During pregnancy, a broodmare's body and hormones go through many changes: Her belly swells, and her insulin and glucose levels vary. When you consider the association between obesity, insulin resistance, and laminitis, this becomes a health concern beyond just delivering a healthy foal. The endocrine changes associated with pregnancy might actually increase the mare's risk of developing the devastating hoof disease laminitis.
Hannah Galantino-Homer, VMD, PhD, Dipl. ACT, senior research investigator at the University of Pennsylvania School of Veterinary Medicine's Laminitis Research Initiative, who said broodmares represent some of the worst laminitis cases the university's New Bolton Center sees, reviewed existing research on this class of horses and offered some suggestions for preventing the disease at the 2013 International Equine Conference on Laminitis and Diseases of the Foot, held Nov. 1-3 in West Palm Beach, Fla.
Pregnancy and Insulin Sensitivity
Insulin is a hormone designed to control blood sugar (glucose) levels. Insulin resistance (IR) is a reduced sensitivity to insulin that makes it harder for cells to transport glucose out of the bloodstream and store it as glycogen. It is typically associated with another condition such as obesity, pituitary pars intermedia dysfunction (or equine Cushing’s disease), or equine metabolic syndrome (EMS).
Pregnancy is known to alter glucose and insulin dynamics, said Galantino-Homer, but researchers have not studied this phenomenon extensively in mares. In studies of women and dogs, insulin sensitivity decreased 60% and 43%, respectively, during pregnancy, she said. And pregnancy can act as a stress test for a woman's future risk of metabolic syndrome.
"It could be very informative to determine if the insulin response to the oral glucose tolerance test (a diagnostic test for endocrine disease) during equine pregnancy likewise has any correlation with later development of EMS and/or laminitis in broodmares," Galantino-Homer said.
She cited a recent study of Thoroughbred mares in which researchers detected decreased insulin sensitivity and increased acute insulin response to glucose in mares at 196 days gestation vs. nonpregnant mares as well as increased and prolonged glycemic and insulinemic responses to being fed a high-starch meal.
In another study, researchers also reported elevated insulin levels in pregnant mares. These levels were highest prior to 270 days gestation and subsequently decreased with increasing gestation.
"Both studies suggest that the mare, like other species studied to date, experiences pregnancy-associated IR from at least Days 150-270 of gestation," Galantino-Homer said. "But to our knowledge, insulin sensitivity in pregnant mares prior to Day 150 of gestation has not been investigated."
For mares diagnosed with IR prior to pregnancy, "the additional metabolic stress of pregnancy could greatly increase the risk for laminitis, although epidemiological studies are needed to verify this suspected risk," Galantino-Homer said.
She cited one case from New Bolton Center in which an obese pregnant mare developed acute laminitis at around 90 days gestation and was ultimately euthanized. "The effects of obesity and pregnancy on IR might have initiated laminitis in this mare and could possibly have been averted by weight management prior to breeding and by avoiding high-starch feed and allowing exercise during pregnancy," Galantino-Homer said.
Endocrine Changes and the Hoof
In bovine, human, and rodent studies, researchers have found that periparturient (around the time of birth) increases in the hormones estrogen and relaxin cause the birth canal connective tissue structure to loosen. Cows, in particular, experience a weakening of the suspensory apparatus of the distal phalanx (SADP), or coffin bone, around calving, known as the "parturition effect." Scientists have tied this to increased susceptibility to solar ulcers and white line disease, especially in cows housed in poor conditions.
Galantino-Homer hypothesized that broodmares might experience a similar connective tissue laxity, increasing their risk of SADP failure and laminitis.
Although further research is needed in all these areas, Galantino-Homer said it's nonetheless important to monitor broodmares for laminitis and endocrine disorders.
"The annual health check or breeding soundness evaluation should include a careful and critical physical evaluation of the feet for evidence of laminitis, such as abnormal growth rings on the external hoof wall, solar abscesses, seedy toe, or dropped soles," she said.
She also emphasized the importance of managing weight and endocrine issues prior to pregnancy, as restricting calories during pregnancy can be detrimental to both the mare and foal. Avoid feeding high-starch feeds, she said, particularly in mares with pre-existing insulin resistance, given the prolonged and increased insulin response observed during pregnancy.
Finally, she raised the question of ethics when it comes to breeding a laminitis-prone mare. While it's not unusual for breeders to retain mares who can no longer perform athletically due to laminitis as breeding stock, she said at some point owners and veterinarians might just need to draw the line for mares with chronic, recurrent laminitis.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.