause Theiler’s disease. Using genetic sequencing, they identified this previously undescribed Flaviviridae virus that shares only 35% amino acid identity with other viruses to which it is related. The newly discovered virus is being called TDAV for “Theiler’s disease-associated virus.”

Theiler’s disease was known to develop in some horses after they were treated with hyperimmune equine plasma or serum after being exposed to a contagious infectious agent. Affected horses showed signs including lethargy, refusal of feed, and jaundice a few weeks after treatment. Several months later, they were found to have elevated serum levels of liver enzymes and bilirubin as well as fever and central nervous system disorders in some horses. These signs were found in up to 18% of horses treated with hyperimmune equine plasma or serum. Among horses with signs of disease, mortality rates were between 50 and 90%. Necropsy results showed significant degeneration and tissue death affecting the liver.

Veterinarians had suspected a relationship between Theiler’s disease and horses treated with hyperimmune equine plasma or serum, but earlier studies had not been able to identify the exact cause of illness. Though TDAV is thought to play a role in the disease, more research is needed to find out the origin of the virus and why it affects some horses and not others.