Do notice any flaking where the rash is located?
Is there any dots?
Is it only on the back?
What type of bedding are you using?
How long has this been going on?
I suspect we are dealing with a Parasitic issue which is very common. There are several types of parasites that can cause this type of problem. The parasites which are either lice or mice that burrow into the skin and cause a rash itching and hair loss. They are very treatable but, to determine what type they are is it best to have Julia into the Vet to have what is called a skin scraping. The Vet will scrape the area and put the sample under a Microscope to determine which type of mite is causing the issue and then he will treat with one of several products. This link goes over Parasites and explains about the treatment involved: http://www.guinealynx.info/parasites.html
I am going to give you two links to help you fine a Vet that can do this for you:
This is a link to locate Guinea Pig Vets: This is by State: http://www.aracnet.com/cgi-usr/seagull/vetfinder.cgiThis is Rabbit Vets who will also treat Guinea Pigs: http://www.rabbit.org/vets/vets.html
The parasite issues are usually treated with Ivermectin or one of the Drops used for fleas called Advantage. It is important to determine what we are dealing with because there may also be something eles going on, so the visit and proper Medication is very important for Julia. Please let me know if you have further questions and I will be happy to continue with you. My goal is 100% Excellent Service. Joan