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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TheHorse.com | The Horse January 2019 21 M y pregnant mare is colicking ... can I give her Banamine? She needs a laceration su- tured ... is it safe for her to get a sedative? What about her fall vaccines? Which common drugs and medications are safe for use in preg- nant mares is a huge topic with more questions than answers, says Margo Macpherson, DVM, MS, Dipl. ACT, professor of large animal reproduction at the University of Florida's College of Veterinary Medi- cine, in Gainesville. This is primarily because very few drugs have been thoroughly evaluated and validated for use in this population. "There is much we don't know about drug safety in pregnant mares," she says. "Safety studies are usually done in geldings rather than pregnant mares, and we can't always extrapolate. Mainly all we have is anecdotal use that suggests the majority of commonly used drugs are relatively safe for use in pregnant mares." Why Does it Matter? Pregnancy changes everything. Physiologic changes in the mare during pregnancy are enormous, says Macpherson, but little pub- lished information about them exists. "Cardiac output is increased by about two-thirds," she says. "The way blood flow is directed in the body, the way the kidneys operate (which is very different than in the nonpregnant female), etc., are all affected by pregnancy. There is a huge increase in fluid volume in pregnant women and pregnant animals. "These changes all affect how drugs are distributed in the body and what the body does with them and the way they are cleared from the body," she continues. "There are a number of metabolic pathways that these drugs undergo. Our database of information about some of the metabolic pathways and the way the drugs are handled in the liver and kidneys is very limited. We don't know enough about some of the pharmacodynamics (effects and mechanism of action) of the drugs." Knowing so little about the physiologic changes mares go through during pregnancy affects veterinarians' ability to not only predict certain drugs' impact but also make proper dosing decisions. Women and Mares Are Different Veterinarians often look at human drug safety data when trying to determine if they should be concerned about using a drug in pregnant mares, says Peter Sheerin, DVM, of Nandi Veterinary Associates, in New Freedom, Pennsylvania. This is mainly due to the time, expense, and challenges associated with studying medications in broodmares. "One issue with doing that, however, is that placentation (placental formation) in the human and horse is not the same, so the things that can cross the placenta and affect the fetus can be different," he says. Jennifer Linton, VMD, Dipl. ACT, assistant professor of clinical studies at the University of Pennsylvania School of Veterinary Medicine's New Bolton Center, in Kennett Square, agrees that there are major differences between horse and human pregnancies: "The human has only three layers of tissue between the maternal blood- HEATHER SMITH THOMAS

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