My wife and I have a 2.5yr old Border Collie. He has had a very soft attitude as he's grown up, however for some time now he has developed a strange issue... Basically, he is fine and playful, even rolls over for you to tickle his belly, however he suddenly gets edgy and starts to snarl. Unless we stop stroking him immediately, he turns. First he warns you by snarling (increasingly) and then eventually (if you haven't moved away) he tries to bite. After this he is immediately sorry and seeks stroking again.

We can't figure out why this has happened. The only thing I can think of is about 1yr ago we were walking and he got a thorn in his paw. He was limping badly and the only way to help him was to hold him still, while my Wife removed the thorn. This is the first time he tried to bite. This tends to fit, as he doesn't like it if you attempt to touch his legs now.

I don't think this is a game as he seems quite aggressive when he turns, but it only lasts about 15 seconds then he's nearly back to normal.

It's become quite upsetting as all we want is to be able to stroke him again without him getting scared / aggresive - any advice would be much appreciated.

I would suggest two things first a full vet check to make sure there is nothing physically wrong with him-I had a GSD who loved to have his tummy tickled who started to grump if you tried it-poor dog had a massive tumour on his liver:(which was causing pain.

Secondly I would use T Touch massage rather than stroking try this link
it releases the bodies natural endophins & relaxes the dog-calming & soothing

JMHO

I agree with Moonmaiden's post - a vet check is an absolute necessity here.

Also if the problem carries on, and makes him difficult to live with, ask for referral to a reputable trainer or behaviourist who has a success record with people aggressive problems. In all probability the dog is scared of pain, or is in pain, esp. if he is normally very soft.

Good luck

Lindsay