Sweet itch the most common chronic skin condition in horses 1. About 10% of horses worldwide suffer from recurrent sweet itch 1. Sweet itch is a seasonal condition and occurs from early April to early December, depending on the environmental temperature. Because sweet itch is related to the temperature of the environment, this skin condition is more common in parts of the world where the average environmental temperature is higher 1. Since 1840, the year the first horse was diagnosed with sweet itch, research has been carried out to find a possible cause of this condition and search for effective treatments 2.
Recent studies have shown that sweet itch is caused by mosquito bites from the Culicoides mosquito, resulting in an allergic reaction on the skin 2. The saliva of the Culicoides mosquito contains proteins which are injected into the horse's body by the mosquito 3. These proteins initiate an allergic reaction in horses that either have a genetic predisposition to this type of skin condition, or in horses that will develop a hypersensitive response after the insect bite 4. The body's allergic reaction to the mosquito bite, triggers the immune system to eliminate the non-body proteins.
The reaction that takes place in the body can be divided into two types. A Type I hypersensitive allergic reaction is a response from the immune system taking place immediately after an insect bite 5. This type of response involves the production of IgE antibodies that bind to mast cells and basophils, two types of white blood cells important to the immune system 6. The binding of the antibodies and white blood cells releases substances, including histamine, which then lead to an inflammatory response in the body 6.
The second reaction, also called a Type IV hypersensitive allergic reaction, is one that is triggered in the body 24 hours after allergen exposure 7. Here, T lymphocytes are triggered to produce cytokines that attract white blood cells to the site of the allergic reaction in the body 7. This then triggers an inflammatory response 7. In most cases, the body responds through one of these two reactions. However, it is possible that both reactions take place in the body 4.
The inflammatory response that follows in the body leads to extreme itching on the horse's skin 8. Because the horse experiences extreme itching, sweet itch is mainly characterised by bald patches around the mane and tail due to excessive rubbing to relieve itching 5,9. In addition, bald spots can also develop on the belly, head and chest but are less common 9. In addition to these bald patches, a thickening of the skin, local alopecia and the appearance of lesions are also common symptoms of sweet itch 9. In addition, the lesions caused by excessive rubbing can also cause secondary inflammation on the skin 10. Management and Preventive Measures
Currently, there is no specific treatment to completely cure sweet itch. Current treatments mainly consist of managing the symptoms of sweet itch and implementing preventive measures to avoid the condition.
Fly rugs that cover the horse's body when they are at pasture in paddocks are a commonly used preventive measure. Besides a fly rugs, an eczema rug can also be used. These rugs are made of a thin fabric that protects the horse from mosquito bites. The difference with a fly rug is that eczema rugs often protect a larger part of the body from mosquito bites. In addition, it is recommended to use a fly repellent body spray with the ingredient pyrethroids to keep mosquitoes at bay 11.
Mosquitoes mainly house near damp and wet places, such as mud, manure or rivers 12. It is therefore recommended to turn horses out in drier pastures, regularly remove manure from pastures and keep the horse away from rivers or ponds to avoid contact with mosquitoes.
Mosquitoes are often most active at dawn and early evening. It is therefore recommended to keep horses inside during this time of day 12.
Mosquitoes cannot fly in a windy environment, therefore a fan at the stable can keep mosquitoes away 11.
Supportive Function of Nutrition and Supplements
In addition to the management measures mentioned above the use of supplements, with for example anti-inflammatory or antioxidant effects, can provide support to horses affected by sweet itch through reducing symptoms and managing the condition.
Omega-3 fatty acids have anti-inflammatory effects and support the maintenance of a healthy skin and coat 13. In addition, research has shown that the use of flaxseed oil, an oil with a high concentration of Omega-3 fatty acids, improves the skin quality of horses with sweet itch and reduces inflammation on the skin 14.
Biotin, or Vitamin B7, is important for maintaining healthy skin. Brewer's yeast contains high levels of B-Vitamins and Biotin so supplementing it can have a positive effect on the skin. A study with humans showed that skin quality improved after taking brewer's yeast 15.
Vitamin E has an antioxidant effect in the body which means it removes free radicals from the skin, thus preventing skin damage. A study with dogs concluded that supplementing Vitamin E results in improved skin quality and recovery from skin injuries 16. Vitamin E supplementation also has a positive effect on immune system function which is beneficial for horses with sweet itch where the immune system is not functioning optimally 17.
Supplements from the Synovium Supplement line that can provide support to horses with sweet itch are Synovium Linseed Oil, Synovium Myocare-E and Synovium Immunomodulator.
