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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A28 TheHorse.com/AAEP2017 AAEP Wrap-Up THE HORSE March 2018 YOUR GUIDE TO THE 2017 AAEP CONVENTION MICHELLE ANDERSON; ALEXANDRA BECKSTETT Mare-ish: Are the Ovaries to Blame? eproductive behavior in mares can range from annoying to performance-limiting. For this reason, many mare owners and their vet- erinarians try to control or eliminate it. However, how do you really know if her reproductive system is to blame? Dirk Vanderwall, DVM , PhD, Dipl. ACT, a professor of horse reproduc- tion at Utah State University's School of Veterinary Medicine, in Logan, addressed this concern and offered ways to manage undesirable behavior. He first cited a survey of more than 750 vets in which 90% believed the estrous cycle affects mares' performance. Based on that survey, the most frequently report- ed performance-inhibiting clinical sign associated with cycle was attitude change. Others included tail-swishing, difficulty training, squealing, excess urination, kick- ing, and diminished performance. But before getting into the possible causes, Vanderwall reviewed mare repro- ductive physiology and seasonality. The key ovarian hormones regulating reproductive behavior are estrogen and progesterone. "The primary hormonal product driving the display of estrous behavior is estrogen," he said. "Progester- one is key to blocking estrous behavior." When a mare is in heat, her estrogen levels are high and her progesterone low. Post-ovulation, progesterone levels rise. Like many mammals, horses are seasonal breeders. In winter, mares' hypothalamic-anterior pituitary-ovarian axis (which regulates reproductive pro- cesses) is inactive, while in summer it's at its peak. During the spring and fall its ac- tivity increases and decreases, respective- ly, with transitional times of prolonged or irregular estrous activity. "In addition to their effects on mare behavior, estrogen and progesterone profoundly affect the physical and ultrasonographic characteristics of the tubular genitalia," said Vanderwall. Thus, ultrasound is a critical tool in determin- ing a mare's hormonal state. Going back to the initial list of per- formance-affecting behaviors vets noted in the survey, it's evident that it includes behaviors associated with both estrogen and progesterone. Furthermore, said Vanderwall, some problematic behaviors that get blamed on estrus are, in fact, signs of submissive behavior, urogenital discomfort, or stallionlike behavior. So, when evaluating a complaint of an estrous-cycle-related behavior or perfor- mance problem, "the veterinarian should first determine whether the problematic behavior is or is not related to the repro- ductive status of the mare," he said. This includes a complete physical exam to rule out other medical conditions and gathering a history to determine if the behavior correlates with reproductive events. Then the vet can perform tran- srectal palpation and ultrasound exam to evaluate the reproductive tract. If the mare is cycling, Vanderwall recommends tracking her reproduc- tive status over one or more cycles. This involves performing transrectal palpation and ultrasound twice a week for three to four weeks. The vet might identify signs of pain during manual evaluation of the ovary, back sensitivity, or coliclike pain. In the latter two scenarios, the mare might benefit from an ovulatory agent to reduce the time the follicle's present on the ovary, or administration of hormones to sup- press ovarian follicular activity, he said. If these serial exams confirm the mare's hormonal status correlates with her prob- lematic behavior, Vanderwall said the vet might prescribe hormone therapy (e.g., altrenogest) to mimic diestrus (the period between estrous cycles), prolong diestrus (e.g., oxytocin), mimic/induce the winter anovulatory state (e.g, via ovariectomy, or ovary removal), or mimic/induce estrus (e.g., the prostaglandin PGF2 α ). Summing up, Vanderwall said vets should perform a careful assessment of a mare's hormonal status/reproductive state to see if there's a definitive, repeatable relationship between it and her behav- ior. If so, it behooves them "to use an evidence-based approach when attempt- ing to regulate reproductive behavior and to choose the most appropriate treatment to eliminate the underlying causes." ISTOCK.COM Mares and Their Hormones Alexandra Beckstett @TH_ABeckstett Dr. McDonnell: ~2/3 of cases referred to their clinic for ovariectomies would not have responded. The root cause was not estrus-related.