It could be a couple of things in that case, the first being what we call a 'scaling collarette' which is a localized bacterial infection in the skin, it forms a ring as the bacteria spread out from a centrally infected pore in the skin. The other option is ringworm, which is a fungal infection, this tends to be a bit scaly and scurfy though. With either condition it may be itchy or not! I would try bathing the area with luke warm salt water, put 1 tsp salt in 1 pint luke warm water, bathe twice daily, you could apply antibiotic cream if you like also. If it spreads or doesn't go away, then its worth consulting the vet to see if its ringworm, which needs an anti-fungal cream like canesten or oral anti-fungal meds.
Hope this helps