References
1. Fettelschoss-Gabriel, A., Fettelschoss, V., Thoms, F., Giese, C., Daniel, M., Olomski, F., Kamarachev, J., Birkmann, K., Bühler, M., Kummer, M., Zeltins, A., Marti, E., Kündig, T. M., & Bachmann, M. F. (2018). Treating insect-bite hypersensitivity in horses with active vaccination against IL-5. Journal of Allergy and Clinical Immunology, 142(4): 1194-1205.
2. Schaffartzik, A., Hamza, E., Janda, J., Crameri, R., Marti, E., & Rhyner, C. (2012). Equine insect bite hypersensitivity: What do we know?. Veterinary Immunology and Immunopathology, 147(3–4): 113-126.
3. Wilson, A. D., Harwood, L. J., Björnsdottir, S., Marti, E., & Day, M. J. (2001). Detection of IgG and IgE serum antibodies to Culicoides salivary gland antigens in horses with insect dermal hypersensitivity (sweet itch). Equine Veterinary Journal, 33(7): 707-713.
4. Wilson, A. D., Heesom, K. J., Mawby, W. J., Mellor, P. S., & Russell, C. L. (2008). Identification of abundant proteins and potential allergens in Culicoides nubeculosus salivary glands. Veterinary Immunology and Immunopathology, 122(1–2): 94-103.
5. Wilson, A. D. (2014). Immune responses to ectoparasites of horses, with a focus on insect bite hypersensitivity. Parasite Immunology, 36(11): 560-572.
6. Wagner, B., Miller, W. H., Morgan, E. E., Hillegas, J. M., Erb, H. N., Leibold, W., & Antczak, D. F. (2006). IgE and IgG antibodies in skin allergy of the horse. Veterinary Research, 37(6): 813-825.
7. Averbeck, M., Gebhardt, C., Emmrich, F., Treudler, R., & Simon, J. C. (2007). Immunologic principles of allergic disease. Journal of the German Society of Dermatology. 5(11): 1015-1027.
8. Jonsdottir, S., Cvitas, I., Svansson, V., Fettelschloss-Gabriel, A., Torsteinsdottir, S., & Marti, E. (2019). New Strategies for Prevention and Treatment of Insect Bite Hypersensitivity in Horses. Current Dermatology Reports, 8(4): 303-312.
9. Björnsdóttir, S., Sigvaldadóttir, J., Broström, H., Langvad, B., & Sigurosson, Á. (2006). Summer eczema in exported Icelandic horses: Influence of environmental and genetic factors. Acta Veterinaria Scandinavica, 48(1): 1-4.
10. Broström, H., Larsson, & Troedsson, M. (1987). Allergic dermatitis (sweet itch) of Icelandic horses in Sweden: An epidemiological study. Equine Veterinary Journal, 19(3): 229-236.
11. Chapman, S. (2019). Get ahead of sweet itch. Equine Health , 46: 36–37.
12. Clarke, A. (2016). The itch that’s less than sweet. Equine Health , 29: 36–38.
13. Elghandour, M. M. M. Y., Kanth Reddy, P. R., Salem, A. Z. M., Ranga Reddy, P. P., Hyder, I., Barbabosa-Pliego, A., & Yasaswini, D. (2018). Plant Bioactives and Extracts as Feed Additives in Horse Nutrition. Journal of Equine Veterinary Science, 69: 66-77.
14. O’Neill, W., McKee, S., & Clarke, A. F. (2002). Flaxseed (Linum usitatissimum) supplementation associated with reduced skin test lesional area in horses with Culicoides hypersensitivity. Canadian Journal of Veterinary Research, 66(4): 272-277.
15. Hibino, S., Hamada, U., Takahashi, H., Watanabe, M., Nozato, N., & Yonei, Y. (2010). Effects of Dried Brewer’s Yeast on Skin and QOL. Anti-Aging Medicine, 7(4): 18-25.
16. Kapun, A. P., Salobir, J., Levart, A., Kalcher, G. T., Svete, A. N., & Kotnik, T. (2014). Vitamin E supplementation in canine atopic dermatitis: Improvement of clinical signs and effects on oxidative stress markers. Veterinary Record, 175(22): 1-5.
17. Petersson, K. H., Burr, D. B., Gomez-Chiarri, M., & Petersson-Wolfe, C. S. (2010). The influence of vitamin E on immune function and response to vaccination in older horses. Journal of Animal Science, 88(9): 2950-2958.
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