Also see Abortion in Horses. Twin pregnancy is the most common noninfectious cause of abortion in mares. In most cases, uterine capacity and subsequent placentation are inadequate to support two fetuses to term. Although twins may spontaneously reduce to a viable singleton early in gestation (<60 days), visible abortions tend to be seen after 7–9 mo. Premonitory clinical signs of impending abortion may be only premature mammary gland development. Fetal membranes should always be examined for large avillous areas typically located between the twin conceptuses.

Because of the risk of abortion and dystocia in a mare carrying twins, if twins are detected by ultrasonography per rectum at <30 days, reduction to one embryo by manual crushing per rectum is recommended. Between 30–60 days of gestation, transvaginal aspiration of one conceptus may result in a viable singleton. At approximately 60 days of gestation, one fetus may be eliminated by cervical dislocation. After 110 days of gestation, one twin may be reduced by intracardiac injection of potassium chloride guided by transabdominal ultrasonography. Success rates vary depending on the procedure performed, the experience of the veterinarian, and the character of the pregnancy. Pregnancy should be monitored repeatedly by ultrasonography after twin reduction.

The most common cause of viral abortion in mares is equine rhinopneumonitis, which is caused by equine herpesvirus 1 (EHV-1). These abortions occur predominantly in the last trimester and usually are not associated with a respiratory infection. All pregnant mares should be vaccinated against EHV-1 at 5, 7, and 9 mo of gestation.

The equine arteritis virus can cause equine viral arteritis (EVA) abortion. An EVA-seronegative mare scheduled to be bred to an EVA-positive stallion that is shedding the arteritis virus in the semen should be vaccinated before breeding. Horses must be isolated for 3 wk after EVA vaccination. Mares should not be vaccinated against EVA during pregnancy.

Sporadic abortions due to placentitis can be caused by bacterial and mycotic infections of the placenta. These are predominantly ascending infections acquired through the caudal genital tract but may also be focal or diffuse. Bacteria involved include Streptococcus zooepidemicus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, Rhodococcus equi, and Actinobacillus equuli. Mycotic organisms include Mucor and Aspergillus spp. Efforts to reduce the incidence of these abortions should include good breeding hygiene, treatment of genital disease before breeding, maintaining good body condition throughout pregnancy, and vulvoplasty (Caslick operation) to prevent pneumovagina. Mares at risk of placentitis should be examined by ultrasonography per rectum periodically during late gestation for evidence of genital infection and placental abnormalities as evidenced by uterine discharge and increased uteroplacental thickness.

Any aborted fetus and fetal membranes should promptly be submitted fresh or cooled (never frozen) to a diagnostic laboratory to determine the cause of the abortion.