my guinea pig mario has a bald circle

Question
my guinea pig mario has a bald circle about 1/2 inch by 1/2 inch right on his neck. Hes very fat so it could be just a fat roll rubbing off his fur but its a perfect circle..he seems to have been acting normal and so has his brother luigi.please answer this questoin im very worried about mario.

Answer
Hi Kirsten

Even though it seems odd its a perfect circle, when I have encountered this before it has turned out to be mites. If that is the case the vet will need to give you some ivermectin.

The other possible is that mario has received some kind of insect bite but I would go for mites.

Andy