Keep everything very clean, and make sure that the entire cage is disinfected. Throw away anything that can't be, like cardboard.
Advantage is very commonly used on guinea pigs for lice, as is ivermectin, although they are not the same product.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry