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Infection Can Prevent Pregnancy or Cause Fetal Loss in MaresBy Kentucky Equine Research Staff · June 30, 2015

Many horse owners have thought about breeding one of their mares, but this seemingly simple goal may not be as easy and problem-free as it first appears. The best chance for conception requires that a mare be healthy, well-nourished, and free of uterine infection or inflammation. Uterine inflammation can prevent pregnancy, and infection of the placenta later in the pregnancy can cause the mare to abort the fetus before it is fully developed.

Breeding, whether by natural cover or artificial insemination, produces a normal but short-lived uterine inflammation that resolves without treatment in a day or two. In some mares, there is persistent post-breeding inflammation that creates an environment in which the fertilized ovum is unable to survive. Other mares may develop an infection from bacteria introduced at breeding or entering with feces that are caught in an inward-tipped vulva. To treat endometritis, uterine lavage is used to flush bacteria from the uterus after which an antibiotic infusion is done to combat remaining bacteria. In mares with poor conformation of the perineal area, a veterinarian can perform a Caslick’s procedure to prevent recontamination. 

Placentitis is one of the most frequent causes of pregnancy loss in mares. This inflammatory condition is caused by environmental bacteria that reach the placenta and cause an infection. A common sign of placentitis is development of the mare’s mammary glands well before she is due to foal. A veterinary examination can detect a thickening of the uterus and placenta that is characteristic of the condition, and therefore an aid to diagnosis.

Oral or injected antiobiotics are used to treat placentitis, and mares may also receive anti-inflammatory medications as well as synthetic progesterone. Treatment of placentitis can be difficult because a number of different bacteria may be involved, and no antibiotic is completely effective against all strains. 

With all the possible problems in producing and maintaining a pregnancy, it’s a good idea to start by scheduling a breeding exam several months before a mare is due to be bred. This period of time allows diagnosis and treatment of any problems that might lead to an unsatisfactory result.