Systemic Therapy in the High-Risk Mare

Several conditions can threaten pregnancy in the late gestational mare. C.S. Bailey, DVM, who is completing a theriogenology residency at the University of Florida and Hagyard Equine Medical Institute in Lexington, Ky., said some of these conditions might warrant the use of therapeutic agents, in spite of limited knowledge about their efficacy or their ability to penetrate fetal membranes.

Current information regarding the effects of antibiotics and anti-inflammatory agents in the gravid mare is based on a model of experimentally induced placentitis in the research setting, added Bailey in his presentation at the Hagyard Bluegrass Equine Reproduction Symposium 2006, which was held Oct. 18-21 in Lexington, Ky.

In the United States, placentitis is considered a leading cause of abortion and neonatal loss, accounting for nearly a third of documented cases. Mares at risk for placentitis include those with poor perineal conformation, a previous history of placentitis, and multiparous mares (those that have had several foals). Three major types of placentitis are recognized in this country, including ascending bacterial placentitis, nocardioform placentitis, and placentitis associated with mare reproductive loss syndrome (MRLS).

Placentitis is commonly caused by bacterial infection ascending through the cervix, with Streptococcus equi subspecies zooepidemicus most frequently implicated. Typically, diagnosis is based on clinical signs and ultrasonographic findings. At present, most successful treatment regimens include a combination of antibiotics, anti-inflammatory drugs, and tocolytic (labor-inhibiting) agents.

According to Bailey, "Treatment outcomes appear to be optimized by a combination approach." Several equine studies have demonstrated the effectiveness of a multitherapeutic approach in the treatment of placentitis in the clinical setting. Studies conducted at the University of Florida have demonstrated the ability of three antibiotics to penetrate to the allantoic cavity (within the chorioallantois, which is the outer layer of the placenta where the placenta and maternal uterus touch and exchange nutrients). When administered at standard doses, potassium penicillin G, gentamicin sulfate, and trimethoprim sulfamethoxazole were shown to reach therapeutic concentrations in the allantoic fluid.

In addition to antibiotics, anti-inflammatory agents, including flunixin meglumine and pentoxifylline, have been studied in the allantoic fluid of mares with experimentally induced placentitis. While flunixin meglumine was not present in detectable concentrations with the testing method that was used, pentoxifylline was found.

In 2006, an ongoing study was evaluating the treatment efficacy of a three-drug regimen including trimethoprim sulfamethoxazole, pentoxifylline, and altrenogest (Regu-Mate). Preliminary data suggest that combination therapy consisting of these three medications is effective in the treatment of placentitis.

"Mares should be treated with all of the drugs from the time of diagnosis until abortion or the delivery of the foal," said Bailey.

While the optimal treatment for equine placentitis is not yet defined, a growing body of evidence suggests that combined therapies with antibiotics, anti-inflammatory agents, and tocolytics are most effective. In the case of placentitis and other conditions that threaten pregnancy in the late gestational mare, treatment of maternal disease is critical to foal survival. Because the benefits of treatment far outweigh the potential risks to the fetus, judicious use of these drugs is indicated in cases in which the survival of the mare and fetus are threatened.

About the Author

Rallie McAllister, MD

Rallie McAllister, MD, grew up on a horse farm in Tennessee, and has raised and trained horses all of her life. She now lives in Lexington, Ky., on a horse farm with her husband and three sons. In addition to her practice of emergency and corporate medicine, she is a syndicated columnist (Your Health by Dr. Rallie McAllister), and the author of four health-realted books, including Riding For Life, published by Eclipse Press and available at or by calling 800/582-5604.

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