Hi Theresa,
You can also buy a horse worming paste product called "Equimec". It
costs about $18 for a tube. You put a rice grain sized amount on a
que-tip (Sp?) and dab it on the rats mouth. You do this once a week
for three weeks and then once every three months after that to make
sure they never get them back. What sort of bedding do you have the
rats on?

Kind Regards,
Megan