My dog just recently tested positive to heqartworms while on Interceptor preventive. In this discussion much information has come to light that shows a misunderstanding of how HW meds work. The information here is directly from the American Heartworm Society and Westwood animal clinic. This is not just an opinion but comes from the Drug companies themselves. Please discuss this with your vet as well.

American Heartworm Society and the other is the Westwood Animal Clinic site. I have read others stating the same thing though.
The information given on the medicine killing for the 30 day coverage has to do more with the life cycle of the larvea not becoming mature and the meds killing it in the firts days rather then the medicine staying in the system of the animal. That is why it is stressed by the companies to admister the medicine within a 30 day cycle, not 45. Being in the medical field even with humans medicines, very few if any stay in the system when orally taken for 30 days. It is digested and expelled. The same occurs with heartworm meds. Vets and reps alike should confirm this information.

Link from the Westwood Animal Clinic on Heartworm preventatives.
http://westwoodanimalhospital.com/MedPages/Parasites/Heartworm%20%20-%20Canine%20Heartworm%20FAQ.htm

This is from the American Heartworm Society.

There many types of medications available for heartworm prevention. What is the difference between the daily and monthly tablets?

ANSWER Until the late 1980s, the only medication available for the prevention of heartworms had to be given daily. These products work by killing the microscopic heartworm larvae deposited by the mosquito, but must be given every single day to be most effective. This is because infective heartworm larvae quickly molt within two to three days into their fourth stage of development. This fourth stage can not be killed by the daily medication. Daily heartworm preventives have largely been replaced by monthly products and are no longer commercially available in the United States. Compounding pharmacies still formulate daily preventives on an "as needed" basis. An important note to keep in mind is that monthly medications are quickly eliminated from a pet's system and do not continue to work for 30 days. Instead they work "backwards" to eliminate the larvae the pet acquired the previous 30 days, in essence, "de-heartworming" our pets each month in many cases, these monthly preventives control other parasites too.
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Here's a link from Lew Olson's site, talking about the various preventatives and how they work.
http://www.b-naturals.com/newsletter/heartworm/

Lorelei, you are correct, the meds work by killing the microfilariae that were acquired from the month prior.

The 45 day dosing is mentioned here also, with links to the research that supports it...this is based on the maturation of the microfilariae, which take longer than 45 days to mature to the next stage.

I wonder if there is any chance that the meds themselves are having issues (ie quality control problems), since from what I have read, it is highly unlikely that the larvae have developed genetic resistance to the meds.

Again, Lorelei, wishing a speedy recovery for your dog...

and another link to an FDA publication, discussing resistance and also the bacteria that is found along with the heartworm.

Lorelei, is your dog getting an antibiotic also (doxycycline)? Might be worth discussing with your vet...

http://www.fda.gov/AnimalVeterinary/NewsEvents/FDAVeterinarianNewsletter/ucm210100.htm

Lynne,

Thanks for the kind thoughts toward my dog. My vet and I are having in depth discussions as to the exact path we are to take. He has been awesome and from what I have studied is up to date on the latest info as well.I have read on the doxycycline both ways so not sure if we are going to do that yet. We have started Advantage multi right now and the shots possibly the first of Nov.