Immunopathological study on equine insect hypersensitivity ("kasen") in Japan.

Equine "kasen" is a chronic dermatitis that occurs especially during the summer months. In the present study, skin lesions of kasen that were collected by biopsy from May to October were classified histopathologically into three stages: initial (Group I, 30 cases), developing (Group II, 48 cases) and regressing (Group III, 13 cases). The characteristic lesions were hyperkeratosis, an increase in the number of T-lymphocytes and Langerhans cells (positive for class II MHC) at the epidermo-dermal junction, oedema of the dermis and perivascular infiltration of eosinophils and mast cells. Ultrastructurally, numerous Birbeck granules were observed in the cytoplasm of Langerhans cells. These lesions were striking in Group II and less so in Group III. Kasen is therefore considered to be similar to Queensland itch and sweet itch as described in Australia and Britain, respectively. The histological features were characteristic of a type I and type IV allergic dermatitis.