Insect Bite Hypersensitivity

What is insect bite hypersensitivity?

Insect bite hypersensitivity (IBH), also known as sweet itch, Queensland itch, summer eczema, and Kasen disease, is an allergic skin disease caused by biting insects of the genus Culicoides (midges, gnats, no-see-ums). When these insects bite, they inject protein-containing saliva that can cause allergies in horses that are predisposed to IBH. Affected horses may also have allergic reactions to other insects such as stable flies (Stomoxys calcitrans), horse flies (Tabanidae spp.), black flies (Simulium spp.) and mosquitoes.

Although IBH has been reported in many breeds worldwide, Icelandic horses exported to other countries have a particularly high incidence of the disease since Culicoides are not present in Iceland. Genetic risk factors for IBH have been proposed and studies have indicated that multiple genes are involved, but further research is needed.

What are the clinical signs of insect bite hypersensitivity?

Clinical signs of insect bite hypersensitivity include itching, skin thickening, lesions, scaling, crusting, hives, and hair loss, which can result in extreme discomfort. Lesions in horses are usually found on the trunk, face, mane, tail, and ears, with locations varying depending on the biting characteristics of the particular insect(s).

How is insect bite hypersensitivity diagnosed?

Diagnosis of insect bite hypersensitivity in horses relies on clinical history, including seasonality and recurrence of clinical signs, and a physical examination to rule out other causes. Intradermal allergy testing can be used to help provide a definitive diagnosis.

How is insect bite hypersensitivity treated?

Treatment for IBH in horses primarily relies on reduction of exposure to midges through stabling and use of blankets, fly masks, and insect repellents. Corticosteroids may be used to reduce clinical signs, but antihistamines have not been shown to be particularly effective for treating IBH. Omega-3 fatty acids can aid in reducing skin inflammation. Allergen specific immunotherapy for IBH usually has been unsuccessful to date.

What is the prognosis for insect bite hypersensitivity?

The prognosis for insect bite hypersensitivity in horses varies. Clinical signs are often seasonal, improving and worsening based on insect behavior patterns. Particularly sensitive horses may exhibit self-inflicted trauma and severe cases may require euthanasia. There is no cure for IBH and affected horses require lifelong management. The financial value and intended use(s) of affected horses may be negatively impacted.

How can insect bite hypersensitivity be prevented?

Prevention of IBH in horses involves strict adherence to insect control protocols. This can include keeping horses stabled during peak times of insect feeding activity (often dawn and dusk), utilizing fans in stalls and barns, and promptly removing manure and standing water to eliminate insect breeding grounds.

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu.