Biting Flies and Horses

Chronic summer dermatitis and “sweet itch” are terms for recurring allergic skin reactions that horses may experience from late summer into early fall, the typical fly season. Bites of small gnats (ceratopogonids, no-see-ums, Culicoides species), horse flies, stable flies, and mosquitoes are among the potential causes of this irritating condition. Crusty skin and loss of hair may result from the animal’s reaction to saliva injected by these small biting gnats as they feed. In addition, animals may damage their skin or injure themselves as they rub against objects to relieve the irritation.

Figure 1. Tiny ceratopogonids slash the skin with blade-like mouthparts and feed on blood that pools at the wound. (Photo:

Figure 2. Feeding by ceratopogonids tends to produce larger, crusty areas (Photo: Itchy Horse Company Ltd.)

Figure 3. Mosquito and horse fly bites tend to produce distinct lumpy bite sites (Photo:

Biting gnats may be the main culprit of allergic skin reactions, but itching may also result from irritants in bedding, other irritant sources, or infections. Careful investigation may identify the cause(s) and direct the best course of action. 

Potential Preventive Actions

  • A variety of fly sheets have been designed to provide physical protection of horses. These fine mesh coverings prevent access to the skin. Face and ear protection may be needed depending on the feeding site preferences of the species in the area.
  • Ceratopogonids are weak fliers, so turn out horses in open areas with good air movement, especially during low light periods (dawn and dusk) when the flies feed.
  • If practical, stable horses or provide refuges so they can escape biting flies that will not enter shelters.
  • Insecticides and repellents may provide temporary protection from biting insects. Apply them to favored feeding sites, especially the belly and backline. A list of labeled insecticides is available in Insect Control for Horses, Horse Barns, and Stables – 2017 (ENT-513)
  • Most biting flies develop in wet areas, so avoid pasturing horses near seepage or wet areas as much as possible. It is impractical to locate and treat breeding sites.

Figure 4. Fly sheets can protect horses from biting flies (Photo:


Sweet itch: signs, treatments and prevention (Horse & Hound, June 21, 2016)


By Lee Townsend, Extension Entomologist




Posted in Livestock Pests