A lot of people ask why they have to test their dog before beginning to give them heartworm preventative. Once infected by heartworm, the adults can produce an enormous number of larvae which circulate in the blood. See the video above to watch heartworm larvae "dancing" in a drop of blood. We recently had a 20# dog that tested positive for heartworm. We saw at least 100 of these larvae in a single drop of blood. Multiply that by the total blood volume of a dog that size, we figure there are at least 1.5 million of those things in that dog's blood stream. If you were to give heartworm medication to that dog and kill all those larvae at once, the release of antigen in the blood stream could cause a severe, possibly life-threatening, allergic reaction. In that case we would still give heartworm preventative, but we would do it under medical supervision so that we could treat any adverse reaction that might occur. If your dog is not on heartworm preventative, it should be. But get it tested for heartworm first!