CHAPTER 181 Placentitis
Placentitis remains an important yet problematic issue in the field of equine reproduction. In most clinical cases, the disease process is well advanced before clinical signs become apparent and a diagnosis can be made. In some cases no clinical signs are observed until the abortion or premature delivery occurs. One study attributed up to one third of abortions or premature deliveries to bacterial placentitis.
A variety of mechanisms have been postulated for equine placentitis. The most common is reported to be the ascending, transcervical form in which the etiologic agent appears to reach the placenta via the cervix and in which infection predominantly involves the cervical star portion of the chorioallantois. The hematogenous route seems less common but can occur when the mare has a systemic illness such as pneumonia, pleuropneumonia, colitis, or leptospirosis. A more randomly occurring type of placentitis is that seen with the nocardioform bacterial agent in which the area of involvement and placental separation involves the ventral aspect of the placenta near the junction of the uterine body and horns. The specific pathogenesis of this type remains unknown, but there has been some speculation that this is a slowly growing organism that may have entered the uterine lumen at the time of breeding.
The mechanisms of placentitis generally involve an inflammatory process, with the effects of inflammatory mediators resulting in premature delivery and placental insufficiency and, when the inflammatory state becomes chronic, growth retardation in the fetus. The time of onset of the inflammatory process is usually unknown, but it can have a significant influence on the outcome of the fetus. Earlier and more extensive involvement of the placental infection may result in more profound effects on the fetus. Even when relatively small areas of placenta are involved in the infection, the common clinical sign of premature mammary development may still be elicited, and the placenta should be examined following delivery to detect any such lesion and to characterize its location and severity as well as to collect samples for ancillary diagnostic testing by culture and histopathology. To date, results of measurements of hormone concentrations in mares with placentitis have been disappointing, with little useful influence on the medical treatment.