These plaques, also known as papillary acanthoma or ear papillomas, are caused by a papillomavirus. Black flies (Simulium spp) are likely the mechanical vector. The flies are active at dawn and dusk, when they attack the head, ears, and ventral abdomen of horses. Clinically, the lesions are characterized by depigmented, hyperkeratotic, coalescing papules and plaques localized to the concave aspect of the pinna. Often both pinnae are affected. Similar lesions may rarely be present around the anus and external genitalia. Lesions are usually asymptomatic, but in some cases the direct effect of the fly bite causes dermatitis and discomfort. Histologically, the lesions are characterized by mild papillated epidermal hyperplasia and marked hyperkeratosis. Increased size of keratohyalin granules, koilocytosis, and hypomelanosis may also be present in the epidermis. Intranuclear viral particles have been seen in electron microscopic studies. Currently, there is no documented efficacious therapy. Anecdotal reports suggest that imiquimod cream is effective in the treatment of aural plaques; however, the severe inflammation induced by the drug makes this treatment difficult to use, with most horses requiring sedation. The recommended protocol consists of applying imiquimod 2–3 times weekly every other week. Frequent applications of fly repellent and stabling the horse during the fly's feeding times are important measures to reduce discomfort and prevent recurrence. Lesions typically do not regress spontaneously.
Last full review/revision July 2011 by Sheila Torres, DVM, PhD, DACVD; Scott A. Dee, DVM, MS, PhD, DACVM