My pup - 11 weeks, keeps nibbling his back left leg- Ive had a look and nothing seems to be there to cause this- but i cant grasp why? only his left leg and he nibbles at it - I dont know how else to explain it?
Any ideas what this could be or what I can do to stop it?
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Where abouts on the hind leg? Nibbling usually means it hurts of it itches so perhaps you should let the vet take a look.

I have heard of dogs doing that when they need their anal glands emptied but at 11 weeks I think that would be pretty young to need that- sorry not much help

Parasites would be my first thought, these could include, fleas, Rabbit mite or mange mites (sarcoptic or demodectic), even ringworm, depending on where the pup came from, though I would expect to see lesions from the latter two.

Also contact allergy, has he laid on not quite dry floor after you have washed with disinfectant/bleach/floor cleaner.  Also air fresheners, especially febreeze and shake n Vac can cause severe reactions.

One of my Papillons is really sensitive to pretty much everything (he's the only one of my dogs that has food allergies for instance) and he very often starts to lick his feet and legs after walks as just walking in and around some types of grass/flowers/weeds etc makes him uncomfortable. Yet I never see any redness or swelling or anything and it always passes.