One of my dogs began limping yesterday for no apparent reason. That wouldn't usually be a big deal, but his breed (Rottweiler) are particularly susceptible to osteosarcoma, which usually remains undetected until symptoms present in the limbs, by which point it's too advanced to really do anything about.
I have found no lumps or bumps on him and he does seem fine today, it could well be nothing, but it is always a worry when this happens as I've lost dogs to osteosarcoma in the past. I'll be keeping a very close eye and there'll be no walks or play allowed for a day or two.
