Skin Problems Bare Patches Around Eye

Yes, the diagnosis could be wrong, and unless something has changed that I don't know about, guinea pigs shouldn't be given Advantage. They can have the plain Advantage, but not Advantage II, which is all that's on the market in the US, I believe.

You don't have to do any extra disinfecting -- just normal procedures should be enough.

For the stuff around his eye, you need to treat with an antifungal medication. See https://www.guinealynx.info/fungus.html. Get one of the creams at any pharmacy, and apply sparingly with a q-tip, being especially careful not to get it IN his eye. Rub it in well so he can't get any of it in his eye when he grooms himself. Do this twice a day until the hair grows back.

Was this an exotic vet? Because it's a waste of time and money to see a vet who isn't, or who hasn't at least educated him/herself about the care of exotics. Small animal vets are not trained to care for them, and some are downright dangerous.

Also, have you turned them all over and compared the private parts to make sure they're all male? Pregnancy and labor are very hard on sows and pups, and the death rate is high. GPs are also susceptible to a number of painful and/or fatal genetic illnesses, and should never be bred. There are enough accidental pregnancies in the world to keep the species going without deliberate breeding.

Thank you soooo much for the information!! This vet was recommended to
me by a friend with a lot of Guinea pigs (one baby in particular who she had alive and wel for 12 years) and ferret experience and knows not all vets are equal when it comes to piggies. With that said, the Dx of fur mites wasn't adding up to me given that his bare patch is getting worse instead of better. We have done a thorough gender check - these are most definitely all three boys. We have no intention of breeding our piggies.