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Placentitis in Mares By Kentucky Equine Research Staff · April 7, 2014

The placenta allows the exchange of oxygen, nutrients, and waste products between a mare and her unborn foal. In a normal pregnancy, a plug of thick mucus seals the mare’s external cervix and keeps environmental bacteria and fungi from reaching the placenta. In some mares, microbes manage to bypass this barrier, setting up a placental infection that can interfere with proper functioning of the placenta. The result may be loss of the pregnancy or premature delivery resulting in small, weak foals that often die soon after birth.

Placentitis can affect any mare but is most common in older mares, those that are underweight, and individuals that have poorly conformed vulvas or have suffered injuries to the cervix in previous pregnancies. Signs of infection include vaginal discharge and mammary gland enlargement before the mare should be ready to foal.

Transrectal and transabdominal ultrasound scans are used to diagnose placentitis. Practitioners can look for thickened placental tissue, separation of the placenta, and changes to the fluids within the uterus. Ultrasound examination can also be used to evaluate fetal activity level and heart rate. Another diagnostic test is measurement of serum amyloid A protein concentrations, as the level of these inflammatory proteins commonly rises as a result of infection.

Many mares diagnosed with placentitis can successfully continue their pregnancies if the infection is diagnosed and treated early in the course of the disease. Specific antibiotics are selected depending on the cause of the infection. In one study conducted at the University of Florida, mares with placentitis that were treated with antibiotics and synthetic progestin to delay premature delivery had better outcomes (83% in one test group delivered viable foals) than mares that were treated only with antibiotics.

Trimethoprim sulfamethoxazole (antibiotic) and pentoxifylline (anti-inflammatory and helps placental blood flow) have been used with good results to combat some types of placentitis. A new form of cephalosporin, ceftiofur crystalline free acid, is active against streptococcal bacteria, and unlike many other antibiotics, it does not cross the placenta to enter fetal tissue.