Agree with the others here.
Also, be careful not to confuse
Lice with
Mites. They are not related and have different symptoms and treatments but sometimes I see people get them confused.
Just wanted to add that I've been that guinea pigs don't just get dandruff.
If the skin is flaking, then there is something wrong that needs attention. If left untreated mites can slowly kill an animal. Here is a good example of what happens when they are not treated:
Guinea Lynx :: Topic - Mites---Before & After w/ treatment of Ivermectin
The only other thing I know of that can cause flaking of the skin is if you bathe them to frequently or with the wrong kind of shampoo, say once a week or more. There is also a slim chance they have been exposed to something that will cause an allergic reaction such as cedar shavings etc. However, shavings are also famous for harboring mites, so you can't even go by that without a visit to a CAVY SAVVY vet.
Do not go to a regular vet. They have to be
certified to care specifically for Cavies. Do not assume that because they are an "Exotics" vet that they know anything about them. Just like regular doctors, most vets have areas of expertise-- so you need to know what those are. Are they more into birds, or reptiles or small mammals? To find out if they are CURRENT and up to date on their guinea pig experience you can ask a few key questions. Use this handy guide to find a good vet in your area. Remember, it's YOUR job to take care of your
Guinea Lynx :: Finding a Veterinarian