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.
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.
Photo 1: Alopecia and crusting of the ear edge in a dog with scabies.
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.
Photo 2: Crusty, seborrheic ear edges in a patient with hypothyroidism.
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.
Photo 3: Alopecia and hyperpigmentation of the ear pinna in a dog with light-responsive alopecia.
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.
Photo 4: Malassezia dermatitis of the ear edge and nasal planum in two sibling Dachshunds.