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Sweet itch
Also known as Queensland Itch, Seasonal Recurrent Dermatitis (SSRD) , Summer Itch or more technically, ''Culicoides'' Hypersensitivity.
Sweet Itch is a medical condition in equines caused by an allergic response to the bites of ''Culicoides'' midges. It may be found in any horses and ponies, especially in the warmer regions. It may also occur, too, in other equines.〔(Queensland Itch by Dr Carl Eden BVM&S MRCVS ) Retrieved 2011-2-6〕 It is also found in Canada, Australia, the US and many other parts of the world.〔GS Anderson, P Belton, & N Kleider (1988) "The Hypersensitivity of Horses to Culicoides Bites in British Columbia", ''Can Vet J.'' 29(9): 718–723〕
==Causes==
A hypersensitivity reaction to specific allergens (protein molecules causing an extreme immune response in sensitised individuals) in the saliva of ''Culicoides'' midges. There are multiple allergens involved,〔W. Hellberga, A.D. Wilsonb, P. Mellorc, M.G. Doherra, S. Torsteinsdottird, A. Zurbriggena, T. Jungie and E. Marti (2007) "Equine insect bite hypersensitivity: Immunoblot analysis of IgE and IgG subclass responses to Culicoides nubeculosus salivary gland extract", ''Veterinary Immunology and Immunopathology'' Volume 113, Issues 1-2, Pages 99-112〕 although some workers claim that the larger proteins (of molecular weight 65kDa) are the most important.〔E. Ferroglio, P. Pregel, A. Accossato, I. Taricco, E. Bollo, L. Rossi, A. Trisciuoglio (2006) "Equine Culicoides Hypersensitivity: Evaluation of a Skin Test and of Humoral Response", ''Journal of Veterinary Medicine'', Series A 53 (1), 30–33〕 These allergens appear to be cross-reactive across many species of ''Culicoides'' - i.e. many different varieties of midges produce similar allergens, giving the same effects upon horses.〔Langner, KFA, Darpel, KE, Denison, E, Drolet, BS, Leibold, W, Mellor, PS, Mertens, PPC, Nimtz, M, Greiser-Wilke, I, (2007) "Collection and Analysis of Salivary Proteins from the Biting Midge Culicoides nubeculosus (Diptera: Ceratopogonidae)", ''Journal of Medical Entomology'', Volume 44, Number 2, March 2007 , pp. 238-248(11)〕
The hypersensitivity response is mediated by IgE, an antibody produced by the horse's immune system which binds the allergens, causing a cascade production of histamine and cytokines which make the horse's skin inflamed and itchy. Of these, histamine appears the most important in the initial phase of reaction.〔A. P. Foster, J. McKelvie, and F. M. Cunningham (1998) "Inhibition of antigen-induced cutaneous responses of ponies with insect hypersensitivity by the histamine-1 receptor antagonist chlorpheniramine", ''The Veterinary Record'', Vol 143, Issue 7, 189-193〕
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