Q: My Quarter Horse gelding has severe fly allergies. He wears a fly sheet, hood and leg wraps from early spring through late fall. Any part of his body not covered is fair game to the flies, and they leave him bloody. I have tried many topical products, as well as home remedies, but nothing has worked.

He is housed in a 15- by 50-foot outside run that has a cover and is protected on three sides. His previous owners ended up keeping him in a box stall during the day with a fly spray system. That arrangement helped; however, it is not possible where I board him. Is there anything else I can try? Name withheld by request

A: It does, indeed, sound like your horse has a severe fly allergy. Controlling the problem requires using as many of the following methods as possible:

  • Stable horses at sunrise and sunset–peak feeding hours for the flies that most commonly cause allergic reactions.
  • Place ultra-fine screens (60 squares to the square inch) in stable windows.
  • Apply permethrin repellent products for horses. In addition, 44 to 65 percent permethrin spot-on formulations marketed for dogs—and therefore “off-label” for horses—have been used successfully.
  • Install overhead or stall fans to create air currents that interfere with insect flight.
  • Outfit horses with sheets and other “clothing” that physically obstruct insects trying to reach the skin.
  • Consider hyposensitization with allergy injections. This is controversial, and success may vary depending on the adjuvant0 or the actual antigen used.
  • Administer oral prednisolone0 to manage the itchiness.

It sounds like you’ve tried many of these at different times, but I would encourage you to implement as many as possible at once. Each will add another “layer” of protection to your horse.

You’ll also want to determine which type of fly you are dealing with. Culicoides spp., commonly known as “no-see-ums” or midges, are the most common cause of equine fly allergies. These insects feed either on the dorsal (top) or ventral (bottom) surfaces of the horse, affecting the mane, saddle area and rump. In hypersensitive horses, the fly’s saliva triggers the allergic response. Larger flies may also affect the horse, primarily by their bite but also presumably with an allergic reaction. These flies include stable flies (Stomoxys calcitrans), horn flies (Haematobia spp.), horseflies (Tabanus spp.), deer flies (Chrysops spp.) and black flies (Simulium spp.).

You can determine which type of fly you are dealing with by finding pictures on-line for comparison or relying on other clues. For example, horn flies produce dermatitis on a horse’s belly while black flies tend to bite the udder, scrotum, prepuce, inner surface of the thigh and upper forelimbs, throat, ears, ventral abdomen, chest and natural body orifices. Stable flies like sunlight, but horn flies do not.

Good sanitation is important with all types of flies, and your other control measures will be more effective if you tailor them to the breeding, feeding and habitat preferences of the species on your property. For example, parasitic wasps, which feed on fly larvae, may help control stable, horn and housefly populations but probably won’t be effective against Culicoides.

Stephen D. White, DVM
University of California—Davis
Davis, California