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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. It is also found in Canada, Australia, the US and many other parts of the world.
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, although some workers claim that the larger proteins (of molecular weight 65kDa) are the most important. 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.
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.
Few treatments are fully efficacious once lesions have appeared. The only effective form of treatment is preventitive - i.e. prevent further insect bites - so these techniques will also be discussed here. Treatments generally fall into one of the following categories:
1) Insecticides and Repellents: These may be applied to the horse or its environment. The most commonly used and effective are permethrins. and benzyl benzoate Citronella has been used, with variable effect. Some sources advocate draining of any stagnant water or pools, to reduce the local breeding potential of the insects. Midge numbers are often larger near water sources, for example ponds, bogs, and slow moving water. Moving the horse away from these areas may help to prevent further problems.
2) Barrier Techniques: Rugs etc., that prevent flies and midges settling on the animal's skin to bite. These include "Boett Rugs" and fly masks. In addition, thin screens may be placed over stable doors and windows to exclude biting insects. Stabling the horse at times of day when the midges are most prevalent is also quite effective.
3) Immunotherapy: A wide variety immunotherapy and desensitisation protocols have been trialled in attempts to reduce or modify the immune response, with considerable success rates. So far, there appear to a significant benefit in more than 80% of equine cases. This particular BioEos product is now available through The National Sweet Itch Centre in the UK and ProVet in the EU. The underlying immune modulation is now proven to shift the immune system from a Th2 to a Th1 mode. BioEos is a research and development company with worldwide patents derived from many years of research at University College London and the many applications are being developed for use in both human, agricultural and aquaculture treatments. Current clinical trials for the treatment of pancreatic cancers (Immodulon Therapeutics) and the treatment of other chronic immune deficiency disorders (ActinoPharma) are putting this research into practical effect.
4) Nutritional supplements: Various supplements may be effective in individuals, including fatty acid supplemantation and linseed oil. However, although owners perceived an improvement, this was not bourne out by objective statistical analysis.
5) Symptomatic Control: Control of symptoms to some degree can be achieved with antihistamines (especially hydroxyzine, and with corticosteroids, although the potential side effects (e.g. laminitis, immune suppression) make this a less preferred option. In addition, antibiotics may be required to manage any secondary infection.
6) Alternative Medicines: A wide variety of herbal, homeopathic and other alternative remedies have been suggested. Among the natural remedies suggested are sulfur, wild geranium (as the base for a shampoo), Lavender oil, Aloe vera (to reduce the itching).
Overall, the wide variety of treatments proposed leads to the conclusion that no one method is universally effective.
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