Infection skin infection? Mites? Fungal?

That may be a classic ringworm hair loss pattern. So start with fungus, but if it doesn't work, you may need a prescription strength cream for it. Just don't get an oral antifungal unless absolutely nothing else works -- that stuff is hard on all living things, pigs or human or whatever else.

ThePigAlchemist said:

I think fungus is a good place to start. What is the medication you got? Monistat is one OTC medication that does the job. You just apply it to the entire bald spot with a q-tip twice a day.

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If you use a spray, put it only on the skin, and not the hair around it. And take a q-tip and rub it in well. You don't want the pig to ingest any of it when they groom themselves.

That is ringworm and is easily treated. My two boys used to have ringworm and got treated with anti-ringworm cream and head and shoulders shampoo. One of my Guinea pigs Oso is in the middle of a ringworm treatment and is clearing up, but Oso has really stubborn ringworm so it is taking longer to clear up than my other Guinea Pigs :/ But, be patient as some ringworm may be more stubborn than others. I hope this piece of information helps! If you have any more questions on ringworm feel free to ask me as I have had to sadly deal with it many times with my pigs :/ Have a nice day!