Many mares which cycle, but fail to conceive, have infections in their reproductive tracts. Hence, they are sometimes called “dirty” mares. The more scientific term for their problem is endometritis, which refers to the acute or chronic inflammatory process involving the endometrium (the inner lining of the uterus). Reduced fertility associated with endometritis—both acute and chronic—has been recognized for many years in broodmares. This subfertility is due to an unsuitable environment within the uterus for the developing conceptus, and in some cases the endometritis causes early regression of the corpus luteum (CL). These changes frequently occur as a result of microbial infection, but they also can be due to non-infectious causes. One of the main obstacles in producing the maximum number of live, healthy foals from mares bred during the previous season is the mare which is susceptible to persistent, acute endometritis following breeding.


The following is a classification system for equine endometritis:



  • Venereal infection;
  • Chronic infectious endometritis;
  • Endometrosis (chronic degenerative endometritis);
  • Persistent mating-induced endometritis (delay in uterine clearance).

The uterine lumen of the normal fertile mare is sterile despite the fact that the reproductive tract is contaminated with bacteria from the act of breeding, foaling, and veterinary procedures. Mares with defective vulval conformation can suck air and bacteria into the vagina, which can develop into endometritis.


The uterus responds to these bacteria with a rapid influx of neutrophils. Normally these neutrophils kill the bacteria rapidly (within 24 hours). These inflammatory by-products then are mechanically removed and the endometritis resolves itself (returns to normal). Failure to resolve this inflammation results in the “susceptible” mare. Susceptible mares have a delay in uterine clearance and the inflammatory by-products accumulate as uterine fluid. Such mares have a reduced pregnancy rate due to an unsuitable environment for the early developing conceptus

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