Q: My vet suggested that we vaccinate the show horses and the colts for equine Herpes virus infections, as he is concerned about the neurological form of the disease. I thought only broodmares needed to be vaccinated for this.
A: Many people share your interest in the protection provided by vaccination for equine Herpes virus infections, which can be highly contagious. Horses are most often affected by two types: Equine Herpes Virus 1 (EHV1) and Equine Herpes Virus 4 (EHV4).
EHV4 causes upper respiratory disease with cold-like signs in horses, frequently spreading through a stable or herd. In contrast, EHV1 infection is associated with broodmare abortions in the second half of pregnancy or the birth of weak foals that seldom survive without very intensive treatment.
In the past, most veterinarians routinely recommended vaccination against EHV1 to reduce the risk of herpes abortion in broodmares, and EHV4 vaccination to prevent respiratory disease. There is some cross protection between EHV1 and EHV4 vaccines, and stronger immunity may occur from the modified live vaccines (Rhinomune and Calvenza). None of the vaccines were believed to protect the horse from the neurologic signs associated with EHV1. However, a new study conducted at Cornell University has challenged this belief. Using the strain of EHV1 that caused an outbreak of neurologic disease in multiple horses in Ohio, the researcher was able to show that prior vaccination with the modified live product, Rhinomune, actually prevented development of the neurologic disease in the majority of the animals when ponies were experimentally exposed to this strain. This vaccine, although originally designed to protect horses from EHV4, also has been shown to offer protection from abortion induced by EHV1 when administered quarterly.
For owners of multiple broodmares, this alleviates the headache of timing vaccination specifically at 5, 7 and 9 months of gestation, as recommended for the killed EHV1 vaccine, Pneumabort K. The quarterly recommendation is made for high risk broodmares only. Most veterinarians vaccinate other horses annually or semi-annually. Until more data is available, it is premature to recommend that all horses get the extra vaccinations.
By Julie Wilson, DVM, University of Minnesota