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ATOPIC DERMATITIS IN DOGS

 

What is Atopic Dermatitis?

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 specialized cells (mast cells) in the skin where they sit waiting for contact with the parasite proteins to which the animal is sensitized. 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 (figure 1).

 

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. The most common 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 fecal pellets of half-digested food and digestive enzymes and it is these minute fecal 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.

 

Clinical signs

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, pruritis dominates the animals' life and specific anti-itch therapy becomes necessary. With increasing pruritis, baldness (alopecia) and redness of the skin become evident and secondary infections with yeast or bacteria become more common.

 

Clues to identify unseen itch

Many people scold their dogs for scratching, almost without realizing. Slowly we train our pets to be quiet and all but the most itchy will choose to scratch and chew in private. Photo of dog's paw showing saliva staining Luckily there are some tell-tale signs that help us to identify the pruritic dog. Saliva staining is a commonly-seen feature in these animals. A red-brown staining of light colored hair is often seen in allergic dogs in the groin, arm pits (axillae) and between the toes (interdigital spaces) and can be seen in figure 2. In addition, with long term problems, the skin itself will also change color. Instead of being pink, a black mottling (hyper pigmentation) will slowly develop, especially if the skin has looked red at the site. This is most commonly seen on the abdomen.

 

Diagnosis

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 as 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. 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 pruritis 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.

 

Skin Testing

Photo of allergic reaction Skin testing is performed to identify the allergens involved in allergic disease. Under profound sedation an area of hair on the chest is shaved and small injections of substances known to be possible allergens made. After 15-20 minutes the reactions are recorded. Figure 3 shows positive reaction to house dust mite allergens in an allergic Labrador Retriever.

The diagnostic approach to the pruritic dog is summarized in figure 4.
Flow diagram of diagnostic approach

Treatment

Photo of allergy spots In treating atopic dermatitis it is imperative to consider the situation as a whole. Bacterial infections will make the animal far more itchy and may even contribute to worsening the allergy through damaging the skins' protective mechanisms. So any bacterial infections seen as a rash or pustular spots (Figure 5), need to be treated promptly, using a combination of shampoos and antibiotics for a minimum of three weeks, and often longer.

 

Corticosteroids medication is best withdrawn throughout the period of treatment as steroids can interfere with the dogs ability to fight infection. Natural products such as Essential Fatty Acid supplements should be considered to supplement the diet. 

 

Yeast infection (caused by the yeast Malassezia pachydermatis) is another complication. Spots are not seen in this disease, but instead the organism causes redness, greasiness and a mousy odour. Dogs can be quite depressed when infected and can be extremely itchy. Treatment is usually with baths containing enilconazole, or miconazole in combination with Chlorhexidine. Tablet therapy is also available, but as a surface infection Malassezia is best treated using baths. Supplementation with Essential Fatty Acids is important to increase the dog's inherent resistance.

 

Similarly, fleas and other ectoparasites will make an atopic dog far more itchy. All allergic animals should have regular and efficient flea therapy using veterinary preparations to treat both the dog and the environment. With bacterial, yeast and parasitic problems under control most dogs will be very much more comfortable.  This is a good time to switch over to a nutritional supplement such as DERMATRIX.

 

Specific Therapy

A variety of drugs are now available for treatment. Generally they are used in combination rather than alone. Their use is summarized in figure 6. Flow diagram of treatment approach

 

Essential fatty acids (EFA) are now widely used for skin conditions. They are known to have few side effects and will help about 25% of allergic dogs significantly. DERMATRIX is a chewable EFA tablet that also contains MSM, an important ingredient for skin conditioning, as well as essential vitamins for skin health. Antihistamines potentiate the action of essential fatty acids (synergy) and so combination therapy would appear to be valuable. Several veterinary products are licensed for use.

 

Antihistamines were widely dismissed as unhelpful in atopic disease until recently when new studies both in the US and UK have shown considerable benefits from their use. No veterinary products are available and the human drugs, chlorpheniramine, hydroxyine, and clemastine have all shown to be useful.

 

Steroids are widely thought to cause side effects which outweigh their potential for good. Their use should be limited to only the most severe cases of atopic dermatitis and should be discontinued as early as possible.

 

Hyposensitizing vaccines ( also known as desensitizing vaccines) are prepared from the allergens identified as important at skin test. By administering these allergens subcutaneously over a long period the immune response to them is modified and pruritis is reduced. They are seen to be beneficial in about 60% of dogs, and take up to nine months to have effect.

 

Allergen avoidance is useful when house dust mites are known to be the problem. Exposure to bedrooms should be avoided by house dust mite allergenic patients to minimize exposure to the allergen. When pollens and moulds are involved avoidance is practically impossible as these allergens travel for miles on the wind, although obviously very large sources of pollens, for instance hay meadows for grass sensitive individuals should be avoided.

 

The future

Studies examining the incidence of atopy in the families where the dam and sire have pruritic skin disease have shown that around 60% of their offspring will have signs of allergic disease. When two unaffected animals are bred the incidence is reduced to 10% and it would appear possible to reduce the incidence of atopy within a breeding program by avoiding those dogs with atopic disease. However identification of the mildly-affected atopic dog can be difficult because we lack a definitive test for the disease. Further studies to better predict which dogs will develop atopy are ongoing and if useful information is forthcoming then there is hope that we can reduce the incidence of this distressing disease.

 

This article was contributed by Stephen Shaw, a Dermatology Research Fellow at the Animal Health Trust.

 

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