Atopic dermatitis is an allergic skin disease of dogs which is caused by immunological hypersensitivity to common substances in the environment such as house dust mites.
What is allergy?
The immune system of mammals makes receptor proteins (antibodies) to substances that are foreign (i.e. not part of the body), each antibody being specific to a given substance. Diagram of alleric process with mast cellAntibodies are of several types, IgG for instance being involved in protection against viral diseases after vaccination whereas IgE, involved in atopic dermatitis, is particularly concerned with protection against parasites. IgE antibodies coat specialised cells (mast cells) in the skin where they sit waiting for contact with the parasite proteins to which the animal is sensitised. If the substance is encountered, perhaps as a result of a burrowing mite, the mast cell releases chemicals (mast cell mediators) which try to destroy the invader. In allergic animals this whole system is oversensitive and the release of mast cell mediators in the skin occurs inappropriately to apparently innocuous substances such as pollens, moulds and house dust mites.
For allergy to be apparent, dogs need to be first "allergic" and then be exposed to substances (allergens) to which they can develop the abnormal immune response. In the UK the main source of allergens is the house dust mite. These tiny creatures live in all of our houses, in carpets, beds and other soft furnishings and feed on skin scales that are constantly falling from people and animals. They litter our environments with fæcal pellets of half-digested food and digestive enzymes and it is these minute faecal particles that contain the most important allergens. Dogs can also become allergic to pollens and moulds although this is much less common, presumably because of less exposure.
Other factors known to be important in atopy in man are certain infectious diseases in the early part of life which modify the response to allergens. In particular it has been shown that children who have more respiratory infections early in life, before any allergy is apparent, have a lower chance of showing signs of allergy. The effect of such infections is not known in the dog.
Atopic dermatitis is often first apparent in the first two years of life. Owners may notice that the dog grooms excessively, with licking or chewing of the paws, abdomen and perineum. The ears may be reddened and hot to touch even though not scratched. The result of this itchiness (pruritis) is that the dog will often be presented a number of times in the first eighteen months of life for a variety of seemingly minor skin conditions. Between these episodes the skin and the coat can look remarkably normal. Spots, acute moist dermatitis, ear infections and scratching may all seem to occur independently and it is only in retrospect that a consistent pattern of disease emerges. As the condition becomes more severe , pruritus dominates the animals' life and specific anti-itch therapy becomes necessary. With increasing pruritus, baldness (alopecia) and redness of the skin become evident and secondary infections with yeast or bacteria become more common.
At present there is no definitive test that will absolutely confirm a diagnosis of atopic dermatitis. Because this is the case, veterinary surgeons may suspect atopy after examining a patient, but have to make sure that other causes of itch are not present. Once these have been ruled out, skin testing can be used a s a pointer to the allergies involved.
So what are these other diseases? Flea infestation and the allergy are the most important causes of itchiness in dogs in the UK. Practically all dogs will have fleas at some time during their lives. The rump and hind end are most often affected. Nibbling and itching gives a rough feel to the coat and, if severe, pyotraumatic dermatitis ( wet eczema) or alopecia will result. Very importantly, dogs with atopic dermatitis are often allergic to fleas as well, so it is pointless making a diagnosis of atopy without taking rigorous flea-control measures. Similarly, other parasitic infestations such as lice or sarcoptic mange may mimic atopy and these should be carefully ruled out.
Food sensitivity ( often called food allergy) is an uncommon cause of allergic skin disease, which accounts for a small percentage of the cases seen by dermatologists. Although a rare condition, all allergic dogs should undergo food trials before being committed to long-term drug therapy. Food sensitivity may coexist with atopy or flea allergy and so partial responses may be seen to food changes. Bacterial infections are a common cause of pruritus in the dog and these can be as a result of atopic dermatitis or any other skin condition that damages the integrity of the skin. Non-allergic causes of bacterial infection include hormonal problems such as hypothyroidism and parasitic problems such as demodex infestation. These are normally non-itchy conditions, but as soon as there is bacterial involvement this changes and it can be difficult to make the correct diagnosis.