Proliferative and necrotizing otitis externa in four cats.

Proliferative and necrotizing feline otitis externa is a rare disorder of unknown aetiology. This condition was diagnosed by skin biopsy in three adult domestic shorthair cats (3-5 years old) and one kitten (6 months old). The affected cats had large tan to dark brown-black coalescing plaques covering the concave surface of the pinnae and external ear canals. Friable material from the plaques and a thick exudate occluded the ear canals. The cats had a secondary bacterial and/or yeast otitis. Prior to the histopathological diagnosis, all cats received numerous otic preparations as well as oral antibiotics and corticosteroids without resolution. Histologically, all cases had strikingly similar changes; acanthosis with pronounced hair follicle outer root sheath hyperplasia and neutrophilic luminal folliculitis, follicular keratosis and individually necrotic keratinocytes in the outer root sheath of hair follicles. One case was documented via skin biopsy to have persisted for 4 years. The adult cats were treated with topical 0.1% tacrolimus and all showed marked improvement although one cat was lost to follow up. The lesions completely resolved with topical tacrolimus alone in one cat and topical tacrolimus in addition to oral prednisolone in another cat.