Hi and welcome
Please have your piggy vet checked for ringworm. If your piggy is newly bought, you can reclaim any vet cost from the seller as part of your lawful customer protection rights for having been sold damaged ware.
Please do not touch! Ringworm is the most contagious species jumping problem that can also transmit to and from humans and other pets. If you or a family member have handled the piggy with bare hands and without thorough hand-washing afterwards there is a good chance that you could contract/have already contracted it yourselves.
Make sure that you keep any children well away; their more fragile immune systems mean that they are more prone to catching ringworm and come down with it badly.
You will need to treat both piggies as your companion is likely in the phase between infection and acute outbreak, which is about 10-14 days for humans as well as pets. If you can catch ringworm during this time, you can prevent any further outbreaks.
Please do NOT diagnose and home treat on spec with creams etc. as you can make it worse very easily and then have to spend a lot more money at the vets on top of what you have spent for your home treatment for a much more advanced problem.
Please note that creams do not prevent any further outbreaks and do not reach the whole affected area, which is much wider than the bald spot. Keep in mind that no online place can replace a hands-on vet examination; and also that our recommended oral treatment is prescription-only.
No vet will thank you when you turn up with a treated bare patch that they find very difficult/impossible to diagnose.
Hygiene is of utmost importance as the tiny, invisible spores are shed in their thousands and are very long-lived. Our ringworm guide will tell you exactly what you need to do to get on top of it once and for all as well as how a typical outbreak takes its due course; it will get worse before it gets better. Your best friend is a vet grade disinfectant like F10.
Our hygiene guide is very practical and precise but it really works if you follow the advice - up to keeping a ringworm outbreak in a room of 30 piggies confined to that one single piggy and without any further outbreaks.
The best, most efficient and least stressful treatment is an oral systemic fungal treatment like itrafungol for the UK/enilconazole for the US; it is not cheap but worth its price and it is prescription-only but well worth its money!
You do not want to treat topically regularly so close to a sensitive organ like the eye as it can swell up badly.
You can find information on both your customer rights and ringworm in these two links here.
What to check for in new guinea pigs (vet checks, sexing, parasites, ringworm&URI)
Ringworm: Hygiene And Pictures
All the best!