Sorry to hear about Jazz I sure hope the treatment is successful.
Like Maggie said, it is important to give the medication EVERY month. But our vet recently told us to be sure and give in ON TIME, that new evidence indicates a few days is critical. I write it on my calendar but there have been months I have missed it by a few days, I am definitely going to try and do better!!! There has a significant increase in the occurrence of heartworm in the last few years even in dogs who have not missed a dose of their medication.
Here is part of an article from this website.
http://www.heartwormsociety.org/veterin ... es.html#14Most LOE (lack of efficacy) claims are explained by compliance issues, either between the clinic and the client or the client and the pet. It is possible for an animal to become infected by missing or delayed administration of just one dose of a heartworm preventive product. The likelihood of this occurring is increased in highly endemic areas. Such areas typically have warm temperatures most of the year, an abundance of standing water, and substantial mosquito populations. These endemic areas also have large populations of infected dogs and wild canids available as a reservoir of infection. In addition, manufacturers have improved the sensitivity of heartworm antigen tests over time and more animals with low female worm burdens are now being detected.
The increase in the number of LOE reports to the FDA during the past several years has led to concerns of possible heartworm resistance to the current heartworm preventives....