In mid-April, former Horse of the Year Zenyatta tragically aborted her Into Mischief foal. This was the sixth pregnancy for the 14-year-old mare. Zenyatta's connections wrote in her blog, “The cause of the abortion was found to be a low-grade placentitis that triggered premature placental separation.” Since the abortion, Queen Z, as her fans call her, is reported to be doing well at Lane's End Farm, but her connections have decided not to breed her this year.
Placentitis is an inflammation of the placenta, the temporary organ that forms during pregnancy to supply oxygen and nutrients to the fetus and carry away fetal waste products. The placenta interfaces with the uterus by villi – minute, finger-like projections that form over blood vessels. The greater the blood supply, the more dense and tall the villi that cover them, so they can take advantage of the rich blood supply.
Villi also form an attachment between the placenta and the uterus, rather like Velcro. If a group of villi are damaged by infection or inflammation and die, the placenta loses its nutrient connection and physically detaches from the uterus at that spot.
A pregnant mare can acquire placentitis in several ways. If the mare is sick with a systemic infection, bacteria can invade the placenta via the blood supply (hematogenous placentitis). Bacteria can gain access to the placenta from outside the mare through her vagina (ascending placentitis), or the mare can become infected with a mucoid pathogen (nocardioform placentitis), and how that is acquired is still a mystery.
Placentitis is difficult to detect because it has few symptoms, other than premature development of the mammary gland. Mares with ascending placentitis also may have a vaginal discharge.
“Most healthy mares will start mammary gland development about three weeks prior to parturition [birth], but they start slowly and get bigger and bigger,” said Dr. Karen Wolfsdorf, a reproduction specialist at Hagyard Equine Medical Institute in Lexington. “The one you worry about is the mare that, when you come in one day, and she has a big mammary gland.”
Wolfsdorf advises owners to have a mare examined with transrectal and transabdominal ultrasound if she develops a discharge or bags up early. If the ultrasound shows a thickened placental and uterine wall, edema, or the beginning of placental separation from the uterus, the pregnancy could be in trouble and further monitoring necessary.
“Premature separation of the placenta can manifest itself as a red bag,” Wolfsdorf said. “The most common type of placentitis that causes premature separation of the placenta at the cervical-star region is an ascending placentitis.”
The cervical star is a pale area of the placenta that abuts the cervix but does not have villi. This is where bacteria enter into the uterus; therefore, the area adjacent to it is the most likely spot for placental separation to begin.
Outbreaks of nocardioform placentitis hit Kentucky in 1999 and again in 2011. This particular type is distinguished from other types of placentitis by the location of its attack on the placenta. Unlike other bacterial placentitis that begins at the cervical star, nocardioform lesions occur at the base of the horns. In a healthy placenta, this area is particularly dense with blood vessels and tall villi.
The nocardioform bacteria cause a thick, brown mucous similar in consistency to peanut butter to cover the placenta at the site of the lesion, choking out the villi and disrupting their interaction with the uterine wall. Once damaged, the placenta is unable to provide sufficient life support to the fetus.
Because veterinarians do not know how a mare is exposed to nocardioform placentitis or the risk factors, no effective prevention exists.
Nocardioform's most common symptom is premature mammary development. Mares who contract this insidious form show no signs of illness until the infection is well advanced. They can be eating contentedly during a watchman's hourly round and aborting the next time he checks.
Treatment for placentitis
Because inflammation causes the uterus to contract, expelling the fetus, the first step is to quiet these contractions using an anti-inflammatory such as flunixin meglumine (Banamine). The drug also halts premature udder development. Pentoxifylline (Trental) often is used to reduce inflammation as well as improve blood flow between the uterus and the placenta.
If the mare has a vaginal discharge, the veterinarian will take a culture to determine the specific bacteria causing the infection and then will begin a regimen of the antibiotic that is effective against that particular strain. The most common bacteria found in ascending placentitis is Streptococcus zooepidemicus, which responds to penicillin. Other antibiotics used to treat placentitis are sulfamethoxazole and trimethoprim (SMZ, Cotrim, or Tribrissen) and gentamicin.
“You're not going to ever have the placenta reattach to the uterus, but you're going to stop it from becoming worse,” Wolfsdorf said. “So it really depends on the amount of compromise to the placenta and to the fetus, and whether or not you're going to be able to treat it and have a live foal…
“The key is early identification and appropriate treatment,” she said. “Then we have quite good success and have a viable foal at the end of it.”
The best step to prevent ascending placentitis is to stitch the vulvar lips together as a barrier against bacteria—a Caslick's procedure. Otherwise, few measures can be taken other than good husbandry.
“That can be sometimes as simple as doing a Caslick's procedure on the mare, making sure she doesn't have a cervical tear, or making sure she doesn't have chronic endometritis—that can also help prevent the disease,” Wolfsdorf said. “We have started monitoring pregnant mares at seven to eight months of gestation, especially high-risk mares, over the last several years to try to identify early signs of abnormalities or ascending placentitis.”
If no complications occurred when the mare aborted, and her body responded correctly to restore a healthy reproductive tract, the placentitis should not impact the mare's ability to conceive or the subsequent pregnancy. But a history of placentitis will place the mare in the “higher-risk” category that warrants close monitoring during subsequent pregnancies.
“When a mare aborts, it usually doesn't preclude them from being bred again that year,” Wolfsdorf added.
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