Maggie has a mild case of heartworms, and has had them for a couple of years now. My mom, who is in charge of giving her heartworm preventative while I'm at school, skipped a month during the winter because she figured it wasn't possible to get them during the winter, and Maggie got heartworms after that. We have been treating using the slow kill method, and every time we "finish" the treatment and get her tested, she still comes up as a mild positive, so we have to start over. Is this normal? I think we've done three treatments. It's a very, very mild positive so we're not really worried about it, but we're a bit frustrated that it's been about two years, and she's still coming up positive.