Researchers have confirmed the importance of including exposed mares when conducting surveillance for contagious equine metritis (CEM) in the recent South African outbreak, along with specific stallion sampling and screening methods in that country for the venereal disease.
Catherine May, BVSc, and colleagues at the Veterinary Faculty of the University of Pretoria, recently completed two studies in the wake of a CEM outbreak in South Africa. They determined the carrier status of Taylorella equigenitalis—CEM’s causative bacterium—among stallions and exposed mares and assessed the distribution of the agent on the stallions’ external genitalia. May presented the team’s results at the American College of Theriogenology conference, held Aug. 7-10 in Louisville, Ky.
Screening for CEM cases and preventing its spread are important because the disease can cause temporary infertility in mares, said May, resulting in breeding losses and negatively impacting the economy when CEM-free countries ban imports from countries that have reported an outbreak. Though exposure to CEM doesn’t impact a stallion pathologically, if his external genitalia are contaminated by T. equigenitalis, he can pass it to mares via natural cover breeding or through semen in assisted reproductive techniques. Mares that become infected, on the other hand, can show physical signs of infection, usually developing a vaginal discharge after breeding to a carrier stallion and/or returning to estrus quickly after breeding due to early embryonic death. Other mares might only be subclinically infected, meaning they show no signs of infection but have the ability to spread the bacterium.
The good news about CEM is that affected mares and stallions respond very well to treatment, although it may take more than one course of therapy to complete a full recovery; it’s just a matter of identifying active cases and, more importantly, subclinical carriers.
Veterinarians have logged CEM outbreaks most recently in the United Kingdom in 2012, the 2011 South Africa outbreak associated with this study, and the United States beginning in 2008. The disease is controlled and/or reportable in many countries, including South Africa, meaning that cases must be reported to an international agency called the OIE (World Organization for Animal Health), and animal health officials must get involved with quarantine of infected horses and elimination of the agent in these animals.
“The hallmark of the South Africa outbreak was that it was mainly transmitted horizontally via fomites,” said May, probably via the breeding phantom and/or artificial vagina. “Traditionally, the disease has been considered as mainly a venereal or sexually transmitted disease, but research into the South African outbreak showed that fomite transmission may be even more important in the spread of this disease. Due to the fact that most mares naturally rid themselves of the infection, and only a certain number of stallions become carriers, the actual extent of the outbreak will never be known.” After the outbreak, researchers instituted a nationwide surveillance program of all breeding stallions in South Africa and determined that 1.35% of males (36 of 2,659 stallions) and 4.41% (three of 68 exposed mares), were positive on quantitative polymerase chain reaction testing, qPCR, a test that detects bacterial DNA.
May noted the reason the prevalence in exposed mares appeared to be higher than in stallions was that researchers only checked mares known to be associated with positive stallions—a much smaller number than the number of stallions checked in total. “If the entire mare population in South Africa was checked, then this percentage would be much lower,” May said. “This percentage did, however, confirm the importance of checking exposed mares during an outbreak situation, as a very small percentage may also become chronic carriers.”
It’s important that veterinarians in South Africa find the optimal test method and swab sites for Taylorella equigenitalis. “South Africa is characterized by extensive distances, meaning that many of our practitioners are far away from approved testing centers, limited postal services, and periodic extremes of environmental temperatures,” May explained, noting that these factors complicate CEM diagnostics reliant on bacteriology. Bacteriology is the OIE-recognized gold standard for testing, but it can be very difficult to grow T. equigenitalis on culture; and when one adds the shipping challenges May described, a robust screening test such as qPCR, which doesn’t have such stringent transport requirements, emerges as very useful.
“PCR therefore provided a practical, reliable, and sensitive method that facilitated the ability to detect T. equigenitalis,” she said.
In regard to the optimal swab sites, “we hypothesized that there would be a single predominant predilection site for the organism in carrier stallions,” May said, noting that the researchers had assessed the frequency of the organism in the urethra (the membranous tube that transports urine from the bladder out of the body), the urethral fossa (circular depression at the end of the glans penis from which the urethral process protrudes), and the lamina interna (the mucosa that covers the penis, visible only when the penis is erect; in the relaxed state it telescopes inward from the preputial opening and lines the penis). In the 24 carrier stallions that were included in this study, the bacterial load in the urethral fossa was significantly higher than in the other two sites.
“The South African outbreak highlighted the importance of having a rapid, robust screening test for CEM in the face of an outbreak situation, in order to effectively identify and control this disease,” May summarized. “Research is still ongoing into the actual source of the outbreak in South Africa. Although CEM is a fully treatable disease, the cost associated with an outbreak situation, including international export and import controls, quarantine and movement restrictions, as well as treatment, mean that a reliable and sensitive test and correct sampling techniques are of the utmost importance in rapid identification of the disease and to prevent the spread in a horse population.”
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.