Feet
Pads
Nails
Dew Claw
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West Highland White, Pads

If a dog chews its paws, it may be walking in or on a chemical that's irritating its feet. Rug shampoos, floor cleaners, or garden sprays, for example, can irritate the paws of dogs. The sore paws may prompt the animal to chew its feet, aggravating the existing irritation and causing pad cracking.

Some other possibilities are that the dog may be having an allergic reaction to a substance it breathes or eats, a chronic yeast infection similar to athletes foot, a nutritional zinc deficiency, or autoimmune disease.

The problem can also be mites, which you can only see under the microscope.

Another possibility is nasodigital hyperkeratosis, an ailment affecting either the nose or foot pads of older dogs. In hyperkeratosis, keratin the tough, fibrous outer covering of foot pads grows excessively. Often, the hard, cracked pads appear to have keratin feathers around their edges. A vet can diagnose this problem by analyzing a section of pad tissue.

Lastly, a dog can suffer from an auto immune disease of the skin, in which the immune system goes haywire and mistakes skin cells for enemy invaders. In the most common type of pemphigus, pus-filled sores which eventually break and form crusts develop on the foot pads, bridge of the nose, and ears.