Placentitis is a term used to describe infection of the fetal membranes and is a fairly common cause of abortion in pregnant mares. Most cases of placentitis are caused by bacteria that have ascended from the mare's vulvar area up to and through the cervix. For this reason, susceptible mares should have a Caslick's surgery after breeding as this surgery partially closes the vulva and helps prevent fecal contamination that could lead to placentitis. To prevent abortion, early detection of the condition is critical and waiting until clinical signs of placentitis develop may be too late. Mares with placentitis may develop premature udder development or a vaginal discharge but by this time, treatment is more difficult. For this reason, mares with a history of pregnancy loss or uterine infections should be checked early for placentitis.

Placentitis can be detected by an ultrasound exam of the placenta and measuring the thickness of the placenta. Abnormal thickness compared to normal measurements has been shown to indicate placentitis. Also, sometimes your vet can see a separation of the placenta from the uterus and can even see infected material. Studies have shown that treatment with an oral antibacterial called trimethoprim sulfamethoxazole, plus a drug called pentoxifylline, plus a double dose of Regumate has been shown to be effective in preventing abortion in some cases. However, it is important to check mares early to be successful in this treatment so it is recommended to start checking mares by ultrasound for placentitis at 8 to 9 months of pregnancy.