It's a good question, the first rat I had, was infested with I thought was lice, as my family was infested with an outbreak of lice caused by my little sister. Well it wasn't her fault, she went to a summer camp and came back with lice, to which as we all know, spreaded fast amongst us. After we were treated I decided to check on my rat, and he was infested with bugs as well. I assumed it was lice from us and treated him, however, now that I'm older, I suspect what he had wasn't lice, but possibly fleas or mites. The bugs in his fur were brown/red colored, and were all over him, causing alot of tiny bites all over his body... Granted, my rat was very old, he was 4yrs old, his immune system was breaking down, so he was an easy target for a bug infestation.
But from what I understand, lice from rats do not spread to other species, just as human lice cannot spread to rats.