Because Equine viral arteritis (EVA) has become an issue for horse breeders in Oklahoma, we have been told to vaccinate our stallion as a preventive measure. We have also been told to quarantine him for three weeks after he is vaccinated. All of our stalls are in the same building. Is it okay to leave one stall vacant next to his? Could he be in a pasture during the day and will this pasture be contagious to other horses in the future?

Some people may believe that the reason to isolate a vaccinated horse is to prevent disease in other horses or abortion in pregnant mares during the time when the vaccinated horse is shedding vaccine virus. This is not the reason. The vaccine virus is a modified live virus with a very low risk of transmission unless the animals are closely co-mingled, and even then, it is not 100 percent.

The primary reason is to prevent antibody production to the vaccine virus in other horses before the owner can document their sero-negative status prior to future vaccination. And that sero-conversion is really only a concern in horses that may one day be exported or from which the owner may one day export semen or embryos.

Yes, a vaccinated stallion could be in a pasture during the day, kept out of contact with other horses. The pasture will not remain contagious with virus sufficient to cause sero-conversion after the stallion is removed, especially if it was left unoccupied for a day.

Transmission of the vaccine virus requires direct, intimate association between animals or mechanical transmission with secretions from horse to horse by a handler or tack.

Common-sense biosecurity procedures are sufficient to eliminate spread of the vaccine virus from horse to horse. As long as the stallion was fed and handled last and there was no sharing of equipment, buckets, tack, etc., and handlers changed clothes and washed up after working with the stallion, there is no reason he couldn't be placed in a stall with empty stalls around him during the 21 days post-vaccination.

As far as being contagious to other horses, especially for pregnant mares, there have been hundreds of pregnant mares vaccinated prior to the last two months of pregnancy when their veterinarian felt there was a risk of exposure to the wild-type virus (i.e., in the face of an outbreak). No adverse effects have been reported.

There is always some concern when a pregnant animal is vaccinated with a modified live virus vaccine. In the face of an
EVA outbreak, the risk of disease is greater than the small risk of a vaccine induced problem. The risks should be discussed between the horse owner and the veterinarian before vaccinating a pregnant mare.

This column is provided by the faculty of the OSU Boren Veterinary Medical Teaching Hospital.

###