A. Biting and chewing to the point of hair loss can be caused by a number of things. Allergies, skin infections, and even metabolic illnesses can all cause the skin to become bothersome, or hair to be lost. In some cases, the behavior of licking or chewing at the hair can become chronic, and lead to infections or sores on the leg.
It is best to have your sister bring her dog in to have the legs looked at. Your vet can check for any signs of infection, allergy or even external parasites and provide treatment for relief. If there is no apparent cause, your vet may also recommend a blood test to check for internal illness as the cause.
Until you can get to the vet, it is best to prevent the dog from licking or scratching at the area. An Elizabethan collar (cone) is best to stop the behavior and allow the legs to rest some until they can be examined. If there is any dirt or debris in the area, a warm clean washcloth can be used to keep the area clean and prevent secondary infection until it can be examined.
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