It's easy to overdose on Ivermectin, use Frontline spray instead.
This is very topical for me, because this weekend we found lice on the rats that we keep at my vet clinic. We took a tape sample and looked it under the microscope and confirmed the presence of the mouse louse. They are sortof clear, but look red when full of blood. I'm not sure how long we had them, but when the white babies were born, the lice became very visible.

We sprayed the adults with a half-pump of Frontline spray, and then (wearing latex gloves) I sprayed my hands and then rubbed each baby. I also sprayed the bedding lightly and mixed it up. There were no visible lice the following day. Frontline is great stuff, much safer and without the neurotoxic side effects of pyrethrins or ivermectin.