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Medical Index
First Posted: Aug 21, 2010
Aug 21, 2010

Horse Ear Plaque, Aural Plaques, Papillary Acanthoma, Ear Papillomas

by Debora Johnson

Ear Plaque Image

I researched this topic at the request of my friend, Colleen.

Have you ever noticed a white, thick and crusty inside your horse's ear? Is your horse sensitive when you try to touch that in his ear? Do you see lesions inside the ear, as well? It is most likely that your horse may have something called aural plaques, papillary acanthom, pinnal dermatitis or ear papillomas. It is a problem that has to be treated and is often quite resistant to that treatment.

What Causes Aural Plaques?

The stable fly, Stomoxys calcitrans, is one culprit in horses."The fly bite causes small papules and wheals with central hemorrhagic crusts that itch. Lesions are found on the tips or on the folded surface of the pinnae...and in horses this fly can cause a hypersensitivity reaction or severe dermatitis resulting in lesions on the dorsal and/or ventral trunk and face in addition to the pinna. Treatment includes fly repellents, controlling the fly population with environmental clean up (manure, etc), and insecticides."...Equine aural plaques (papillary acanthoma, ear papillomas) are caused by a papillomavirus. Black flies (Simulium spp) are likely the mechanical vector. These 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 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 mildy papillated epidermal hyperplasia and marked hyperkeratosis. Increased size of keratohyalin granules, poikilocytosis, and hypomelanosis may also be present in the epidermis. Intranuclear viral particles have been seen in electron microscopic studies. Treatment includes frequent applications of fly repellent and stabling the horse during the fly's feeding times. Lesions typically do not regress." Vet Merck Manual

Insects and parasites are also culprits. They can cause pinnal dermatitis, too. The parasite or insect bite cause the horse to have a reaction that is hypersensitive. Ticks can also cause this problem. "Nonburrowing psoroptic mites cause a pruritic otitis externa in horses. Horses may present with head shaking and a drooping ear. Diagnosis is confirmed by finding the mites on skin scraping or in otic exudate, but mites may be difficult to find in the ear canal. Psoroptic mange is a reportable disease in some regions..." Vet Merck Manual You may see your horse shaking his head, rubbing his head, ears and face, or drooping ears. Both the animal and the environment should be treated. Pyrethrin/pyrethroid products are effective. Of course, you will see the white, thick crust inside your horse's ear as described above.


The University of Minnesota has done research on this subject. The following was taken from their website: "Many treatments have been tried to remove the aural plaques but none have been shown consistently effective. Recent research at the UMN suggests imiquimod (AldaraTM) can help clear the aural plaques and lessen the ear sensitivity..." Aldara

Also the Vet Merck Manual suggests that "Ivermectin at 200 mg/kg, PO, every 2 wk for 2 treatments has been shown to be effective."

Some say that it is quite resistant to treatment. Always check with your vet.

Medical Index