Several times, during my long career as a veterinarian, I have had clients call me because, they said, their horse was over-reacting to stable flies by violently shaking their heads. I remember asking why other horses in the stable were not displaying similar reactions to flies.
Not knowing what else to recommend, I suggested using fly repellant sprays every morning, and also getting fly masks or head nets for the affected horse.
Towards the end of my practice career, I read about headshaking syndrome in one of the scientific veterinary medical journals I routinely read. It is apparently caused by the effect of sunlight upon the trigeminal nerve, one of the cranial nerves. The sunlight is most intense during the summer months, and, of course, that’s also when flies are most active, so it is understandable why the flies were blamed for the head shaking. But, the head shaking is simply the horse’s reaction to the effect of sunlight on the cranial nerves.
The most effective treatment, therefore, is to minimize the patient’s exposure to sunlight, especially during long, sunny, seasonal days.
Providing shade obviously helps, as do fly masks. Even when riding, a nose mask can be provided. Supplementation with Magnesium has been reported to help, along with other supplemental or pharmaceutical products, but avoiding direct sunlight is the major solution. There have also been reports of acupuncture providing relief.
My wife’s gelding started headshaking during middle age. But, now knowing what to do, we keep a mask on him for half of the year. He is comfortable, and we are happy.
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