Q: Every winter my 6-year-old mare develops bilateral alopecia around the hip area. The condition does not seem to be related to tack, and she loses her hair whether she is blanketed or not. Each spring the hair grows back, but in January she begins to lose it again and by February the area is almost bald. Can this be related to a melatonin issue? —Patti Drake; Eagle Creek, Oregon
A: It sounds like you’re describing seasonal flank alopecia, a poorly understood condition characterized by patches of partial or complete hair loss that recur cyclically, at certain times of the year. In other words, something about your horse’s annual cycle of hair growth and shedding is changing, but only on part of her body.
It is thought that an imbalance in the pineal gland, which is the source of melatonin, can cause this hair loss because of the role the hormone plays in the body’s circadian rhythm. Melatonin treatment has been used with some success in dogs with alopecia, but to the best of my knowledge, this approach is still unproven in horses.
In some cases, the lighting the horse is kept under during the winter can contribute to the problem. Is your barnyard and/or stall lit by fluorescent or quartz halogen lights? Some of these light sources approach the wavelengths of natural sunlight and could encourage the horse’s pineal gland to produce too much melatonin.
I’d suggest that you try keeping your horse under natural lighting conditions instead of artificial light to see if that might prevent another episode. Provided that the hairless areas are protected from sunburn and frostbite, the issue is a cosmetic one and should not affect the horse’s health.
William H. Miller, VMD
College of Veterinary Medicine
Ithaca, New York