Preventive medications can be harmful or even fatal to a dog that has an adult heartworm infection, so it is very important that dogs be tested before starting a preventive program.

Preventive medications can be harmful or even fatal to a dog that has an adult heartworm infection, so it is very important that dogs be tested before starting a preventive program. Adult heartworms reproduce by releasing millions of microfilaria into a dog’s bloodstream. When we give a preventive medication, these microfilariae can suddenly die and a shock-type reaction can occur. This reaction can be triggered within a matter of minutes or hours and without prompt veterinary attention, death can occur. But even if an infected dog does not have an immediate response to preventive medication, the pet often remains infected for several years. As long as adult worms remain, they will continue to cause progressive disease of the heart, lungs, and pulmonary arteries which can lead to irreversible life-threatening problems.