Each year throughout the United States, mares lose their pregnancies due to placentitis or an infection in the placenta. Placentitis causes lesions in the placenta, which provides nourishment from the mare to the fetus. When that nourishment is disrupted, the fetus might be compromised, or die.
Kentucky is about the only place where nocardioform placentitis is diagnosed as a cause of abortion each year, with some years having higher losses than others. Nocardioform placentitis is a distinct form of this disease characterized by the area that is damaged. Most placentitis cases are caused by an infection near the cervix. However, with nocardioform placentitis, the lesions are seen at the cranial or top of the placenta rather than at the cervix or bottom of the placenta. The lesions often extend onto the area where the uterus joins the uterine horns. The affected area usually is large, but isolated. The surface of the placenta is covered with a thick, brown exudate, which contains dead placental cells, white blood cells, and bacteria.
With regular placentitis, mares often will prematurely "bag up" and have a vaginal discharge; with nocardioform placentitis mares might bag up, but there usually is no vaginal discharge and therefore little outward sign that the mare has a problem.
At the University of Kentucky's Livestock Disease Diagnostic Center, James M. Donahue, PhD, has been isolating the bacteria that cause nocardioform placentitis for more than 15 years, and he has saved bacteria from about 250 abortions. He noted that on group of bacteria associated with cases of placentitis and abortion were recovered from in about two-thirds of placentitis abortions. He sent isolates of that bacteria to David P. Labeda, PhD, at the USDA's National Center for Agricultural Utilization Research in Illinois to undergo DNA relatedness studies. The species was found to be close to--but not exactly the same as--a bacterium named Crossiella cryophila
. Therefore, the species most associated with nocardioform placentitis in Kentucky has tentatively been named Crossiella equi
The name will not be officially accepted until the verification is published in the International Journal of Systematic and Evolutionary Microbiology, to which the paper outlining the identification process has been submitted.
Donahue said Labeda is working identifying and naming other isolates received from the Kentucky laboratory, but that they are not nearly as important in causing nocardioform placentitis as Crossiella equi
At its peak, nocardioform placentitis caused 94 and 144 abortions in 1998 and 1999, respectively, according to statistics compiled by Neil Williams, DVM, PhD, of the Livestock Disease Diagnostic Center. In other years, the number of cases fluctuated from two to 32. Prior to 1998, the Kentucky Livestock Disease Diagnostic Center diagnosed an average of 20 cases of nocardioform placentitis each foaling season.
During 1998 and 1999, nocardioform placentitis was the most common cause of placentitis in Kentucky, outnumbering placentitis cases caused by leptospires and streptococci. Researchers are unable to explain why the fluctuations occurred.
Mares of all ages and breeds have been affected by nocardioform placentitis. A mare might abort in late gestation; have a stillborn or weak foal prematurely or at term; or can produce an apparently normal foal. The mare usually will rid herself of the bacteria rapidly after abortion or delivery, and future gestations don't seem to be affected.
Cases of nocardioform placentitis appear in the horse population sporadically, but not as an epidemic. Over a nine-year period on farms in Central Kentucky with cases, 83% had two or fewer cases, and 66% had only a single case, according to information from the Livestock Disease Diagnostic Center.
The reason nocardioform placentitis shows up on some farms and not on others remains a mystery. In 1998 and 1999, veterinarians and farms that submitted nocardioform placentitis cases received questionnaires from the Livestock Disease Diagnostic Center gathering information such as prior reproductive health of the mare, breeding practices, stallions used, and mare management and environment. Development of the disorder was not linked to any farm management or veterinary practices. Faculty at the University of Kentucky's Veterinary Science Department have tried to recreate nocardioform placentitis in mares so they can explore its tendencies further, but have been unable to establish the infection. Scientists will continue to look for factors that play a role in the development of this disease.