Are Heartworms getting worse? A Seminar: Part 1 of 6

Heartworm map 07 The Problem

In the past three years, we have seen a significant increase in the number of dogs where heartworm preventive medication has not been 100% effective.  In addition, it has been more difficult to clear infected dogs of the worm burden with treatment. It has also been more difficult to clear microfilariae from dogs post treatment.  Some dogs who have been treated and cleared have heartworms again the next year, despite year-round preventive medicine. These reports come from the Mississippi valley, starting about 100 miles south of St.Louis, and getting worse as one goes south.

For my own part, it was as though in the year 2006 "the rules changed".  Many veterinarians and dog-owners have become convinced that a resistant population of heartworms has developed.  With no new drugs in the pipeline, this is a major concern.

The Experts

As late as October of 2008, educational meetings, publications and discussions tended to dismiss the problem as merely one of perception, rather than an actual change in the incidence of the disease.  

In April of 2009, I was privileged to be one of six veterinarians from private practices invited to a round-table discussion seminar.  Our input was sought in regard to our experience with heartworm disease, its prevention and treatment, and our problems with all of these.

Three leading parasitologists, all involved with heartworm research, opened the discussion by saying, “We know that something has changed, but we don’t know what it is.  There is a problem, but the underlying cause has not been determined.  We are here to gather and share information.” 

This was a refreshing change.  You know what they say: the first step is admitting you have a problem.

Epidemiology Considerations

Everyone now concedes that something has been different in the past three years.  There was a sudden “uptick” to a higher level of problem (observed in my practice in 2006 and continuing over 2007 and 2008), rather than a slowly growing problem.  In other words, we went from “Plateau A” to “Plateau F” without going through B,C,D,&E.  The change seemed abrupt, rather than gradual.  This speaks against the development of a resistant population of heartworms.  There are other epidemiological objections to this, as well. 

The parasite has a very long life cycle: six months minimum.  It requires a vector, the mosquito.  Reservoirs are widespread and act as carriers for years, and more than half of the reservoir hosts are never treated.  Coyotes are a big reservoir of the disease, but their rate of infection has not been studied anywhere except in California.  There are, of course, also many, many dogs in communities and the countryside that do not receive preventive medicine, and act as reservoirs of the disease.

This makes the development of a resistant population of worms by “Darwinian” selection almost impossible.

On the other hand, as yet no other reasonable explanation has been put forward.  An increase in the numbers of mosquitoes might explain it, but there is no data to support or refute this.  There just is no helpful mosquito data. Our subjective experience locally is that the mosquitoes are terrible, but they have been so for many years.

11 thoughts on “Are Heartworms getting worse? A Seminar: Part 1 of 6

  1. PBurns says:

    When you say “In April of 2009, I was privileged to be one of six veterinarians from private practices invited to a round-table discussion seminar,” what does that mean?

    Who was the seminar for? Was it actually an “education seminar” paid for by Merial or some test company? Were you financially compensated or compensated by travel or goods? Who was the audience? Who was the sponsor? What was the goal of the sponsor and why did the audience care?

    Also, what data sets were used to support your conclusions? Surely it was not simply “I think I’m seeing more in my practice” kind of thing?

    Basically, what I’m saying is that the information here is too light to take seriously, but I take the implication seriously so I am curious as to to what spurred this seminar and this post. Is there really a problem, or is this simply clever sales and marketing by tapping folks and telling the they are “kley opinion leaders” and by the way, here’s a discount, a free sample bin, a $500 speakers fee, and free travel to Ohio … That happens all the time.

  2. Doc says:

    Hello, PBurns,

    Here is a link to a post from one year ago that detailed my some of my concerns at that time.

    For the last three years, I have had the unfortunate task of telling a significant number of dog-owners that their dog has heartworms, despite giving preventive year-round (as best I can determine, then DID give it). I have had to pursue the “Satisfaction Guaranteed” policy from Novartis, Merial, and Pfizer many times. The clients aren’t happy and neither am I.

    When I attended a seminar in Memphis, Tennessee in October, there were sixty or so very disgruntled veterinarians wanting to know “why the preventive isn’t working like it used to”. The speaker was Dr. Tom Nelson, a past president of the American Heartworm Society. [The meeting was sponsored by Merial. I drove 100 miles and got to eat some cold dinner; I was late.] While there was open discussion, it was basically a didactic presentation. “Here’s what’s really going on with heartworm disease and medicines. Doesn’t that clear everything up for you?” [NO, it doesn’t.] There was a lot of good information on the heartworm life cycle, how preventives are tested, and treatment strategies (for infected dogs, not preventives. However, his thesis was that we are just detecting more cases with better testing, and that if there ARE more cases, it’s because of more tropical storms hitting the gulf basin. The veterinarians in attendance weren’t buying that explanation, as I mentioned in the post. It wasn’t a “lynch mob” atmosphere, but nobody felt like they got any help in dealing with the apparent problem of preventive medicine failures.

