EQUINE | Equine Disease Update
EQUINE | Equine Disease Update
The Asymptomatic Carrier Stallion : Critical Role in Venereal Disease Transmission
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 T . 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 / 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 USA , 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 na . ve pregnant mares , and establishment of the carrier state in a variable number of exposed stallions . It must be emphasized that stallions that continue to harbor equine arteritis virus , equine herpesvirus 3 or T . equigenitalis are asymptomatic or clinically inapparent carriers . With the exception of infection with T . equiperdum , there is no means of knowing whether a stallion is a carrier of a particular venereal pathogen or not without subjecting it to appropriate testing protocols for whatever the agent under consideration might be .
Regardless of what venereal infection is being screened for , however , it is critically important that such testing is carried out by a reputable veterinary diagnostic laboratory with an established record of competency and experience in testing for that infection . The reliability of laboratory testing is crucially important to the success of any prevention and control program especially in the case of equine viral arteritis and contagious equine metritis . A further confounding factor when dealing with stallions that are asymptomatic carriers of equine arteritis virus or T . equigenitalis is the fact that the majority of na . ve mares to which they are bred may subsequently exhibit minimal , if any , clinical evidence of infection . This leaves the breeder / mare owner unaware that transmission of infection has occurred and that the stallion in question is a carrier of either of these two venereal pathogens . This could have significant consequences in the case of equine arteritis virus should such an acutely infected mare be pastured with na . ve pregnant mares .
The 2008-2010 CEM event in the USA illustrated how easily a stallion that was a carrier of T . equigenitalis could escape detection at time of importation and ultimately be responsible for the very costly event that first came to light in 2008 . Past and recent experiences underscore the importance of screening breeding stallions regardless of breed , for presence of the carrier
22 • Equine Health Update •