Equine Aural Plaques Study Guide

Overview and Clinical Importance

Aural plaques (also known as papillary acanthoma, pinnal acanthosis, or hyperplastic aural dermatitis) are a common, benign papillomavirus-induced skin condition affecting the inner surface of the equine pinna. These lesions are characterized by well-demarcated, depigmented, hyperkeratotic plaques that rarely spontaneously resolve. While typically asymptomatic and primarily a cosmetic concern, aural plaques can cause significant ear sensitivity and behavioral issues in affected horses, making this a frequently tested topic on the NAVLE.

The clinical prevalence of aural plaques in horses is significant, with studies demonstrating that at least 22% of horses are affected, and some regional studies in Brazil have found prevalence rates as high as 14.8% among examined populations. Understanding the etiology, clinical presentation, and management of this condition is essential for veterinary practice and board examination success.

Etiology and Pathogenesis

Causative Agent

Aural plaques are caused by infection with Equus caballus papillomavirus (EcPV). Multiple viral types have been identified in association with aural plaques, with EcPV types 3, 4, 5, and 6 being most commonly detected. Recent molecular studies have demonstrated that EcPV-4 is the most prevalent type, found in up to 84% of affected horses, followed by EcPV-3, EcPV-6, and EcPV-1. Coinfection with multiple EcPV types is common, occurring in approximately 59% of cases.

Equus caballus Papillomavirus Types Associated with Aural Plaques

Vector Transmission

Black flies (Simulium spp.) serve as the mechanical vector for EcPV transmission. These hematophagous insects are most active at dawn and dusk, targeting the head, ears, and ventral abdomen of horses. The flies breed in fast-flowing, highly oxygenated water sources such as rivers and streams. The location of aural plaques on the inner pinna correlates with the preferred feeding sites of Simulium species, supporting the vector transmission hypothesis.

Additional factors potentially contributing to transmission include:

• Direct contact with contaminated fomites (shared tack, grooming equipment)
• Environmental contamination
• Culicoides biting midges (less commonly implicated)

Clinical Presentation

Signalment

Aural plaques can affect horses of any age, breed, or sex, though horses under 1 year of age are rarely affected. No definitive breed predisposition has been established, although some studies suggest Mangalarga Marchador horses may have higher susceptibility compared to Quarter Horses. The condition is more commonly observed in horses managed in semi-intensive systems and those subjected to ear grooming or clipping.

Clinical Signs and Lesion Characteristics

Behavioral Signs

While many horses with aural plaques are asymptomatic, affected horses may exhibit:

• Ear sensitivity or head shyness
• Resistance to bridling or haltering
• Head shaking or tossing
• Ear-laying or pulling away when ears are touched
• Aversion to ear clipping or cleaning

Diagnosis

Clinical Diagnosis

Diagnosis of aural plaques is typically based on characteristic clinical appearance and does not routinely require laboratory confirmation. The pathognomonic presentation of well-demarcated, depigmented, hyperkeratotic plaques on the inner pinna is usually sufficient for diagnosis.

Diagnostic Methods

Histopathologic Findings

When biopsy is performed, characteristic histopathologic findings include:

• Mild papillated epidermal hyperplasia (acanthosis)
• Marked orthokeratotic hyperkeratosis
• Koilocytosis (cytopathic effect of papillomavirus - enlarged cells with perinuclear clearing and hyperchromatic, irregular nuclei)
• Hypergranulosis with enlarged, irregular keratohyalin granules
• Hypomelanosis (decreased pigmentation)
• Intranuclear viral particles (visible on electron microscopy)

Memory Aid - "KHAKI" for Histopath Findings: K = Koilocytosis, H = Hyperkeratosis, A = Acanthosis (epidermal hyperplasia), K = Keratohyalin granules (enlarged), I = Intranuclear viral particles

Differential Diagnosis

The most important differential to distinguish from aural plaques is the auricular sarcoid. This distinction is critical as management differs significantly.

Treatment

Treatment of aural plaques is generally not required in asymptomatic horses, as the condition is benign and primarily cosmetic. The decision to treat should be based on whether the plaques cause functional problems (ear sensitivity, bridling difficulties) or significant owner concern.

Treatment Options

Imiquimod Treatment Protocol

Imiquimod 5% cream (Aldara) is an immune response modifier with potent antiviral activity that represents the most effective treatment for aural plaques when intervention is desired.

• Mechanism: Toll-like receptor 7 agonist; stimulates local cell-mediated immune response
• Application: Apply thin layer 2-3 times weekly on non-consecutive days, every other week
• Duration: 1.5-8 months (median 2.9 months)
• Pre-treatment: Crust removal required; sedation needed in 75% of horses
• Adverse effects: Marked local inflammation, exudation, thick crust formation; may temporarily worsen head shyness

Prognosis and Complications

The prognosis for horses with aural plaques is excellent from a health standpoint, as the lesions are benign and do not affect systemic health. Key prognostic considerations include:

• Chronicity: Lesions are typically non-self-limiting and persist indefinitely without treatment
• Recurrence: High recurrence rate even after successful treatment due to viral persistence
• Malignant transformation: Rare; one case report of progression to SCC in a 28-year-old horse with chronic aural plaques and EcPV-4 detection
• Secondary infection: Plaques are susceptible to fly bites and secondary bacterial infection
• Economic impact: In some regions, horses with visible aural plaques may be excluded from exhibitions or have reduced commercial value

Prevention

Prevention strategies focus on reducing exposure to the vector (black flies) and minimizing potential fomite transmission:

• Fly masks with ear covers: Physical barrier to prevent fly access to ears
• Fly repellents: Apply to ears and head; products such as Deosect (permethrin-based)
• Stabling at high-risk times: House horses indoors during dawn and dusk when black flies are most active
• Environmental management: Avoid pastures near fast-flowing water sources; use fans in stables
• Equipment hygiene: Disinfect shared tack and grooming tools with povidone-iodine
• Avoid ear clipping: Ear grooming has been associated with increased prevalence of coalescing lesions