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