Aural Plaques (Ear Papillomas) in Horses: Causes, Risk Factors & Treatment

Aural plaques in horses are a skin condition characterized by thick, pale, crusty patches that develop on the inner surface of the ears. These plaques may begin as small, raised bumps and can gradually merge into larger areas of raised, rough tissue.

Although they are usually painless and do not affect a horse’s general health, aural plaques are often persistent and can sometimes lead to increased sensitivity around the ears and head. The condition is linked to a type of papillomavirus (warts) infection, and while not fully understood, transmission is believed to involve biting insects such as black flies.

In most cases, aural plaques are considered a cosmetic issue, but they can complicate handling and grooming if the horse becomes head shy. Treatment is not always necessary, though several topical and surgical options are available when intervention is warranted.

Read on to learn more about aural plaques in horses, including how to recognize symptoms, understand risk factors, and implement effective prevention strategies.

Aural Plaques in Horses

Aural plaques in horses are a skin condition where light-colored, thick, rough patches form on the inner surface of the horse’s ear. ^[1][2][3]

Aural plaques are also referred to as: ^[1]

• Ear papillomas
• Hyperplastic dermatitis of the ear
• “Ear fungus” – a misnomer, as aural plaques are not fungal
• Papillary acanthoma

When aural plaques first develop, they often present as one or more small, disconnected bumps. As the condition progresses, these bumps may grow and merge into larger patches. In severe cases, the entire inner surface of the horse’s ear is covered with aural plaques. ^[4]

Aural plaques are usually considered a cosmetic defect and cause no damage to the horse’s overall health. In some cases, they can cause ear and head sensitivity. There have been a few cases of aural plaques that have progressed into squamous cell carcinoma in horses, but this phenomenon is rare and not fully understood at this time. ^{[1][2][5][6][7]}

While aural plaques are generally a cosmetic concern, horses may be disqualified from joining specific exhibitions and sporting events if plaques are visible. ^[8]

“While aural plaques are rarely harmful in a medical sense, they can lead to increased ear sensitivity, handling challenges, and behavioral issues like head shyness. In many cases, prevention through insect control is more effective than attempting to remove the plaques once they’re established.”

— Dr. Jennifer Skaggs, DVM
Equine Veterinarian

Symptoms

The characteristic symptom of aural plaques is one or more areas of raised tissue on the inside surface of the horse’s ears. ^[6]

Aural plaques begin as discrete bumps or nodules about 1 to 2 mm in diameter. These patches often join together to form thick, rough patches. They are usually greyish-white due to depigmentation of the tissue, with smooth pink tissue under the epidermal layer. They tend to have a shiny surface and are clearly distinct from the normal tissue surrounding them. ^[1][6]

Aural plaques may occur in one or both ears, and usually cause localized hair loss. ^[1][6] In most horses, plaques do not cause the horse pain, itching, or other discomfort. In some cases, affected horses develop increased sensitivity around the ears and head. This sensitivity can continue as prolonged head shyness even after the aural plaques are resolved. ^[5][6][8]

Causes

Aural plaques are associated with equine papillomavirus. ^{[2][4][5][6][7][9]}

In healthy horses, the immune system typically functions well enough to control the spread of papillomavirus within the body. This immune response generally keeps the virus at a manageable level and effectively prevents the development of noticeable clinical signs. If symptoms do arise, the immune system usually eliminates the virus effectively, leading to a resolution of symptoms. ^[1]

On the other hand, horses that suffer from recurring or chronic episodes of this condition are thought to have some level of immunosuppression. ^[1]

Transmission

How this condition spreads between horses or from the environment to the horse is still not fully understood at this time. However, it is currently theorized that transmission may occur through the activity of black flies, specifically Simulium spp.. ^[6]

Simulium Fly

This theory is supported by observations from geographical regions where these fly species are not present. For instance, in countries like New Zealand, where black flies do not exist, there have been no reported cases of this condition, which lends credibility to the hypothesis regarding their role in transmission. ^[6]

Risk Factors

Aural plaques are a common condition in horses. ^[6][9] They typically affect adult horses over the age of 1 year. ^[6]

There does not appear to be any sex or breed predisposition to developing aural plaques. ^[4][6]

Diagnosis

Since the appearance and location of plaques are the hallmark of this condition, diagnosis of aural plaques is usually made based on physical examination. ^[1][6][8]

A biopsy may be warranted to investigate underlying pathology. The biopsy sample is examined for inflammation, infection, or abnormal cells. Additionally, any viruses, proteins, or different cell types may be identified. ^[6]

In cases where diagnosis is confirmed and plaques are causing complications or behavioral changes, treatment may be considered based on the severity and persistence of symptoms.

Treatment

Treatment of aural plaques is not usually necessary as this is usually a cosmetic issue. Some experts suggest that aural plaques tend to resolve on their own in 3 months from the onset of symptoms. ^[1][6] Others indicate that they do not resolve on their own. ^[3]

In cases where treatment is deemed necessary, the treating veterinarian may recommend a topical medication.

Imiquimod

Among the medications available on the market, the most effective is reported to be imiquimod (Aldara^™). This drug is a topical immune response modifier, which means it stimulates the immune system, bolstering its response to abnormal cells and viruses. ^[1][5][6][8]

For imiquimod to reach the viable tissue in the plaques, the topmost crusty layer needs to be removed (debrided). In cases where this is not done thoroughly, the results of treatment are variable. In many cases, debridement of plaques in horses is performed under standing sedation. ^[5]

In most cases, it takes several months of imiquimod treatment for symptoms to improve. This medication may cause side effects, including: ^[5][6][8]

• Redness
• Open sores
• Oozing
• Swelling
• Irritation
• Inflammation
• Crusting

Careful consideration is recommended before choosing this treatment. ^[5][6][8]

Other Topical Medications

There are anecdotal reports of other medications being effective treatments for aural plaques in horses, but research supporting their efficacy is currently lacking. Compounds that may be considered as part of topical preparations for treating equine aural plaques include: ^{[1][10][11][12][13][14][15][16][17]}

• Podophyllin: A cytotoxic plant resin that inhibits cell division; may help remove hyperplastic plaque tissue by promoting localized necrosis
• Trifluoroacetic acid: A strong organic acid that may remove superficial skin layers and stimulate regrowth of normal tissue
• Tretinoin: A topical retinoid that promotes cell turnover and may reduce plaque thickness by normalizing keratinocyte behavior
• Tincture of benzoin: Used for its adhesive and antiseptic properties; may create a barrier and reduce irritation in plaque-prone areas
• Mycobacterials: Immunostimulant preparations derived from bacteria; may enhance local immune response to the papillomavirus
• Propionibacterium acnes products: These bacterial extracts may act as immunomodulators by activating macrophages and other immune pathways
• Cisplatin: A chemotherapy drug used topically in veterinary oncology; may destroy proliferating plaque cells by damaging DNA
• IL-2: Interleukin-2 is an immune cytokine that may be used topically to boost the local T-cell response to virus-infected cells

Certain medications can permanently discolor the treatment site, so it is important to carefully consider potential adverse effects before starting treatment of solely cosmetic cases. ^[1] In many instances, treating plaques may reduce the appearance of the lesions without fully resolving cosmetic concerns.

Surgery

In rare cases, aural plaques are surgically removed to improve the appearance of the horse. This can be performed with cryosurgery, where liquid nitrogen is used to freeze the plaques into necrosis. ^[1]

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Prognosis

Aural plaques are persistent. Once they develop, some do not resolve. However, other than the cosmetic change to the horse’s ear, they do not usually cause any other harm or discomfort. ^[3][6]

In some cases, after resolution of the plaques, the area remains discolored or hairless. ^[5] In rare cases, aural plaques can develop into squamous cell carcinoma. ^[6][7]

Topical treatment of ear plaques requires handling and drug application around the head and ears of the horse. If not applied carefully and patiently, this process can traumatize the horse, which can result in long-term head shyness. ^[5]

Prevention

A vaccine against autogenous tumor cells (also known as warts) is available. Early research suggests this vaccine may be effective in preventing equine viral papillomatosis and therefore aural plaques, although it has yet to be fully studied. ^[1]

Papilloma virus is contagious. Therefore, unaffected horses – especially those with weaker immune systems – should be kept separate from horses that develop aural plaques. Stalls, grooming equipment, and tack should not be shared between affected and unaffected horses until they have been disinfected using povidone-iodine or lye compounds. ^[1]

Since transmission of aural plaques is suspected to be linked to flying insects, particularly by black flies (Simulium spp.), the use of insect sprays and sheets as well as farm-wide insect control protocols is recommended. ^[1][6]

Frequently Asked Questions

Here are some frequently asked questions about aural plaques in horses:

Summary

Aural plaques are a persistent skin condition in horses associated with equine papillomavirus. They are characterized by rough, depigmented patches on the inner surface of the ears. While they are typically cosmetic and non-painful, aural plaques can cause sensitivity and behavioral issues in some horses.

• The characteristic symptom is raised, grey-white patches that may coalesce and cause localized hair loss, sometimes leading to ear and head sensitivity.
• Aural plaques are common in adult horses, with no specific breed or sex predisposition.
• They are linked to papillomavirus and suspected transmission by black flies and contact with contaminated surfaces or equipment.
• Their severity can range from mild, isolated bumps to severe cases covering the entire ear surface; chronic cases may reflect immune suppression.
• Treatments include topical immune stimulants like imiquimod, other topical agents, or surgery in rare cases.
• While they are generally persistent, plaques are not harmful to overall health, though the cosmetic effects of this condition may be long-lasting.
• Aural plaques are best prevented through insect control, hygiene practices, and possibly vaccination.