The equine placenta consists of the allantochorion, the allantoamnion, and the umbilical cord. The chorionic portion of the allantochorion is attached to the endometrium by microcotyledons that are present throughout the uterus, with the exception of a small area at the internal os of the cervix called the cervical star. The allantochorion supports the fetus in utero. This structure provides respiratory and nutrient exchange between the mare and the fetus and is an endocrine organ for maintenance and normal development of the fetus. The “free floating” allantoamnion allows the fetus to move freely within the uterus. The only attachment between the fetus and the allantoamnion is at the umbilicus. The umbilicus contains two umbilical arteries, one umbilical vein, and the urachus. The length of the cord and the length of the allantoic and amniotic portions can vary, but is normally 50 to 100 cm long.
Pregnancy loss during late gestation can be the result of fetal illness, placental dysfunction, maternal illness, or a combination of these factors. A functional placenta is necessary for a normal development of the fetus. Any insult or disruption of normal anatomy or physiology of the placenta may result in placental insufficiency and abortion. Compromised placental anatomy or function is the most common cause of abortions in late gestational mares. Placental insufficiency may be noninfectious (e.g., twin pregnancy) or infectious. Effective management of twin pregnancies has reduced this cause of abortion, and placentitis has become one of the most common cause of abortion in late gestational mares.
The most common route of infection is believed to be ascension through the cervix. An ascending infection may be the result of a failure of the external genital barriers to protect the uterus from bacterial or fungal invasion (e.g., defective perineal conformation, nonfunctional vestibulovaginal fold, or cervical lacerations). The possibility of bacterial contamination entering the uterus at the time of breeding or the presence of a preexisting low-grade endometritis with clinical signs that develop several months later has not been critically investigated. The characteristic location of the lesions away from the cervix in mares with placentitis caused by a Nocardioform actinomycete raises the question of whether the microorganism may enter the uterus before, or at the time of breeding, without causing a clinical problem until later during the pregnancy. Hematogenously spread placentitis occurs but is considered to be less common than an ascending route of infection.
The most commonly isolated microorganisms from mares with placentitis are Streptococcus zooepidemicus, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Nocardioform actinomycete, Aspergillus spp., and Candida organisms. The mechanism of abortion as a result of placentitis is not fully understood, but it most likely involves infection of the fetus, hormonal changes, the release of inflammatory mediators, and deprivation of the fetus of nutrients.
Placentitis: Treatment And Prevention
Treatment of mares with placentitis should focus on elimination of the infectious agents, reduction of the inflammatory response, and reduction of the increased myometrial contractility in response to the ongoing inflammation. No controlled studies have been reported on the efficacy of treatments for mares with placentitis, and the following recommendations are based on clinical experience and extrapolation from other species.
Urine pooling, cervical lesions, and poor perineal conformation should be corrected to prevent an ascending route of infection during pregnancy. Mares with abnormal placental findings on ultrasonographic examination or clinical signs of placentitis should be treated with broad-spectrum antibiotics, antiinflammatories (flunixin meglumine, 1.1 mg/kg ql2h; or phenylbutazone, 4 mg/kg ql2h), and tocolytics (altrenogest, 0.088 mg/kg q24h; or clenbuterol, 0.8 μg/kg ql2h). Pentoxifylline (7.5 mg/kg PO ql2h) is thought to increase oxygenation of the placenta through an increased deformability of red blood cells. A bacterial culture should be obtained in mares with vaginal discharge for isolation of a causative agent and sensitivity to antibiotics. After foaling or abortion, the uterus of the mare should be cultured and the mare should be treated for endometritis if the culture is positive.
Mares have been reported to deliver normally developed foals several weeks or even months after successful treatment of placentitis. No current diagnostic method exists, however, to predict how the compromised uterine environment in a mare with placentitis will affect the development of her fetus in individual cases.