Leg licking,sore mouth,thick saliva...poorly dog

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Hi can anyone offer some advice please?I have a 7 year old Siberian Husky who started licking his left hind leg causing a sore.I took him to the vet who couldnt see anything obvious and gave him some painkillers.The wound dried up after lots of licking discouragement but he still seemed to be struggling like his back legs were hurting.I took him back as he had started licking other paws,again the bet couldnt see anything obvious but his saliva has become thick like raw egg white,his mouth is now really sore and red and all the fur on his mouth has disappeared.Hes had three runs of anti inflammatories,some antibiotics and some flea/mite treatment but nothings helping.The vet is now talking about wanting to take skin scrapings.Any advice would be gratefully received