Heartworm preventive medicines may not be 100% effective.

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.

Comments

[Brian Gardner]

Are there any dietary supplements which will help strengthen the dog when this disease is diagnosed? Will such treatment help at all?

[Doc]

The field of nutritional supplementation has a lot of gray areas. Certainly if a dog is not in good physical condition, it will be more difficult for him to withstand the disease, as well as the stress of treatment.

Dogs eating a well-balanced diet, who look and act healthy, probably do not need additional supplementation. There are many nutritional supplements on the market, and I do not believe that any of them are magical. There are certainly good ones available, and there are certainly dogs that benefit from them.

That said, I am not aware of a specific nutrient or supplement that would support the dog through a thrombo-embolic crisis.

On the human side, we are suckers for virtually any type of nutritional supplement because we know that (as a nation) most of us eat lousy diets. Modern, name-brand, commercial dog foods provide good, well-balanced nutrition for most dogs that do not have some medical problem.

Thanks for reading and writing.

[Sheila]

In the last few months, I have had 4 of my dogs test positive for HW, in spite of absolutely monthly year-round doses of Interceptor. 2 are large breed dogs, one is a beagle that has been mine since she was a puppy in 1999, one is a 25 pound mixed breed dog I received as an adult about 5 years ago. All of these dogs were negative at their last test. Two of them live outside, 2 of them live inside. In addition to the HW problem, we also have difficulties ridding them of whipworms, which the Interceptor package insert say it will handle adult whips. If you find a safe, effective alternative preventive, please blog about it, we would love to know.

[Doc]

I wish that I had an answer for you. To the best of my knowledge,there are no new drugs in the pipeline. I was giving my own dog Heartgard on the first, and Revolution on the 15th, (i.e. dosing twice monthly)and she has heartworms this year.

While I have had generally good luck with the Interceptor for whipworms, I'd probably use Panacur monthly for 3 months in the dogs that have this problem. Knowing that you generally have a fairly sizable dog population, it is possible that there is a heavy re-exposure rate from fecal contamination of the soil.

Thanks for reading and writing.

[WendyAnn]

When I lived in Florida I had a dog come up positive for heartworm when he had never missed a dose of Frontline Plus in his entire life.

I had six dogs at that time and he was the only one to come up positive. I know none of the other dogs ate his dose because I administer them personally.

Now that I live in New England again, my fear of hearworms has lessened, but I'm still afraid one of my dogs will get them even though they get heartworm preventative.

He was treated successfully and is a very healthy dog.

[WendyAnn]

oops - that should have read -- Heartguard Plus, not Frontline... Duh!

[Bettina Hart]

I have 3 dogs who have been on Heartgard Plus their entire lives; never missed a dose. My large breed, mainly outdoor dog was just diagnosed with a positive heartworm (antigen test). He starts treatment Tuesday.He's very active, barking and jumping, and I'm very concerned about keeping him quiet to avoid PTE. I live in Northwest Alabama. People should be told that the preventive is NOT 100% effective and should get their dogs tested twice a year. Thanks for your website-

[Doc]

Hello, Bettina,

I guess nothing is 100%, but I really felt that our
heartworm preventive were nearly that until 2006.
This has been an almost daily source of disappointment
for me in the last two years. I really appreciate the
companies standing behind their guarantee, but it's
still not fun.

Thanks for reading and writing.

[Pietoro]

I thought Ivermectin was a treatment, not a preventative? It will kills worm larvae if they're there, but it doesn't stop them from appearing at all.

[Pietoro]

In other words, even if a dog gets some worm larvae while between doses, as long as he gets the next dose he's due for on time, he should be just fine. Since the drug doesn't -prevent- worms, of course they're going to appear once in a while. But the drugs will shut them down pretty fast before they even start to hurt the dog. It seems like not that big of a problem, it's just the drug working the way it always has...

[Doc]

Ivermectin is considered a "heartworm preventive drug" in that it is supposed to prevent the development of adult heartworms.

Mosquitoes infect the dog with microscopic larvae daily and it takes these larvae at least six months to mature into adult worms in the heart.

When one gives the ivermectin each month, he is (it is to be hoped) killing all the heartworm larvae that the mosquito has brought there in the previous few weeks.

The apparent change in my geographic area is this: there is a significant increase (tenfold) in the number of dogs who develop adult heartworms, despite regular monthly doses of ivermectin and other heartworm preventive drugs (selamectin, milbemycin oxime, etc.). If the ivermectin were killing all the developing larvae, this would not be happening.