Alopecia Areata with Lymphocytic Mural Folliculitis Affecting the Isthmus in a Thoroughbred Mare

A 13-year-old thoroughbred mare exhibited multifocal, generalized, noninflammatory alopecia over 8 years, with recent alopecia, erythema, and scaling on the forehead and muzzle. Histopathological examination revealed a lymphocytic inflammatory infiltrate around anagen hair bulbs, indicating alopecia areata. Notably, the forehead biopsy showed atrophic hair follicles with perifollicular and mural mononuclear folliculitis at the isthmus, confirmed as T-lymphocytic via CD3 marker staining. The simultaneous lymphocytic infiltration at both bulbar and isthmic levels in the same horse was unusual, suggesting a variation in the histological presentation of alopecia areata.