Sweet itch is the most common chronic skin disease affecting horses 1. Approximately 10% of horses worldwide suffer from recurrent sweet itch 1. It is a seasonal condition that typically occurs from early April until early December, depending on the environmental temperature. As sweet itch is closely associated with environmental temperature, it is more prevalent in regions of the world with warmer climates 1. Since 1840, when the first case of sweet itch was diagnosed in a horse, research has focused on identifying the underlying cause of the condition and developing effective treatment strategies 2.
Recent studies have demonstrated that sweet itch is caused by bites from Culicoides midges, which trigger an allergic skin reaction 2. The saliva of Culicoides midges contains proteins that are injected into the horse by the mosquito 3. In horses with a genetic predisposition to sweet itch, or in horses that develop hypersensitivity following insect bites, these proteins can initiate an allergic response 4. The body's allergic reaction to the mosquito bite, triggers the immune system to eliminate the non-body proteins.
The immune response can be divided into two types. A Type I hypersensitivity reaction is an immediate immune response 5. During this reaction, immunoglobulin E (IgE) antibodies are produced and bind to mast cells and basophils, two types of white blood cells that play an important role in the immune system 6. Binding of IgE antibodies to these cells triggers the release of inflammatory mediators, including histamine, resulting in an inflammatory response 6.
The second mechanism, known as a Type IV hypersensitivity reaction, develops approximately 24 hours after exposure to the allergen 7. During this response, T lymphocytes are activated and release cytokines that recruit additional white blood cells to the site of the allergic reaction 7. This subsequently leads to inflammation 7. In most cases, the immune response is dominated by either a Type I or a Type IV hypersensitivity reaction. However, both mechanisms may occur simultaneously in some horses 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, summer eczema 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 summer eczema 9. In addition, the lesions caused by excessive rubbing can also cause secondary inflammation on the skin 10.
Management and Preventive Measures
At present, there is no specific treatment capable of curing sweet itch. Current management strategies focus on controlling the clinical signs and implementing preventive measures to minimise exposure to biting midges.
Fly rugs that cover the horse while it is turned out in the field or paddock are widely used as a preventive measure. In addition, specially designed sweet itch rugs can be used. These lightweight rugs provide a physical barrier against midge bites and generally offer more extensive body coverage than standard fly rugs.
The use of fly repellents containing pyrethroids is also recommended to deter biting midges 11.
Culicoides midges breed and congregate in damp environments such as muddy areas, manure and near rivers or standing water 12. It is therefore advisable to graze horses on well drained pastures, remove manure from fields regularly and avoid grazing horses close to rivers or ponds to reduce exposure to biting midges.
Biting midges are generally most active around sunrise and during the early evening. For this reason, it is recommended to stable horses during these periods of peak insect activity 12.
As Culicoides midges are poor fliers in windy conditions, the use of fans in stables can help reduce midge activity around horses 11.
The Supportive Role of Nutrition and Dietary Supplements
In addition to the management practices described above, dietary supplements with anti-inflammatory or antioxidant properties may help support horses with sweet itch by reducing clinical signs and assisting with long-term management of the condition.
Omega-3 fatty acids possess anti-inflammatory properties and contribute to the maintenance of healthy skin and coat condition 13. Furthermore, research has shown that supplementation with linseed oil, which is naturally rich in omega-3 fatty acids, improves skin condition and reduces skin inflammation in horses affected by sweet itch 14.
Biotin (vitamin B7) plays an important role in maintaining healthy skin. Brewer's yeast is naturally rich in B vitamins, including biotin, and supplementation may therefore have beneficial effects on skin health. A human study demonstrated improvements in skin quality following supplementation with brewer's yeast 15.
Vitamin E functions as a powerful antioxidant, neutralising free radicals and protecting the skin against oxidative damage. A study in dogs concluded that vitamin E supplementation improved skin quality and promoted the healing of skin lesions 16. In addition, vitamin E has beneficial effects on immune function, which may be advantageous for horses with sweet itch, where immune responses play a central role in the development of the disease 17.
Products within the Synovium supplement range that may help support horses with sweet itch include Synovium Linseed Oil, Synovium Myocare-E and Synovium Immunomodulator.
Referenties
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.