A group of researchers looked at factors in the stallion that may contribute to the likeliness of the EAV persisting in that stallion, leading to the possibility of that stallion shedding the virus in his semen. Studies in their lab showed that horses can be divided into two distinct groups based on whether they were found to have a certain population of lymphocytes (CD3 T lymphocytes-a type of white blood cell) that are susceptible to infection by EAV in the lab (in vitro). It was also shown that these same stallions with this type of lymphocyte were also at a higher risk of becoming persistent carriers compared to those stallions that lack this particular cell type.
When a genome study was done, it was shown that this phenotype (those horses having this susceptible lymphocyte) is associated with a particular gene (CXCL16) on chromosome 11 that produces a protein that acts as a receptor for the equine arteritis virus (the virus will bind to that protein). The researchers were also able to analyze tissues from the EAV carrier stallion and determined definitively that the vas deferens (a particular portion of the stallion's reproductive tract) is the primary site of EVA persistence and that the infection is localized exclusively in the reproductive tract of the stallion.
EVA is of current concern not just to Friesian owners and breeders but to anyone who is involved in the equine breeding business. Equine viral arteritis begins as a respiratory infection in the horse for which symptoms can be so subtle as not to be noticed by the owner. One of the unfortunate complications for a mature stallion is the possibility that the infection may persist and result in the shedding of the virus in the semen of the stallion.
This bit of research provides us with some additional insight not only with respect to the individual susceptibility of a particular stallion but also with the concern over viral transmission by the carrier stallion. Because the virus was found only in the reproductive tract of the carrier stallion, these stallions are not a risk for transmission when participating in public events in which there is NO SEXUAL ACTIVITY by the particular stallion.
Using the results of research such as this may help with early recognition of susceptible horses before they become infected and the ability to take proactive management steps with at-risk horses to avoid this complication altogether. It may also allow us to better understand when and how we should be concerned when it is determined that a stallion has the persistence of EAV after a natural infection.
Balasuriya U, et. al. Host Factors that Contribute to Equine Arteritis Virus Persistence in the Stallion: an Update. J of Eq Vet Sci. Aug 2016, vol. 33 Supplement, pp. S11-S17. on line at DOI: http://dx.doi.org/10.1016/j.jevs2016.05.017