Aural Plaques in Horses

Equine aural plaques, also known as papillary acanthoma or ear papillomas, are caused by papillomaviruses. Aural plaques in horses have been associated with Equus caballus papillomavirus (EcPV) types 1, 3, 4, 5, and 6; intranuclear viral particles have been observed in electron microscope studies.

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.

Lesions are characterized by depigmented, hyperkeratotic, coalescing papules and plaques localized to the concave aspect of the pinna (see plaques image). Often, both pinnae are affected. Lesions are usually not associated with systemic clinical signs; however, in some cases, the fly bite itself causes dermatitis and discomfort.

Equine aural plaques

Image

Coalescing plaques in the concave aspect of the pinna in a horse.

Courtesy of Dr. Sheila Torres.

Histologically, lesions are characterized by mild, papillated epidermal hyperplasia and marked hyperkeratosis. Increased size of keratohyalin granules, koilocytosis, and hypomelanosis might also be present in the epidermis.

Various treatment regimens yielding minimal to no response have been reported anecdotally. Two studies have found imiquimod cream to be effective in treating equine aural plaques (1, 2); however, the severe inflammation induced by the drug (see inflamed plaques image) makes imiquimod difficult to use, and most horses require sedation. The recommended protocol consists of applying a thin layer of 5% imiquimod cream 2–3 times weekly (on nonconsecutive days) every other week or every 48 hours until clinical resolution (1, 2).

Inflamed equine aural plaques after imiquimod therapy

Image

Cutaneous reaction to imiquimod 5% cream applied to the pinna of a horse with aural plaques.

Courtesy of Dr. Sheila Torres.

Frequent applications of fly repellent and stabling the horse during the feeding times of flies are important measures for alleviating discomfort and preventing recurrence. Lesions typically do not regress spontaneously.