About 10% of horses worldwide suffer from IBH, an allergic, recurrent seasonal dermatitis. | Photo: The Horse Staff

The season is upon us when approximately 10% of horses worldwide are faced with one of the most frustrating skin diseases in the equine industry, insect bite hypersensitivity (IBH), more commonly known as “sweet itch” or “summer eczema.” This an allergic, recurrent, seasonal dermatitis develops when the immune system overacts or becomes “hypersensitive” to allergens in the saliva primarily of biting midges (Culicoides) and other biting insects.

Although there is no cure for IBH, there is ongoing research to assess immunotherapies as future treatment options for horses suffering with IBH.

While IBH has a multifactorial cause, it is acknowledged that the immune system plays a role in its development. Hypersensitivity reactions are classified into four types, and IBH is a combination of Type I and Type IV. Type I hypersensitivity reactions develop when antigen specific allergens bind to IgE and cross-link receptors on mast cells, basophils, and eosinophils. These cells release vasoactive amines, such as histamine, that affect blood vessel size and leakage and produce inflammatory cytokines and other mediators, which result in inflammation and pruritus (itchiness). Type IV reactions, often referred to as delayed-type hypersensitivity reactions, involve cell-mediated responses particularly of activated Th-2 lymphocytes which produce IL-5 cytokine and large numbers of eosinophils, both contribute to clinical IBH manifestations