I have not had a ferret that has had heartworm, but a few months ago I saw a picture of heartworms in a dead ferret's heart. They took up almost the entire cavity. It was enough to scare me straight, and our ferrets will never be without their monthly dose of Revolution (cat, 5-15 lb. size) during the months when mosquitos are out and alive in our area (New England). Heartworm is spread by mosquitos, so if a mosquito can get to your ferrets, then they can get heartworms. Yo can get Revolution for about $10 per month. Pretty cheap insurance for this nasty parasite that, from what I understand, is almost certainly a death sentence if your ferret gets it. Jeff In Memory of Neo Caring for Trinny, Morphy, and Baby Girl [Posted in FML 5668]