One of the most serious infectious diseases we see in horses is the neurological form of the herpes virus. It can affect the respiratory tract and generally causes mild disease. But when it affects the neurological system it can be deadly.

Researchers at Michigan State found that young horses generally develop respiratory disease, while older horses usually develop the neurological form of the disease because of different immune responses.

At the American Association of Equine Practitioners (AAEP) convention, researchers said that 10% of horses develop the neurological form of the disease. It is unclear why some horses develop this form and not others.

Some predisposing factors in horses that develop the neurological form include increasing age, female sex, pregnancy, or nursing. Certain breeds seem more susceptible including standardbreds, warmbloods, thoroughbreds, quarter horses, paints, appaloosas, Spanish horses, fjords, draft horses, and the Lipizzaner.

According to the publication The Horse, the first infection begins in the respiratory tract, and the virus is picked up by the lymph nodes where it enters the bloodstream. This is when the neurological form occurs. The virus attacks the blood vessels resulting in vasculitis, in which white blood cells infiltrate the area, developing blood clots that cause tissue destruction.

In the study, the young horses that developed only the respiratory disease had two different temperature spikes while the horses that developed neurological disease only showed the secondary fever spike. Horses that developed either the respiratory form of the disease or the neurological form had classic responses consistent with the form of disease they had developed.

Scientists are looking to alter the immune response to decrease the neurological form of the disease.