Approximately 20,000 mares are bred in Kentucky each year, which means that it is literally “raining foals” in the spring, as long as things go as planned. Unfortunately, late-term abortions and even abortion storms can occur, resulting in massive economic and emotional mayhem.
The most common cause of late-term abortion is placentitis—an inflammation of the placenta. Frequently, placentitis is caused by bacteria found in the environment ascending though the vagina and cervix, subsequently spreading from the part of the placenta referred to as the cervical star. Signs of this “ascending placentitis” are early mammary development and lactation and eventually vulvar discharge.
Another, but less common, type of placentitis is called nocardioform placentitis.
"Nocardioform placentitis involves specific types of filamentous, branching bacteria, called actinomycetes, and the disease is very different from an ascending placentitis in that only the interface between the placenta and the uterine lining is affected and only the lower (ventral) part of the uterine body and the base of the uterine horn is affected,” explained Mats H.T. Troedsson, DVM, PhD, Dipl. ACT, ECAR, the director of the Gluck Equine Research Center at the University of Kentucky. "In the case of an ascending placentitis, the lesions are found around the cervical star and bacteria invade the placenta, fetal fluids and infect the foal."
Nocardioform placentitis therefore does not directly affect the foal. Instead, it deprives the foal of nutritional support from the placenta, resulting in starvation and abortion or the birth of a small and underdeveloped foal.
Diagnosis and Treatment
In suspected placentitis cases (e.g., if a mare bags up early), Troedsson recommends a transabdominal ultrasound (across the body wall rather than through the rectum, which is needed for an ascending placentitis).
“The ultrasound will show a separation of the two layers of the placenta and a very, very thick mucus,” he described. Following diagnosis, mares can be treated with antibiotics, an anti-inflammatory, and a high dose of a progestinlike hormone (e.g., Regumate).
“When the condition is caught early, the majority of mares do not abort and the foals survive,” Troedsson said.
Stacey Oke, DVM, MSc
“Nocardioform placentitis has been diagnosed at several locations in the U.S., and we only intermittently see a larger number of nocardioform placentitis cases here in Kentucky," relayed Troedsson. "In this area, where we breed at least 20,000 horses, we usually only see about 15-25 case a year. In a bad year, the number of cases can increase to between 100 and 200, which is what happened last in 2011.”
Subsequent to the nocardioform placentitis surge in 2011, research in this field has progressed, yet exactly how the infection is established remains a mystery. According to Troedsson, researchers have conducted a number of experiments to establish how the nocardioform actinomycetes cause placentitis.
Examples of such studies included the following:
- Inoculating mares’ uteri with the bacteria at the time of breeding;
- Inoculating mares’ uteri at Day 270 of gestation;
- Using an intranasal spray containing the bacteria to infect mares;
- Feeding the bacteria to mares; and
- Directly injecting the bacteria into mares’ bloodstreams.
“Not one of the mares treated via these routes developed a placentitis,” lamented Troedsson, who added, “Without a model of the disease, it is not possible to study the mechanisms of the disease. Our data suggest that the bacteria associated with nocardioform placentitis is fairly harmless in most instances, suggesting that there is something else in either the mare or the environment that makes these bacteria harmful.”
He adds, “Even more puzzling is the fact that we have not even been able to culture the bacteria from the environment in Central Kentucky. We obviously have a problem and have associated the bacteria with a disease, yet we can’t find it or reliably cause the disease to study and create an effective vaccine or other treatment or management strategies to prevent nocardioform placentitis.”
Stacey Oke, DVM, MSc, is a freelance medical writer based out of Canada.
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Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.