If scabies treatments don't work, you may have to dig deeper.
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.
Ear edge vasculitis
Ear edge vasculitis is common in Chihuahuas and Dachshunds. "Punched out" lesions are usually present in the pinna, which appear clinically as smudging of the pinnal vessels, resulting in alopecia and/or necrosis of the ear edges (Photo 5). Early vascular smudging is evident when the lateral aspect of the pinna is backlit with an otoscope. Make the diagnosis by clinical appearance, breed affected and skin biopsies.
Photo 5: Smudging of the pinnal vessels in a dog with vasculitis.
Fly strike and frostbite may occur in dogs housed outdoors. The distal pinnal tip is commonly affected, resulting in crusting, ulceration and necrosis. Other conditions that more commonly affect the ear pinna than the ear edges include pemphigus foliaceus, dermatomyositis, collagen disorders and cold agglutinin disease.
Although canine scabies is the most common differential diagnosis for ear edge changes, be sure to take the whole picture into consideration. A thorough history, consideration of the breed affected and evaluation of any other clinical signs will help in the diagnosis.
Dr. Jeromin is a pharmacist and veterinary dermatologist in private practice in Cleveland, Ohio. She is a 1989 graduate of The Ohio State University College of Veterinary Medicine and an adjunct professor at Case Western Reserve University's College of Medicine in Cleveland.