If your rat has mites or lice, you may need to get some Ivermectin. You just add a drop the size of an uncooked grain of rice in your rat's food. It's mostly used for treatment of mites, *but* it kills arthropods, so it should work just as well for lice. Here are two articles on the subject, plus a rat drug chart:
What's Bugging Your Rats and Mice?
Mites, Lice, and Fleas
Drug Usage Chart for Rats
I haven't needed to try this on anyone yet, but it might be less stressful than bathing in powders and such, and it has a very good success rate.