Pruritus in Horses: Causes and Control

Pruritus often leads to self-inflicted trauma, alopecia, and moderate to severe secondary skin lesions resulting in horses which are visually disfigured and/or unsound for work. Because of the many possible diagnosis of a pruritic skin disease the history, especially the environment, seasonality, age of onset and progress of the disease, is very important in making a diagnosis. Time spent obtaining a detailed, complete history is rewarding and often equally as important as the examination and diagnostic tests. A large percentage of horses affected with aseasonal pruritic dermatitis are hypersensitive to the bites of insects. Any biting insect, including Culicoides spp. (No-see-ums, midges, punkies), stable flies (Stomoxys calcitrans), horn flies (Hematobia sp), black flies or "buffalo gnats" (Simulian spp.), horse and deer flies (Tabanidae and Chrysops spp.), mosquitoes (primarily Culex and Aedes spp.), can cause insect hypersensitivity dermatitis. "Chiggers" (Trombiculids) and ticks can also lead to hypersensitivity reactions. Insect bite induced hypersensitivities are characterized by intense pruitus which often leads to excoriation, extensive hair loss, secondary infections and chronically to hyperkeratosis and lichenification. Several different clinical syndromes have been associated with hypersensitivities to different insects. Queensland or Sweet Itch, initiated by the bites of hematophagous Culicoides species, is classically characterized by a diffuse dorsal distribution with severe involvement of the mane and tail area. However, there are several species of Culicoides in different geographic areas and different species have different preferred areas of feeding. There are, for example, over 1000 species of Culicoides worldwide and as many as 30 or 40 species active in any one area. Work by Greiner et al in Florida has demonstrated multiple species feed on horses in different regions of Florida. These different species have variable feeding patterns with some species having a predilection for biting the ventral aspect of the body instead of the dorsal. a The only region generally not affected by Culicoides is the flank. An hereditary predisposition to develop hypersensitivity to Culicoides spp. has been shown in some breeds of horses and clinical evidence supports a familial tendency for hypersensitivity in many, if not all, breeds . Bites of horn flies have been implicated in Ventral Midline Dermatitis, which is characterized by sharply demarcated focal lesions which often ooze serum. Bites of Simulian spp. are most commonly associated with lesions on the head and within the inner pinnae, but in "swarm attacks" may bite any region of the horse. A generalized ventral distribution of lesions may also be caused by black flies. Simulian flies are blood suckers and bites are covered with small accumulations of dried blood. In mass attacks urticaria and angioedema may occur and can be associated with systemic signs of weakness with an increased pulse and respiratory rate, particularly in small young animals. Stable flies are vicious biters (as opposed to house flies (Musca spp) that feed on body secretions). Initial lesions may consist of erythema, wheals, and raised nodules with a central crust. Exudative dermatitis of the legs due to reactions to extensive stable fly bites and/or urticaria and large nodules that are pruritic characterize hypersensitivities to stable flies. b