原文 There is no "preventive" medicine for Heartworm. Despite what your veterinarian may have told you, there is NO "prevention" for heartworm infection; there is only heartworm treatment. ALL heartworm medicines work the same way -- they kill heartworm microfilaria present in the body of the dog. . Heartworm "prevention" medicines are actually toxic poisons. The drugs used to kill heartworm microfilaria are Ivermectin or Milbemycin. Both drugs are nematode poisons, and in both cases a single dose will kill all microfilarial infection that occurred up to 90 days earlier.