Robin you beat me to it.
OP, i know when we have a flea issue, Riley will tear himself to shreds and it always starts at the base of his tail and if he cant get to his tail, he goes after his legs and stomach. he's got pretty bad flea allergies. When we first found out about it, we were at the vets office a couple times a week for a month before we found the first flea. he's a big black dog so it was very difficult to find fleas on him. Vet said the chewing raw at the base of the tail, literally obsessively, is a good indictator you have fleas and that you dog has an allergy to them. Normally just the scratching but causing further damage to themselves will sinch the deal. I hope you get it figured out.
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Shasta GSD 5/5/10 CGC, ITD, TC
"Dax" Thor z vom Weberhaus CAX, CGC, ETD, HCT, RATI, RATN, TC 3/18/2013
Zena GSD 6/1/03-2/16/2016
Riley GSD/BC 1/10/05-2/1/2013