The most common cause of hair loss in dogs is caused by external parasites, such as fleas, lice, mites and ticks. Bites from any of these pests can cause intense itching and skin inflammation, causing hair loss.
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Fleas
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Fleas, a common pest, will infest a dog, biting it to feed on its blood. The flea's saliva triggers a reaction from the immune system, causing itching and irritation. Hair loss generally appears toward the rear of the dog, though it can occur anywhere.
Lice
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Lice infest a dog and irritate the skin much like fleas. Hair loss occurs when the irritated skin becomes infected or inflamed. Hair loss is somewhat patchy, in areas that are reached by the dog's mouth and claws during scratching.
Mange
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Mange mites burrow into the hair follicles and cause inflammation, and thus itching and scratching. Hair loss associated with mange mites tend to be concentrated around the eyes, ears and feet, but if allowed to progress, will encompass the body.
Mosquitoes
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Mosquitoes cause hair loss by triggering an overreaction of the immune system, much like in humans. A mosquitoes injects a small amount of saliva into its prey before biting. The saliva causes the itching, inflammation and hair loss.
Treatment
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Treatment for each of these conditions includes insecticide treatment, oral antibiotics for skin infections, and oral or injectable steroids or antihistamines to reduce the immune reaction.
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