    The seminar I attended in April was completely different. [Again, sponsored by Merial] I do not have personal permission to quote the parasitologists, so I did not name them, but they are big guns. This time it was “We know something is different, but we don’t know why. Tell us what you’re seeing.” They DO know something is different, because they are getting a mountain of claims against the guarantee (and paying for the treatments, too, to be fair). The manufacturers do have a vested interest in figuring out why things have changed. They are paying out the wazoo on these guarantees.

    From an epidemiological standpoint, it is interesting that Merial’s reporting shows some clinics in the “Red” areas that never report claims, even though surrounded by clinics that do. I have been in personal contact with colleagues over a 100-mile radius and have found nobody who is not experiencing the same frustration that I am.

    Even though these dogs have very few heartworms, and seldom suffer ill effects from the treatment, it becomes a trust issue between veterinarian and client. “You told me to do this and I did it and it didn’t work.” I cannot tell you how tired I am of going through this with disappointed clients. It is no fun. I want to return to the days of (near) 100% effectiveness of the preventive. I don’t like being on a first-name basis with the call-in techs at the guarantee program.

    In re compensation and audience: I did receive an honorarium for attending, which surprised me. I have been so frustrated with the situation that I would cheerfully have paid for the opportunity. I closed my office for a half-day and drove 90 miles to Cape Girardeau. We talked from 3:00PM until 8:00PM.

    I don’t know what use that Merial or the parasitologists (who are University professors, not corporate flacks, though I’m sure they get grants and sponsorships to speak) will make of our clinical impressions.

    I can tell you this: they weren’t there to tell me I have a problem I didn’t know about. I was there to tell them that I have a problem that I don’t know what to do about. I suggest you reserve your judgment until I can get the whole discussion posted.

  3. etta denton says:

    I is just found out yesterday that my lab does indeed have heartworms for a 2nd time. He was treated in 2007 and was clear at his last testing in2008 . Yes, Meril is the company and they did not pay one cent for his treatment. My husband is a pharmacist and retail drugstore owner so he ordered Heartgard for our 4 dogs through his wholesaler. We had invoices, I marked the date every time I gave the drug. So It made me sick to continue buying heartgard for the past two years but I did it. And before you ask, the vets I use are 40 miles from my home so we ordered the drug from the wholesaler. Meril won’t pay a cent this time either. This sucks!

  4. etta denton says:

    Upon rereading my post I realized I had omitted the most important point. I love my dog and it really hurts me that this is happening to him. He is the one true innocent in all this.

  5. cheri says:

    Same story -Coco- my lab was treated in 2009 and 24 months later has tested positive. She has had her medication on the 15th of every month and it was purchased at my vets office- having trouble getting the company to help us because she was not tested in 2010. We live in Memphis right in the heart of heartworm country. I’m thinking the the meds are a waste of time and money and it needs to be admitted.

  6. Doc says:

    Hello, Cheri,

    While it is super frustrating to give the medication and have a dog come up with heartworms anyway, it is not a waste of time and money.

    These dogs get a few heartworms, but without the meds, they would have a LOT of heartworms and have much more trouble with the treatment.

    We are switching our problem dogs and high risk dogs to Advantage Multi in the hopes that it will have fewer failures.

    Good luck.

  7. Debbie Fowler says:

    My 17 month old Westie is currently receiving heartworm treatment and has been on heartworm preventative since 7 months old with NO missed dosages…In light of this becoming more and more of an issue and since we are in central Geargia..does the research support ghb going forward with any particular brands in effort to avoid repeating this very unfortunate event in the future..Thanking you in advance for your tesponse.

    • Doc says:

      Hello, Debbie, I am sorry to be so late in replying, but our website was re-vamped and the blog comments went into limbo. In our area we had a lot of problems for three years, from 2006 through 2009. At that time we switched problem dogs over to Advantage Multi and we haven’t had any failures with the monthly Advantage Multi. You might also consider ProHeart-12 injections. It is the same drug (moxidectin) and you wouldn’t have to worry about missing a dose.

  8. Renae says:

    Dear Dr Mobley-

    I hope this doesn’t clot up your message boards but I just wanted to THANK YOU so much for all of this extremely helpful and thoughtful information. I have been reading your blogs non-stop since I found it. My rescue was diagnosed with heartworm despite being in preventative. I’m learning so much without being excessively worried. Thank you for spending the time to write these straight-forward and informative posts and respond so kindly to all of these worried people. Your patients are very lucky!!!

  9. Renae says:

    Dear Dr Mobley-

    I hope this doesn’t clog up your message boards but I just wanted to THANK YOU so much for all of this extremely helpful and thoughtful information. I have been reading your blogs non-stop since I found it. My rescue was diagnosed with heartworm despite being on preventative. I’m learning so much. Thank you for taking the time to write these straight-forward and informative posts and respond so kindly to all of these worried people. Your patients are very lucky!!!

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