Ear edge dermatitis: Look beyond scabies

Ear edge dermatitis: Look beyond scabies

If scabies treatments don't work, you may have to dig deeper.
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Mar 01, 2010

Ear edge dermatoses in dogs can consist of alopecia, seborrhea, crusting, ulceration and necrosis. Depending on the cause, pruritus may or may not be present. When pruritus is present, canine scabies is the most likely cause of ear edge crusting in dogs. However, not all ear edge dermatitis is due to scabies. Differential diagnoses, depending on the presence of accompanying clinical signs, include hypothyroidism, light-responsive alopecia, atopy, seborrhea, ear edge vasculitis, frostbite, fly strike and pemphigus foliaceus.

Scabies


Photo 1: Alopecia and crusting of the ear edge in a dog with scabies.
Canine scabies (Photo 1) most commonly affect the ear edges, hocks, elbows and ventral abdomen. If the ear edges are affected, alopecia is the initial skin change followed by crusting, but be aware that some owners may pick off the ear edge crusts. One or both ears may be affected, and the patients are usually pruritic. Always check ear edge scrapings if alopecia or crusting is present, especially if the dog is pruritic. Often a positive pinnal/pedal reflex is present, but this may also be true in atopic patients, particularly in Boxers and Labrador Retrievers.

Endocrine disease


Photo 2: Crusty, seborrheic ear edges in a patient with hypothyroidism.
Endocrine diseases that can produce ear edge changes include hypothyroidism and light-responsive alopecia. The key to differentiating between the two is assessing whether any other clinical signs are present. With hypothyroidism (Photo 2), weight gain, exercise intolerance, hyperpigmentation, seborrhea and symmetrical hair loss may be present. A free thyroxine blood concentration test (free T4) by equilibrium dialysis will help confirm the diagnosis. Light-responsive alopecia (Photo 3) can also present with symmetrical ear edge alopecia, seborrhea and hyperpigmentation. The results of blood work are usually normal, and, in contrast to hypothyroidism, no other systemic clinical signs are usually present. This disease is seen primarily in the Midwest and in northern climates during dark winters, resulting in a lack of natural sunlight exposure. Specific breeds affected include Doberman Pinschers, Labrador Retrieverss, Boxers and English Bulldogs.

Atopic dermatitis


Photo 3: Alopecia and hyperpigmentation of the ear pinna in a dog with light-responsive alopecia.
Atopy in some breeds such as Boxers and Labrador retrievers can manifest with crusty, alopecic ear edges. This is often mistaken for scabies since these dogs are pruritic and can have a positive pinnal/pedal reflex. It is certainly appropriate to treat for scabies, as anytime scabies is suspected, treatment should be initiated. However, if the condition persists after the appropriate scabies treatment is completed, atopy should be considered.

Seborrhea


Photo 4: Malassezia dermatitis of the ear edge and nasal planum in two sibling Dachshunds.
Ear edge seborrhea due to Malassezia yeast or keratinization disorders can present in breeds such as Yorkshire Terriers and cocker spaniels, respectively. Skin smears can confirm Malassezia yeast, and a skin biopsy confirms keratinization defects such as vitamin A-responsive dermatosis. Yorkshire Terriers and Dachshunds are most commonly susceptible to Malassezia ear edge dermatitis (Photo 4). Scaling of the dorsal nasal planum may also be present. Vitamin A-responsive dermatosis usually results in a thicker ear edge scaling sometimes accompanied by crusting, follicular casts and fissuring.