be ingested in high doses as the liver is able to neutralize small amounts of photodynamic compounds. Grey horses and the white markings on coloured horses are more prone to photosensitization as white areas lack . Melanin in the dead keratinised layer absorbs UV radiation preventing it from penetrating to deeper, more vulnerable layers. . Horses with liver damage are also more prone as the liver is compromised and unable to neutralize toxic compounds. Culicoides Hypersensitivity Culicoides hypersensitivity is known to occur in horses. It is considered the most common hypersensitivity reaction in horses. Culicoides hypersensitivity is referred to by many different names as it is so widespread. Some of the common terms used for Culicoides hypersensitivity are “sweet itch”, “summer eczema” “seasonal dermatitis” and “Queensland itch”. Culicoides hypersensitivity is a type I hypersensitivity reaction to antigens in the saliva of female midges or of the Culicoides spp. group. Blackflies, stable flies & horn flies have also been reported to cause the same reaction.
Culicoides is strictly an allergic reaction, therefore no transmission.
Type I hypersensitivity reactions involve the release of IgE and inflammatory mediators such as histamine and cytokinins into the area causing inflammation & intense pruritis.
Horses that are turned out from in the summer are more prone to developing culicoides hypersensitivity as these are the gnat’s preferred feeding times. Horses turned out near swampy or stagnant water areas will also be at increased risk. Thoroughbreds & Iceland ponies are the most reported breeds affected. to In response the horse tends to rub and itch the bite areas due to intense pruritis & pain causing hair matting, alopecia, scale and crust formation, and excoriation or ulcers. Chronic cases of culicoides hypersensitivity may result in lichenification of the withers and rump areas with hyperkeritanization. There are three distinct feeding patterns of gnats and midges. The first pattern is restricted to the dorsal midline, the second to the ventral midline and the third includes both the dorsal and ventral midlines. The dorsal pattern of feeding tends to be more prevalent.
Diagnosis of culicoides hypersensitivity can be made through clinical signs and symptoms or by skin biopsy showing basophilia, eosinophilia & increased IgE.
Treatment is . Administration of oral or topical antihistamines and corticosteroids help to suppress the immune response and decrease inflammation. Topical antibiotic may be required for secondary bacterial infections that develop in excoriated skin.
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