Equine Allergic Dermatitis

Horses exposed to bites of certain Culicoides species commonly exhibit an allergic skin reaction. This typically occurs as a seasonal dermatitis affecting the withers, mane, tail, and ears. The back, ventral midline, and other body regions also can be affected, presumably reflecting the feeding sites of different biting midges involved. Equine allergic dermatitis was first attributed to Culicoides bites in Australia in the early 1950s, where it was known as Queensland itch. It is now known to occur widely throughout the world and is known by various names such as sweet itch, summer recurrent dermatitis, summer eczema, equine Culicoides sensitivity, Dhobie itch (Philippines), and Kasen disease (Japan). A similar, seasonal dermatitis in response to Culicoides bites also has been reported in sheep.

The dermal response apparently is a sensitivity reaction to components of salivary fluids introduced to the bite wound while the flies are feeding (Fig. 10.19).

FIGURE 10.19 Allergic dermatitis in neck region ofhorse in response to injections of Culicoides extracts. (Courtesy of Yehuda Braverman, Kimron Veterinary Institute, Israel.)

Normal, unsensitized horses usually react to these bites by developing small welts with relatively little associated discomfort. Sensitized horses, however, react more severely by developing intense local inflammation and pruritus; this can result in irritability, rubbing and scratching of involved areas, open wounds, and secondary infections. Ponies are especially sensitive. Affected animals often are unsuitable for riding and, in the case of show horses, may decrease substantially in commercial value because of their irritable behavior, hair loss, and skin blemishes.

Once sensitized, horses experience either an immediate hypersensitivity response, peaking within 4 hr, or a delayed hypersensitivity response in which large welts develop after 24 hr, with inflammation persisting up to 3 weeks or more. There is good evidence to show that Culicoides-'\nA\iccA hypersensitivity is a polygenic hereditary trait which predisposes certain animals to this response. This sensitivity occurs primarily in older horses, usually after 4—5 years of age.

A number of Culicoides species have been implicated as the cause of equine allergic dermatitis. Most are based on correlations between seasonal occurrences of the midges and clinical signs, biting sites on horses, and positive reactions to intradermal injections of horses with extracts of the respective biting midges. The following species are suspected of being involved: C. insignis, C, ob-soletus, C. spinosus, C. stellifer, and C. venustus in the United States; C. pulicaris in England; C. nubeculosus and C. punctatus in Ireland; C. chiopterus, C. impuncta-tus, and C. obsoletus in Norway; C. imicola in Israel; and C. brevitarsis in Australia.

Treatments for equine allergic dermatitis in the form of antihistamines and corticosteroids usually provide only temporary relief of symptoms. Desensitization of animals with injections of Culicoides extracts has not proved to be effective. Horse owners in areas where this condition is recognized as a problem should avoid breeding their animals with lineages of known sensitivity. Insecticides applied directly to horses to repel or kill biting midges afford some protection and can substantially reduce the severity if administered on a regular basis throughout the fly season. Ivermectin, however, is ineffective. Stabling horses at night or pasturing them away from the attack of biting midges can also help to alleviate the problem.

Larviciding generally has not been effective in reducing populations of biting midges. Often the breeding sites are not easily located and may be so dispersed that the application of insecticides to kill the immature stages is not practical. In some situations modifications

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