Foal Heat and Contaminant Bacteria⁵

Although the foal heat in mares is fertile, it is associated with a high incidence of late embryonic mortality. This is attributed to the presence of microbial contaminants associated with incomplete uterine involution at the foal heat.

Eighty-nine pregnant mares were identified at the Tandil military equestrian facility and randomly divided into two groups. All mares were treated the day after foaling with either a single dose of intravenous SETTLE or placebo. Twenty-five mares were sampled for intrauterine bacteria and cytology at the time of treatment and seven days later (n[SETTLE]=13, n[placebo]=12). Mares were bred at the foal heat to stallions of known high fertility.

There was no difference in bacterial contaminants for mares in either group on the day after foaling, with
b-hemolytic streptococci and E. coli being the most common contaminants. Sixteen different organisms were cultured from 12/13 mares in the SETTLE group and 16 from 12/12 in the placebo group. Seven days later, only two mares in the SETTLE group had mild bacterial infections, whereas nine mares in the placebo group were still infected (p<0.001).

The overall reduction in total bacterial colony forming units (cfu) was 99% for SETTLE and 45% for placebo. Similarly, total PMN counts were reduced by 92% in SETTLE treated mares compared with 59% in the placebo treated animals. At day seven, 11/12 SETTLE mares had <5 PMN/400x whereas the placebo group had only 2/12 animals with <5 PMN/400x. Eleven of 12 sampled mares in the SETTLE group ovulated at the foal heat and were bred. Only 6/12 placebo-treated, sampled mares ovulated and were bred within three weeks of foaling.

Results for all foal heat breeding pregnancies were 28/43 (65%) for SETTLE treated mares and 9/38 (24%) for the placebo treated group (p<0.001).

With large numbers of mares and indifferent mare management, this facility has historically obtained poor pregnancy results (average pregnancies / cycle less than 25%) when compared to well-run broodmare farms. That being said, this facility offered a single site, with numerous endometritis susceptible mares available for study and the foal heat breeding results obtained were statistically significant (p<0.001).