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This is often caused by an allergy or a bacterial infection between the toes. It can also be a symptom of a serious thyroid condition.
I suggest that you see the vet as soon as you can to rule these out.
Until you can, you can clean the paws well with peroxide. Then dry them well and apply neosporin ointment. Then you can pull a baby or child's sock over the foot and tape it lightly at the ankle. If she'll leave it on, this will keep the foot clean and dry and prevent her from chewing.
Let me know if you still have questions.