Herpes Virus Vaccination in Horses

October 20, 2008 (published) | August 25, 2014 (revised)

One of the most serious infectious neurological diseases that affects horses is the neurological form of equine herpes virus. This virus caused all the problems at a big Quarter Horse event in Utah several years ago where several horses died. Equine herpes virus 1 causes this form of herpes, and there are vaccines available to protect against herpes virus. However, the vaccines are only labelled to protect horses against the respiratory form and the one that causes abortion; none are labelled for protection against the neurological form. In fact, the neurological form has occurred in horses that were well vaccinated. Some people that believe vaccination may actually increase the chance of horses developing the neurological form. So the ability of vaccines to protect against the neurological form is unknown.

Because of this lack of understanding, a study was performed at Oklahoma State to attempt to determine the ability of vaccine to be protective. Six aged mares were vaccinated three times with Pneumabort-K, which is a high antigen vaccine, and another six mares were used as controls and not vaccinated. There are several different vaccines available for herpes virus and some have higher antigen loads than others. After three vaccinations at monthly intervals, all horses were exposed to a neurological strain of the virus. Results showed five of the six unvaccinated horses had more severe disease versus only one of the six vaccinated horses having severe disease. In this study, horses vaccinated with a high antigen vaccine had less severe clinical signs than unvaccinated horses. Ask your veterinarian about vaccination against equine herpes virus.

VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email

Information and opinions expressed in letters to the editor are those of the author and are independent of the VIN News Service. Letters may be edited for style. We do not verify their content for accuracy.