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Dogs usually chew and lick at themselves because of allergies. Allergic skin disease is a common problem in dogs, and, although not curable, it is controllable.
Dogs can be allergic to hundreds of different substances. but the most common are pollens, flea saliva, grasses and weeds, mold spores and some food ingredients. These substances can trigger reactions by direct contact or by being eaten or inhaled.
Whatever the route of entry, allergens trigger a hypersensitive reaction in the skin. Your dog feels as if its skin is on fire and responds by chewing, scratching or licking to try to relieve the irritation. This self-mutilation leads to red skin, hair loss, sores and scabbing, often progressing to oozing, infected areas and chronically thickened, stinking skinfolds. Affected dogs tend to lose weight and be irritable, and they smell bad from the secondary skin infection (pyoderma). Dogs that have heavy, matted coats and oozing sores from allergies can become infested with maggots.
Even without such horrible side effects, you want to give your dog relief from the irritation and frustration of itchy skin. This requires veterinary attention from a professional who is willing to treat the problem until it is under control.
Ideally, treatment of allergic skin disease would involve removal of the allergen. This is feasible if the culprit is fleas, but not if the problem is caused by such things as pollens or grasses.
Note where the irritation takes place. Fleas feed most often on the back and around the base of the tail, so look for itching and sores in those areas.
Some veterinary clinics perform skin tests to identify allergens. Once these are identified, injections can be given to desensitize the dog to the irritating substance. Unfortunately, this is expensive and can be frustrating, because the dog can develop new allergies over time.
The most common way of controlling allergies is to suppress the allergic response in the dog when its allergies are at their worst and to treat the skin infection with antibiotics. In mild cases, antihistamines given orally once or twice a day can bring relief. For open or oozing sores, topical sprays or creams that contain antibiotics and cortisone can soothe the inflammation.
Some dogs have such severe allergies that antihistamines don't bring relief. In those cases, a short treatment of oral cortisone can reduce the skin response and give the dog relief. Cortisones (such as prednisone) are generally safe if taken orally and if use is carefully monitored by your veterinarian. The usual treatment starts with high daily doses until symptoms are under control (which takes three to six days). Then the drug can be given every other day. The dosage is slowly reduced until it can be withdrawn altogether.
If cortisone doesn't help, a food allergy is a possibility. Food allergy treatment involves feeding a special diet.
Author(s): Wilcox, Bonnie, D.V.M.
Canine Library: Allergy