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By Karen Wolfsdorf, DVM, DACT

Placentitis is an inflammation of the placenta usually caused by an infectious agent. It has emerged as a leading cause of reproductive loss in the equine breeding industry, with considerable economic impact when a pregnancy ends in abortion or the birth of a small, ill-thrift foal. 

A mare’s placenta comprises the amnion, which surrounds the fetus, and the chorioallantois that attaches to the endometrium (uterine lining). These structures protect the fetus and provide gas and nutrient exchange, allowing the foal to grow. Placentitis usually affects the chorioallantois, compromising the attachment of the placenta to the endometrium or causing infection and inflammation, all of which can be harmful to the foal. 

Bacteria use one of three mechanisms to gain access to the placenta and potentially the fetus: 

Ascending infection

This occurs when the pathogen gets past the vulva and the vestibulovaginal sphincter (tissue separating the vestibule and vagina) and enters the cervix (it ascends through the reproductive tract). The bacteria then destroy the microvilli, which are tiny fingerlike projections, and this prevents gas and nutrient exchange in that area. Abortion occurs following fetal death from septicemia (infection of the blood), placental insufficiency (characterized by a lack of nutrients reaching the fetus and reduced gas exchange), or the inflammatory process and its resulting prostaglandin (a hormone) release causing the uterus to contract. 

Hematogenous infection

This occurs when a mare is systemically sick and the organism seeds within the blood supply of the uterus/placenta and fetus. 

Unidentified infection

The last cause has been attributed to a Gram-positive branching bacillus, and is described as a mucoid (resembling mucus) or nocardioform placentitis. Vets do not yet know how bacteria gain access to the uterus and placenta in these cases, but one theory is they enter the uterus when the mare is in estrus—possibly at the time of breeding. 

Clinical signs of placentitis include vaginal discharge and premature lactation. Mares with ascending infections might show one or both of these signs, whereas mares with hematogenous and mucoid infections usually only show premature lactation. A veterinarian can make a diagnosis using transrectal and/or transabdominal ultrasound in combination with a culture if vaginal discharge is present. The culture can be used to identify the infective organism and provide antibiotic sensitivity; this allows the veterinarian to select an antibiotic that will be optimal for treatment. Maternal progestogen hormone levels (indicative of pregnancy status) and total estrogen measurements can help the veterinarian determine that the placenta and fetus are intact and healthy. 

For a mare to maintain her pregnancy, her fetus needs to develop in a quiet environment free of infection and inflammation, with the placenta providing adequate blood flow for nutrition and gas exchange. Although bacterial infection initiates disease, researchers know (from their recent work using an experimental model of ascending placentitis in pony mares) that premature delivery might occur secondary to inflammation of the chorion (the outermost placental membrane) rather than as a consequence of fetal infection. With nocardioform placentitis, however, it appears that placental separation/disruption due to the mucoid exudate (pus) causes placental insufficiency, leading to fetal compromise. Therefore, veterinarians direct therapies at resolving infection and decreasing inflammation and uterine contractions that cause a mare to abort. Systemic treatment of the mare can include antibiotics, exogenous (from outside sources) progestogens, anti-inflammatories, tocolytic agents (which are designed to decrease uterine contraction), and medications that improve uterine perfusion (blood flow). 

In high-risk mares, such as those with a history of fetoplacental compromise, cervical incompetency/ lacerations, chronic disease, old age, or poor reproductive conformation, vets can detect placental and fetal problems early in pregnancy by performing routine placental ultrasonographic evaluations in conjunction with fetal viability assessments and serial hormonal evaluations. Detecting problems early in the course of infection can mean earlier treatment, resulting in the birth of a healthier foal. Therapeutic protocols, however, are still mostly derived from human literature, nonpregnant mare research, and clinical impressions, so their efficacy is varied, and further studies are needed. 

AAEP Forum article courtesy of The Horse magazine, an AAEP Media Partner.