Biting Flies And Insect Hypersensitivity In Horses

With the warmer weather comes the onslaught of our favorite fair weather friends, biting flies. These obnoxious insects can wreak havoc on horses and without proper care and management, your horse could become the victim of these annoying little critters.

Many types of biting flies can cause skin problems in the horse. They include mosquitoes, gnats, biting midges, sand flies, “no see ums,” black flies, horse flies, deer flies, stable flies, and horn flies. There are several ways these flies can cause damage. In some cases the bite itself is the problem. The larger flies can cause an irritating and painful wheal or nodule. Also, many of these flies transmit various viral, bacterial, and fungal diseases. In this case, the horse isn’t bothered by the bite. Rather, they develop a problem because of the virus, bacteria, or fungus which has been transmitted via the fly bite. Lastly, the horse can develop a hypersensitivity to the salivary antigens or the injected toxins of the fly. The most common culprit in insect hypersensitivity is Culicoides (gnats, biting midges, sand flies, “no see ums”).

Insect hypersensitivity is one of the most common pruritic skin diseases of the horse so I will focus attention on this specific topic. Because hypersensitivity causes intense itching, most of the lesions are the result of self-trauma as the horse continually rubs the affected areas to relieve the itching sensation. Lesions can include alopecia (hair loss), crusting, scaling, and small, oozing papules. The most common distribution occurs on the forehead, neck, withers, shoulders, rump, and tail. Hypersensitivity, which may be inherited, varies from horse to horse and tends to worsen with age.

The best treatment for insect hypersensitivity is a reduction in insect exposure. Yes, I know, this is easier said than done, but if you have hypersensitive horses, it is critical to take aggressive measures to reduce the fly population. There are topical, oral, and injectable medications that can be used on a short term basis to control the intense itching associated with insect hypersensitivity. However, the best long term treatment is a reduction in insect exposure. Culicoides breed in standing water such as that found in lakes, puddles, swamps and stagnant watering troughs, so moving hypersensitive horses away from these areas will help. Stabling during Culicoides peak feeding times (early morning and dusk) particularly when the humidity is high, can reduce exposure if the stall is completely enclosed with a fine wire mesh that is sprayed frequently with insect repellent. Time release overhead fly spray systems can be useful and do not have to be elaborate to be effective. Another means of keeping flies away from your horse is to introduce air currents into the barn by using big fans. The small flies can’t manage in strong air currents. Also, make sure to keep watering buckets and troughs clean to reduce fly breeding sites.

Various commercial products can also help hypersensitive horses. Frequent application of fly spray is critical, especially prior to peak feeding times. Look for products that contain pyrethrins with synergists. Fly masks that cover the eyes and ears are also helpful if the flies are bothering your horse in these areas. In some cases it may be beneficial to purchase a nylon fly sheet. Frequent bathing with specially formulated shampoos may be helpful in reducing scaling, crusting, and itching, making the horse more comfortable. In extreme cases that do not respond to the previous therapies, corticosteroid treatment may be necessary.

Biting flies can cause many types of skin diseases in the horse. Their bite may be significant enough to cause oozing sores in which case the wounds only needs time to heal. They may transmit a viral, bacterial, or fungal infection. These types of infections can require specific treatment. Or, your horse may by hypersensitive to the fly’s saliva or toxins which requires aggressive reduction in fly exposure. Because biting flies can cause such a variety of problems always consult your veterinarian when you notice any type of skin condition affecting your horse. It is important to get a correct diagnosis so your treatment plan is appropriate.

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