Atopic Dermatitis (Atopy)
What is Atopic Dermatitis?
Atopic dermatitis is a genetically predisposed inflammatory and itchy skin disease with characteristic clinical features associated with antibody production due to environmental allergen challenge. It affects 3 to 15% of dogs and is involved in up to 50% of all skin cases. Atopic dermatitis normally commences in dogs between 1 to 3 years of age and can affect any breeds, though there is an increased incidence in some breeds such as Terriers, Setters, Golden Retrievers, Dalmations, Boxers, English Springer Spaniels and Chinese Sharpei.
What are the Causes of Atopic Dermatitis?
Atopic dermatitis is caused by air borne allergens that are thought to gain access to the body via the percutaneous route. The majority of cases develop a hypersensitivity reaction to house dust mites (leading sometimes to a non-seasonal dermatitis), and less commonly to pollens which may typically lead to the seasonal spring-summer dermatitis. Moulds and various other allergens can also be contributory causes.
It is thought that an inherited defect in the outer skin that compromises its role as a functional barrier facilitates access of the allergens. The allergens are then captured or attached to special cells bearing the specific antibody to the allergens concerned. A variety of inflammatory mediators also get involved at this stage. The defective skin barrier overgrows, further adding to the allergic load. The skin starts to develop severe seborrhoeic changes as the condition becomes chronic.
What are the Clinical Signs of Atopic Dermatitis?
There is a varying degree of itchiness, ranging from mild to intense, with the itchiness being most prevalent on the bottom of the face, the trunk and the feet. In early cases the skin is just red with little red papules developing over time. Small pustules develop when secondary infection occurs. The pustules may clear up after treatment with antibiotics but the itchiness from the underlying allergen is still present. Otitis (ear infection) accompanies the atopy in approximately 80% of cases where the inner surface of the ear flap and vertical ear canal are often thickened, inflamed and affected by debris.
Seborrhoeic skin changes are often seen in chronic cases where a greasy skin and hair coat with massive scaling accompany the varying degrees of hair loss.
How is Diagnosis Made?
The condition needs to be differentiated from several other conditions of itchy skin that can present with similar signs, such as flea allergic dermatitis, food allergies and allergic contact dermatitis. The location of the itchiness may assist with the diagnosis, as can the rule of elimination. The gold standard is the intradermal test where reaction to a variety of allergens injected into the skin is observed. This test enables the detection of the various allergens that may cause the allergy and subsequently assists in the treatment of affected dogs.
How are Affected Dogs be Treated?
A number of systemic medications are used in the control of atopic dermatitis. These include:
Corticosteroids - They are very effective in relieving the itchiness, but their effect is only transient. Ideally a low dose should be employed for as short time as possible due to their side effects.
Cyclosporin - It is as effective as corticosteriods when dosed at 5 mg/kg bodyweight. However, its effectiveness in reducing itchiness is slow.
Antihistamines - They can give part relief, or occasionally complete relief, but results are normally fairly limited.
Essential fatty acids - If used correctly with the correct combination of fatty acids they can have a significant anti-inflammatory action and assist in restoring the barrier function of the skin.
Antibiotics - They are useful where there is a presence of secondary pustules.
Immunotherapy - This is the cornerstone of therapy in cases where control of the atopy is not achieved with medications other than corticosteroids, or with a short term low dose of corticosteriods. It involves treating the patient with allergen solutions determined by the intradermal tests. The response can be quite fast (within 3 months) or delayed for up to 9 months. Many of the cases treated this way also require concommitent treatment.
Overall most cases of canine atopic dermatitis can be well controlled with the right combination of treatment.