Atopic Dermatitis

Atopy is the most common allergic disease I see in dogs , although some texts say that flea allergy dermatitis is more common.  The  condition is also thought to  exist  in cats, although it is more difficult to diagnose.

Most cases start between 6 months and 3 years of age although occasionally they can be seen either side of this age range.  The disease has had its definition change over the years and as our understanding of the disorder becomes clearer it will no doubt continue changing, but the current definition is a “an inherited pruritic skin disorder characterised by the production of allergic antibodies to environmental allergens”

The classic signs are an itch and diffuse erythema (red inflamed skin) affecting the inside of the ear flaps, the upper ear canals, the  skin around the eyes and muzzle, the underside of the body and the feet.  Many cases will only show one or two of these signs and approximately 5-10% of cases show the inflammation and itch only in their ears.   Affected dogs will often scratch and rub their faces and chew at their feet.

The disease is very frequently complicated by the presence of secondary bacterial and yeast infection, and these infections will also contribute greatly to the amount if pruritus the dog is suffering with and also change the appearance of the skin, producing scaling and crusting. It is not uncommon for an atopic dog to go into a subclinical state if the secondary infection is controlled.    Chronic cases will develop thickening of the skin and a dark pigmentation (lichenification and hyperpigmentation).

Much research has been done on this disease over the last 10 years  and it has been discovered that dogs suffering with atopy have different skin to non-atopic dogs.  They have  a defect in their skin barrier due to differences in the lipid barrier and there are defects in the protein filaggrin that  helps bind skin cells together.  These defects can allow environmental allergens such as house dust mite proteins and pollens to permeate the skin and be picked up by cells called langerhans cells which can then present them to the immune system.  In atopic dogs , due to differences on the responses of immune cells called T lymphocytes, this can result in the production of allergic antibodies ( IgE)   and a host of other reactions which can occur when the animal is exposed to the allergen.  In addition, it has been found that the bacteria responsible for the vast majority if skin infections , Staphylococcus pseudintermedius, which normally resides in the nose and around the anus, can colonise the skin  which is one reason why bacterial infection is so commonly seen in atopic dogs.

The diagnosis of of atopy is based on history, age of onset, clinical signs and after ruling out all other causes of pruritus such as scabies and other parasites, food allergy and contact dermatitis/allergy.

Once a provisional diagnosis is made, allergy testing is useful if one is considering allergen specific immunotherapy (ASIT)

Atopy is an incurable disease and although the ultimate goal with treatment is to abolish all symptoms entirely, a more realistic goal is to allow the patient to be “comfortably itchy” i.e. devoid of most of the severe signs of the disease even though a mild itch may persist.  There is no “one size fits all” treatment for this disorder and each case is treated individually according to the severity of the disease and the resources of the owner.  A recent paper published in Veterinary Dermatology  in April 2010 found that the only treatments that are effective in their own right are ASIT, cyclosporine or corticosteroids.  Treatment with antihistamines and essential fatty acid supplements  and other pharmaceuticals may help as an adjunctive treatment, for instance, lowering the dose of steroids that are required.  Intermittent treatment with antibiotics and antifungals (both topical and systemic) may be necessary throughout the life of an atopic dog and all affected animals should be placed on a permanent programme of flea control as a picking up a flea can raise the “pruritic threshold” below which an animal may be itch free.

Allergen Specific Immunotherapy

After allergy testing, the relevant allergens are supplied in a vial  and are injected subcutaneously starting with a very small dose  which is gradually increased  over a period of weeks.  The injection is then given monthly  for the rest of the animals’s life if it proves successful.  Response can be seen between 2-9 months.   Approximately 75-80% of atopic dogs will experience a 50% or greater improvement with their signs of atopic dermatitis with this method of treatment.  This is a very safe therapy, the only side effects being in a small number of cases which may experience a transient worsening of itchy skin for a few days after the injection.