This is the sort of "can of worms" one hates to open. Yet I feel the need to open a discussion. I am concerned that heartworms (at least in our area) are starting to become resistant to all the preventive medications that we have.
I have been practicing in a severely endemic area for heartworm disease since 1978. Starting in 2006, it seems as though "the rules have changed" with regard to the prevention and treatment of canine heartworm disease. The problems that I have encountered are mirrored by other veterinarians in my area. They are experiencing the same frustrations. My Novartis representative has been working from Missouri south to Louisiana. In an unofficial conversation, he indicated that he had heard it was primarily a problem in the Mississippi valley, with the problem being worse as one goes farther south.
We routinely test all dogs for both microfilariae (baby heartworms in the blood) and heartworm antigen (adult heartworm protein in the blood) at their annual examination. In years past, I always detected a small number of heartworm-positive dogs which had allegedly been on year-round prophylaxis (as we recommend). Some few of these would prove to have an obvious deficiency in compliance. When I asked, "Is it possible that he missed a dose?", the folks would start looking at the floor and shuffling their feet. "Well, you see, Aunt Tillie was sick, and the boy was on a traveling team with the baseball season, and the motor in our car blowed up and…". Which is to say, yes, it is possible that he missed a dose.
On the other hand, each year we had four or five patients whose owners had bought plenty of preventive medication. These were folks whom I believed perfectly honest in their report of administration, and competent in their administration. With this small number of apparent product failures each year, it was easy to attribute the failure to things like the dog clandestinely vomiting under a bush. Nobody watches his dog 24 hours a day. So even though the owner was 100% conscientious in giving the medicine, and the medicine near 100% effective, these rare failures were easy to attribute to the weak link in the chain: the dog.
In 2006 and 2007, we have gone from 4 or 5 apparent product failures per year to 4 or 5 per month. Almost all are large-breed, outside dogs. We have the same percentage of failure among all the different products that we use. Interceptor/Sentinel (which we use in the majority of our patients), Heartgard 30, and Revolution. With this ten-fold increase in incidence of the problem, I am very concerned that the parasites are becoming resistant to the avermectins (EVERY heartworm preventive on the market except for Sentinel and Interceptor) and to milbemycin oxime (the active ingredient in Sentinel and Interceptor).
The drugs are far from useless, however. In fact, I believe that they are still preventing almost all of the heartworm infections from reaching maturity. In a previous post, I noted that a dog’s problem with heartworm treatment is pretty directly related to how many adult worms he has to deal with. These dogs who have been on year-round preventive (and turn up positive for heartworms anyway at their yearly physical) rarely show signs of thrombo-embolic complications after adulticide treatment. This would indicate that these dogs have very few adult worms present. For this reason, I believe that the preventives are largely effective, though no longer 100%.
Again, though the number of heartworm prevention failures has risen dramatically, it is still a very small percentage of our patients. Almost all have been large-breed, outside dogs, and our practice is in an area with a heavy mosquito population during the warmer months (and a few skeeters nearly all year). I certainly would not stop giving the preventive medications.
I certainly would give the medicine year-round, and I certainly would have my dog tested every year. When we can document the dog’s previous "clean" status, and document the preventive medicine purchases, the manufacturers have been very good about paying for the dog’s treatment under their guarantee programs.