Health: what is this bald spot on my guinea pigs chin?

[interestingthoughts]

My guinea pig is 2, he has a bald spot under his chin. As you can see the front of his face including his nose, eye area, and ears are all healthy. His cagemate doesn't have the bald spot. I also saw that it might have to due with wet food getting under his chin but honestly I don't know. The skin doesn't seem to be inflamed, scabby or patchy. It might be mites and I have liquid ivermectin, the thing is that I'm afraid to use it because I've heard that if you accidentally overdose guinea pigs can get poisoned from it. Please let me know if oyu have any advice, thank you

 

[Piggies&buns]

Please do see a vet if you are concerned and please don’t use the ivermectin without a diagnosis and the vets say. It can be overdosed but also there is no point in treating him for something he may not even have - we never recommend treating anything on spec as you can do more harm than good (if you treat before seeing a vet can wipe the scene of evidence a vet needs to make a diagnosis this risking the wrong diagnosis being made).