Although you are taking really good care of your dog, sometimes things crop up that are not a reflection of you, or the quality of care you provide. It also has nothing to do with the health of your other dogs.
Paw-licking and -chewing are common signs of disease in dogs. Many owners assume these behaviors are normal and fail to recognize the extent of the problem. Genetics play a role in many cases, but overbreeding is not a specific cause. Purebred and mixed-breed dogs can chew any combination of paws, but chewing both front feet is most common. The degree of itch and obsession with the feet can vary with time of year, weather, age, diet and other factors.
The most common cause for the itch you describe is atopic dermatitis, or inhalant allergy. This is similar to hay fever in humans except that the result is usually foot-chewing rather than respiratory signs. Allergens include dust and dust mites; pollens from trees, weeds, grasses and other plants; molds; mildew; animal or human dander, including wool; and insects. Usually more than one substance is involved.
Allergies can develop from food, even if the dog has been fed the same diet without any problems for years. The same can be said of shampoo. Even if it's been used on the dog before without problems, it is possible that she has become allergic to it.
Foot chewing can lead superficial infection (bacteria and yeast like warm moist skin) , which leads to deeper penetration of the organisms into the tissues, creating a very itchy cycle that is hard to break.
The cause of the itch has to be established. If you address only the itchy symptoms this problem may never be cured. It is important to establish a specific diagnosis and rule out complicating factors to properly address foot-chewing in dogs.
Prednisone is often prescribed by vets for this kind of problem because it will relieve the itch associated with most allergies and infections, at least temporarily. Unfortunately, improper usage of prednisone and other corticosteroids may worsen or allow infections to develop.
Diagnostics to help define the problem beyond simple allergy include thyroid testing, cytology (examining smears of the skin and nail beds under a microscope for infectious agents) and skin scrapings with microscopic exam to rule out mites. Blood or skin testing can be done to specifically identify offending allergens, thus allowing an owner to decrease exposure if possible.
Trial courses of medications can also be used once it is deemed safe. As with allergies in humans, the situation can be complex and may require trying several different medications to find the best drug or combination of drugs.
Dogs with atopy (inhalant dermatitis) may require further anti-inflammatory treatment. This is where corticosteroids such as prednisone come into the picture. It is important to realize that steroids are not inherently "bad" medications; in fact, they are essential in the short-term and emergency treatment of many patients. The itch relief gives damaged and infected tissues a chance to heal by breaking the lick cycle. Long-term reliance on steroids and failure to pursue other causes of the itching should be discouraged. Unfortunately, a few dogs need steroids for sustained allergy relief. Even in these individuals, efforts should be made to use the steroids only intermittently and at the lowest possible dose, and to spare the need with other medications and supplements whenever possible.
Antihistamines may provide complete or at least adequate relief for many dogs, especially when used in combination with omega-3 fatty acid supplements. Ask your vet about the use of the Omega fatty acids. Each dog is different, so you may have to try several types and dosages to find the best one.
Food allergy also has to be considered because it can cause the same signs as atopy. Allergy can develop to any food, though protein sources are most often the problem. Hypothyroidism can also contribute to many allergic conditions.
You should have your dog evaluated by your vet as soon as possible, since your dog has only recently developed this problem. Early treatment can mean a quick recovery which is important in something as potentially chronic as an allergy.