Try non-toxic DE,Diatomaceous Earth (HUMAN-GRADE otherwise you may get the stuff they use in swimming pools and THAT grade IS poisonous ).
DE is a powder,made up of diatoms' skeletons. Be careful not to get it into your or the pet's eyes and don't inhale it. It's not poisonous,but because it's gritty (which tears up the insects' exoskeletons and kills them).
It WILL irritate eyes and lungs if you are not careful.
First,wash and dry your rats' cage. Then sprinkle or pat on(using a paper towel) the DE throughout the cage,especially in the corners. You can't overdo it. If the rat ingests some of the powder,it's a good thing as it acts as a wormer and nutrient too. It also deodorizes. Farmers buy huge sacks of the stuff to use in their chicken coops and other pens.
I just spent a lot of money and time getting rid of scaly mites from one of my new birds. I could have gone with the DE only and accomplished eradication much cheaper and faster.
I'd shampoo the rat with a vet approved wash and then when the rat was fully dry, I'd very carefully (to avoid eyes and lungs) rub some DE into the rat's fur. Bye bye lice!
People with fire ants pour this stuff into the red ants' nests. It does a great job of killing them.
DE kills,fleas,ants,lice,mites,bugs that bother house plants,bugs that bother garden plants. Snails and slugs don't like to cross over DE because it hurts their slimy feet.
I bought my DE at my pet store.