It could be any number of things. Dogs repel of most things that would cause skin irritations to humans. Things like poison ivy. They also tend to go places where we wouldn't like under the bushes and through the sticks. Your pet may have picked up something that irritated the skin. Does it appear that your dog is itching those spots? If so, I would take him to a vet to see what it maybe or ways to control it. If it is not scratching, leave them alone. Dogs are very much self healers and sometimes we humans intervene and make things worse than what they really are.
