Why does my dog/cat need to have a blood test before starting heartworm medication?

Your pet should be tested for heartworm infection before he or she is placed on a preventive to avoid any harmful or possibly fatal complications. For instance, if a heartworm-infected dog is started on a monthly preventive, immature heartworms (called microfilariae) can die suddenly, causing a serious, shock-type reaction. In addition, preventives won’t kill adult heartworms, so an infected dog needs to be started on a treatment plan.