Published June, 2012

Breeders who utilize live cover can breathe a sigh of relief: While live cover breeding facilities sometimes face pathogenic bacteria outbreaks, results from a study led by University of Kentucky PhD student Katheryn Cerny, MS, demonstrated bacteria on stallions' external genitalia does not affect stallion or mare fertility.

The study's purpose, explained Cerny, was to investigate the occurrence of potentially pathogenic bacteria on stallions' external genitalia to determine if bacteria present on the stallion impacts pregnancy and pregnancy rates. She also studied the occurrence and type of bacteria in the mare's uterus after live cover breeding to stallions with or without positive bacterial cultures.

Two Central Kentucky Thoroughbred farms (15 stallions and 206 mares) participated in the study during the 2010 and 2011 breeding seasons. The research team collected samples for bacteriological evaluation from the stallions' prepuce and post-ejaculate urethra. In mares they took uterine swabs 18 to 36 hours after cover. They also tracked mares for pregnancy at Day 14 and pregnancy loss after Day 14.

Culture results showed 22.4% of stallions tested positive for potentially pathogenic bacteria, with Streptococcus zooepidemicus accounting for more than half (51.1%). In mares, 29.2% of uterine cultures tested positive, the vast majority also with S. zooepidemicus (90.9%). Cerny determined, however, that these positive bacterial culture results did not impact breeding. "This study found that there was no difference in pregnancy rates between mares bred to a stallion that had a culture positive for potentially pathogenic bacteria compared with mares bred to a stallion that had a negative culture," she explained. "Also, the bacteria that was cultured from the uterus after breeding was not likely to be the same bacteria that was found on a positive-culture stallion at the time of breeding."

These results call into question the practice of routine cultures of stallions and post-breeding treatment of mares, Cerny noted. "Future studies should be directed at investigating the benefits of post-breeding treatments administered to all mares," she said. "Identifying susceptible mares and treating accordingly should therefore be a priority. Additionally, there is a concern for the development of antibiotic resistance when these practices are implemented. Studies have found that some bacteria have increased resistance to one or more antibiotics. These findings have been population-dependent, but with the increased use of intrauterine antibiotic infusion in Central Kentucky, the risk of increasing antibiotic resistance is a concern and a call for future research."

Cerny completed her MS in equine reproduction in the spring of 2012 in the Gluck Equine Research Center's Department of Veterinary Science under the guidance of Ed Squires, PhD, Dipl. ACT (hon.), director of UK Ag Equine Programs and executive director of the UK Gluck Equine Research Foundation. She is currently working on a PhD in reproductive physiology in the Department of Animal Science with Phillip Bridges, PhD. She will present the abstract of this study, "Presence of Bacteria in the Reproductive Tract of Healthy Stallions and its Relation to the Fertility of Mares," at the August 2012 Society for Theriogenology conference in Baltimore, Md.

Natalie DeFee Mendik, Freelance Journalist