A range of venereally transmissible agents—viral, bacterial, and protozoal—have long been known to establish persistence or the carrier state in stallions, mares, or both. Some of these agents (e.g. Pseudomonas aeruginosa, certain capsule types of Klebsiella pneumoniae, and Streptococcus zooepidemicus) are commonplace in most domesticated horse populations. Others such as equine herpesvirus-3, equine arteritis virus, Taylorella equigenitalis, or Taylorella asinigenitalis are less frequently encountered. Of additional significance is Trypanosoma equiperdum, the causal agent of dourine, which though rarely reported nowadays, is reputedly still present in certain regions and countries of the world.

Even though some, but not all, of the foregoing agents can establish persistent infection in both the stallion and the mare, it is the carrier stallion that plays a more important role in the epidemiology of the respective infections. Not only has it the potential to disseminate a particular infectious agent among the mares to which it is bred, but of even greater long-term significance, it ensures the transfer of infection from one breeding season to the next.

While some of these agents, such as equine arteritis virus and T. equigenitalis, can be transmitted either through natural service or artificial insemination, the risk of more widespread transmission is much greater through the practice of artificial insemination with fresh-chilled or frozen semen from a carrier stallion. This was borne out in the course of the 2006 equine viral arteritis disease event in the United States, when fresh-chilled semen from a well-known Quarter horse stallion in high commercial demand was responsible for spread of the virus to breeding stock in 18 states and two provinces in Canada, all within a two- to three-week period. This resulted in outbreaks of equine viral arteritis, abortion in naive