A lot of horses are allergic to flying insect bites.  Culicoides (a type of gnat), black flies, stable flies, horn flies, and mosquitos can all cause an allergic reaction in some susceptible horses.  Generally these horses have intense itching to the point they rub their hair out and this rubbing leads to skin damage, secondary infections, and chronic skin damage with thickened skin.  In horses, the effects are commonly seen on the entire horse but especially the neck, back and tail head while donkeys seem to have the problem only occur on the lower legs.  Dr. Susan White from the University of Georgia indicates in Equine Disease Quarterly that the distribution of the lesions depends on the biting characteristic of the insects involved.  However, most of these allergies in climates like that of Texas occur in the warmer months and are not a problem in the winter.  If your horse is still itching in the winter in Texas, it is likely another type of allergy involved. Urticaria, also called hives, is common with biting insects but can also occur as a reaction to any kind of allergy, even a food allergy.

Treatment of the itching and hives centers around fly control and there are multiple options that can help, including sprays, wipe on products, fly sheets, face masks, fly predators, and feed-through fly control; the latter is a product that contains a pesticide not digestible by horses but that kills fly larvae in the manure. Some of these methods are more effective than others, depending on the type of fly involved, so speak to your veterinarian if you’re not sure what type of insects you’re dealing with. 

Dr. Amanda Adams from the University of Kentucky is working on a vaccine that targets an inflammatory chemical called il-5, which is involved in the allergic reaction to flies. Hopefully this vaccine will prove to be successful.