New Type Of Placentitis In Mares

For several years a unique form of placentitis, referred to as “nocardioform” or “mucoid” placentitis, has been diagnosed at the University of Kentucky Livestock Disease Diagnostic Center. While the number of cases of mucoid placentitis has remained fairly constant over the preceding years, for unknown reasons during the current foaling season there has been a dramatic increase in the number of these cases.

This type of placentitis is unique due to the location of the infection, the type of material that accumulates on the placenta, and the bacteria that is usually involved. In a departure from other forms of placentitis, the infection is localized on the body of the placenta in the area of the junction of the body and horns. The affected area varies in size and appears to begin on the ventral aspect of the placenta, with extension cranially onto the horns, caudally on the body, and circumferentially around the placenta.

Typically there is a large accumulation of thick brown mucoid material on the surface of the placenta (chorionic surface). The underlying chorionic villi of the placenta are reduced in size and, in the central portion of the lesion, may be completely absent. Variable numbers of inflammatory cells invade the placental membrane. There is no communication of the placentitis with the cervical portion of the placenta and, therefore, it does not appear to be an ascending infection as usually is the case in mares with placentitis.

Associated with mucoid placentitis are gram-positive branching filamentous bacteria, which are often numerous in the exudate. These bacteria appear to be a heterogenous group, in that there are variations in morphology and in vitro growth characteristics between cases and even within the same case. At present these bacteria are unclassified. Invasion into the placental membranes by the bacteria and spread to the fetus are not observed; instead, the infection likely harms the fetus by causing placental insufficiency through disruption of the uterine-placental interface.

While most mares that have this type of placentitis are normal in appearance, some exhibit early development of the mammary gland and lactate prematurely. Ultrasound examination of the reproductive tract at that time may reveal areas of separation of the placenta from the uterus. Usually one of the following four outcomes of this type of placentitis are seen: 1) some mares abort the fetus in late gestation; 2) other mares produce a stillborn foal at term; 3) have an alive but small, weak, malnourished appearing foal; or 4) have a normal, healthy appearing foal. Mares that have had this form of placentitis typically do not have problems with subsequent conception and pregnancy and are not at an increased risk of redeveloping placentitis.

The commonly accepted mechanisms by which placentitis occurs include infection of the placenta by bacteria or fungi that gain access to the uterus by ascension through the lower reproductive tract or by gaining access by spread through the blood stream. The consistent location of the placentitis in mucoid placentitis and the lack of communication with the cervical area suggests that a different pathogenesis may be involved in this type of placentitis.

Studies are in progress to help elucidate when and how this form of placentitis occurs and, consequently, if intervention may be possible.

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