If you have access to Frontline flea and tick spray or drops, you can put a small amount (drop) on their back between the shoulder blades. You will have to do this to all of the rats if that one has been anywhere near the others. (as in same building).
I know there are others to use, I can do a search and see what I find.
I've heard Revolution for Kittens is probably one of the safest. It's fast acting and it does the job. One tiny drop between the shoulder blades is enough.
Do remember though, don't feed off any rodent that you have applied a topical poison to for at least 30 days. I'd recommend even longer, say 45 days.
Since Provent-A-Mite kills mites, I did a treatment in all my rat enclosures and had no more mites. I cleaned the enclosures, spread out my aspen, and did a very light spraying of it on the aspen, let it COMPLETELY dry. No more rat mites.