"Just wanted to clarify.... There are NO heartworm preventatives. Only heartworm insecticides which kill the larvae already circulating in the blood. When you give a dose of heartworm meds, you are killing heartworms that are already present before they grow into the adult heart-clogging worms. There is no medicine that PREVENTS heartworms. The meds we have just kill the worms that are there." I thought that in a ferret that is tested negative for heartworms, that Revolution is actually a preventative, because it will prevent heartworms from being able to grow up in the ferret's body for 30 days once it's given. So you are saying that it is not considered a preventative? I'm not arguing, just trying to understand. You are absolutely right - no ferret should be given meds without being tested first, which is a simple test to do at the vet. L [Posted in FML 7044]