strange bald spot

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Hello, I noticed today that Tsunade has a strange bald spot on her back, in 
between her shoulders.  i suppose she could have scratched it or rubbed up 
against something.  Any thoughts on this, should I be worried??  She does not 
have fleas, I doubt it would be mites, it is cold 30-60 degrees F and last time 
she went to the vet the vet said her fur looked fine.
thanks
Chelsea