No to be a downer to a happy thread...I've had at least 3 dogs for 20+ years, currently have 6.Our bull terrier/lab came down with heartworms last fall. Heartgard her entire life(like the rest). Anyway, did the arsenic at the vet(partially paid by Heartgard). At the 6mo check...positive for worms.Re-check next month. If positive again(vet said "could be false positive"), Heartgard isn't paying anything(already checked, would be a "improper dosing problem").Not sure(me or vet) if arsenic on a second round is a good idea. Young dog(5) and shows no signs of heartworm(no coughing, labored breathing, choking when eating).Anybody tried the 18mo remedy? There are two drugs(lyme and what Heartgard essentially is) you can get at Tractor Supply and do it yourself...Anybody been through this?