https://www.petmd.com/dog/parasites/evr_dg_how_dog_heartworm_treatment_preventive_medications_work
excerpts below (out of context)
The heartworm life cycle is complex. The dog is infected by early stage larvae that are transmitted by a mosquito carrying infected blood. This larvae goes through multiple stages of development within body tissue before migrating to the heart and lungs as an adult heartworm.
These adults produce microfilariae, the earliest life stage that circulates within the dog’s blood. Prevention kills only early stage larvae and microfilariae. This is why it is important to give your dog heartworm prevention every month. It kills the larvae before they develop into a stage that is immune to the medication in heartworm prevention.
Most heartworm medications require monthly administration, while others work longer (up to six months with an injectible product called moxidectin or Proheart®). There are many choices of heartworm prevention available, from topical products to chewable oral medications; many come in both dog and cat versions.
Monthly heartworm preventative medications do not stay in your dog’s bloodstream for 30 days. The active ingredients work to kill any larvae that have been in the system for the past 30 days, clearing the body each month. The medication is only needed once a month because it takes longer than a month for the larvae to develop to a stage where they reach the body tissues.
Heartworm medications available today work to kill off the larval heartworms that made it into the dog’s body during the past month. The heartworms in dogs will die at certain stages of development, before they can become adult heartworms. However, heartworm preventives will not kill adult heartworms that are already present.
