After using flea/tick spray on the rat, I would not feed them off for 30 days or more. Flea/Tick Poisons don't really wash off, no matter how hard you scrub. I'd let it pass through their system.

I personally use Revolution for kittens or Advantage, one drop behind the shoulder blades does the trick.

I've also heard of others success using Prevent a mite, spaying it on a couple cotton balls, and let the rats nest in them. Preventic collars are also useful, clip an inch or so off of the collar and place it in the rats enclosure.

It wouldn't hurt to double check the snakes to make sure they don't have mites also. While snakes mites don't feed on rats, they've been known to hitch a ride.

Good luck with those pesky mites!