The Horse

JAN 2019

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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22 January 2019 The Horse | TheHorse.com stream and the fetal bloodstream, so it's easy for anything in the maternal blood- stream (bacteria, chemicals, etc.) to cross into the fetal bloodstream, whereas in the horse there are six layers," she says. "The fetus is better protected in the horse." To Administer or Not To Administer? Safety data for commonly used equine drugs in pregnant mares is basically nonexistent. "The concerns when admin- istering a drug to a pregnant mare are the risks to the developing fetus and risks to the mare," says Macpherson. "The fetus is particularly at risk, given its poor ability to metabolize and eliminate most foregin compounds." So when deciding whether to give a pregnant mare a certain drug, first consider whether she really needs it. "The second thing is that you can't have a successful pregnancy and a live baby if you can't keep the mare alive," says Linton. "For instance," she says, "one group of antibiotics called tetracyclines (oxytet- racycline, doxycycline, minocycline, etc.) is often used to treat Lyme disease, anaplasmosis, and Potomac horse fever. Anaplasmosis and Potomac horse fever cause high fevers and can make the mare very sick. Without appropriate treatment she might lose the pregnancy and pos- sibly her life. We know tetracycline can damage … fetal bones. If the mare may die or lose the pregnancy from being sick, however, we use tetracycline and gamble on how it might affect the fetus." Let's look at specific drug classes and how they might affect the mare and fetus. Antibiotics Based on what research has been done in pregnant mares, we know some antibiotics cross the placenta and some don't. "A lot of the first studies showing which ones do cross the placenta were done in research with placentitis (inflam- mation of the placenta)," says Sheerin, who notes that you need an antibiotic that will cross the placenta and address the infection causing the inflammation in such cases. "But one thing to remember is that in mares with placentitis, the placenta is ab- normal; effects of the antibiotic might be different than in a normal pregnancy," he adds. "There is still a lot we don't know." Antibiotics Linton says veterinar- ians have used without significant side effects in pregnant mares, particularly to treat placentitis, include sulfas or trimethoprim/sulfa combinations. Macpherson, who's done much of the research on these drugs in mares, agrees that these antibiotics, along with MEDS FOR MARES FDA GUIDELINES FOR DRUG USE IN PREGNANT WOMEN CATEGORIZED BY RISK, AS ADAPTED FOR MARES CATEGORY FDA DEFINITION EXAMPLES OF EQUINE DRUGS CATEGORY A Adequate and well-controlled studies have failed to demonstrate risk to the fetus in the first trimester of pregnancy (and there is no evidence of risk in later trimesters) N/A CATEGORY B Animal reproduction studies have failed to demonstrate a risk to the fetus and there are no adequate and well-controlled studies in pregnant women Azithromycin, erythromycin, penicillins, cephalosporins, metronidazole, amphotericin B, omeprazole, cimetidine, ranitidine CATEGORY C Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant the use of the drug in pregnant women despite potential risks Clarithromycin, imipenem, gentamicin, fluoroquinolones, trimethoprim sulfadiazine, rifampin, fluconazole, itraconazole, ketoconazole, some non-steroidal anti- inflammatories (NSAIDs), butorphanol, dexamethasone, prednisolone, fluticasone, albuterol CATEGORY D There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant the use of the drug in pregnant women despite potential risks Tetracyclines, aminoglycosides (except gentamicin), some NSAIDs, angiotensin- converting-enzyme inhibitors, diazepam, phenobarbital CATEGORY E Studies in animals or humans have demonstrated fetal abnormalities and/ or there is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience, and the risks involved in use of the drug in pregnant women clearly outweigh potential benefits Misoprostol S. Giguére, 2014, courtesy Dr. Margo Macpherson

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