It's my understanding that rats almost always have it but healthy, active rats who clean their body's regularly will never have a noticeable outbreak.

In my colonies there is only one breeding pair that I've ever noticed rodent mites on. They never seem to be on any others. I've checked them very well. Those 2 are the oldest and rather lazy. I've done some research on it myself and it seems the rat mites/lice will not stay on healthy, active rats.

As for treatment, I'm not sure if anything is safe for them. One rat breeder did tell me once that if the rats are kept on cedar for one week (not long enough to cause health issues for the rats) then the bugs will disappear. But I'm not sure I trust this method of treatment since cedar is known to dangerous. Especially reptiles.