Neurological disease associated with equine Herpes virus-1, EHV-1 (Rhinopneumonitis) infection has been recognized as a sporadic occurrence for a long time. However, the emergence of a new neuropathogenic strain in recent years has really caught the attention of researchers. This strain may have no respiratory symptoms, is highly infectious and causes severe, even fatal, neurological disease.
A study from the Maxwell H. Gluck Equine Research Center in Kentucky, published in the American Journal of Veterinary Research, gives some very important insight. Test horses were inoculated with either the regular EHV-1 strain or the new neuropathogenic one. Researchers looked at factors such as age, breed, circulating antibody titers, level of virus infected circulating white blood cells and the level of circulating cytotoxic T lymphocytes specific for EHV-1 virus. They found that the horses with the highest levels of the EHV-1 specific T cells were the least likely to have high circulating levels of virus and be able to protect themselves from developing the neurological disease.
Cytotoxic T lymphocytes are part of the sophisticated, ”cell mediated” arm of the immune system, which destroys body cells that have been invaded by a virus. Knowing how critical their role is in preventing this disease will hopefully direct vaccine production toward this type of response.
Most rhinopneumonitis vaccines are killed vaccines, which trigger little, if any, cell-mediated response. This may in part explain why even heavy vaccination schedules with these vaccines fail to protect from the neuropathogenic strain. The ability of live viruses to stimulate cell-mediated immunity might explain the preliminary findings that Pfizer’s Rhinomune, an attenuated live-virus vaccine, provided better protection.
The findings might also explain why horses on frequent doses (every one to three months) of the killed vaccines seem to be at high risk. If high circulating antibodies are blocking the ability of the EHV-1 respiratory strain to enter cells and trigger the T-cell response, they may have good protection from respiratory disease but not from the neuropathogenic strain.
Avoid frequent doses of the killed vaccine and use the modified live Rhinomune, available at the moment, for the best protection against Herpes neurological disease.