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Heartworm Medication Part 1: Truths, Omissions and Profits

Heartworms are Spread by Mosquitoes.
Heartworm Meds are Spread by Fear.

It’s getting warmer outside — time for sellers of heartworm medications to start scaring you to death.Television and print ads, which used to push meds only during warm summer months, now urge you to keep your dog on medication year round. The question is: why the change?

Drs. David Knight and James Lok of the University of Pennsylvania School of Veterinary Medicine, addressing recommendations for year round meds, warned:  “The practice of some veterinarians to continuously prescribe monthly chemoprophylaxis exaggerates the actual risk of heartworm transmission in most parts of the country and unnecessarily increases the cost of protection to their clients.”

So, is the change to year round meds all about money? Or is there more to this story?

Heartworm “prevention” is a major health decision for pet parents and multi-billion dollar Big Business for drug companies, veterinarians, testing laboratories and on-line sellers of medication. When health intersects money, there’s a lot of room for conflict of interest. Only by understanding the business aspects and the truth about heartworm transmission can you make an informed decision about if, how and when to protect your dog with commercial products.

While everyone agrees that heartworm infestations can be life-threatening, infestation is far from inevitable nor is it the immutable death sentence advertisers would have you believe. (Otherwise, all dogs and cats not on meds would die of infestation. But they don’t.)

Every holistic vet I’ve consulted had concerns about the long-term safety of heartwormmedications. Well-known vet, author and columnist Martin Goldstein wrote in his wonderful bookThe Nature of Animal Healing that he sees heartworms as less epidemic than the “disease-causing toxicity” of heartworm medicine.

Dr. Jeff Levy, vet and homeopath, concluded “that it was not the heartworms that caused disease, but the other factors that damaged the dogs’ health to the point that they could no longer compensate for an otherwise tolerable parasite load.” Those factors include, “… being vaccinated yearly, eating commercial dog food, and getting suppressive drug treatment for other symptoms….”

Heartworm meds do not, by the way, prevent heartworms. They are poisons that kill heartworm larvae (called microfilariae) contracted during the previous 30-45 days (and maybe longer due to what is call the Reach Back Effect).

The heartworm industry authority, The American Heartworm Society (and their cat heartworm site) offers a wealth of information. Their website is a public service but also a marketing tool aimed at buyers and resellers of heartworm meds. Sponsors of this website are a Who’s Who of drug companies. Fort Dodge Animal Health (Wyeth), Merial and Pfizer are “Platinum Sponsors.” Bayer merits Silver. Novartis, Schering-Plough, Virbac and Eli Lilly get Bronze. Most of these companies have sales reps that regularly call on vets and show them how to sell you heartworm meds. With any purchase of any drug, we recommend you ask for information regarding possible adverse effects, the necessity for taking this drug and available alternatives.

How Heartworms Infect Dogs: It’s Not Easy!

Well, now that we’ve looked behind the scenes of the heartworm industry, let’s take a look at how the heartworms themselves (called Dirofilaria immitis) do business. Seven steps must be completed to give your dog a dangerous heartworm infestation:

Step 1: To infect your dog, you need mosquitoes (so you need warm temperatures and standing water). More specifically, you need a hungry female mosquito of an appropriate species. Female mosquitoes act as airborne incubators for premature baby heartworms (called microfilariae). Without the proper mosquito, dogs can’t get heartworms. Period.

That means dogs can’t “catch” heartworms from other dogs or mammals or from dog park lawns. Puppies can’t “catch” heartworms from their mothers and moms can’t pass heartworm immunity to pups.

Step 2: Our hungry mosquito needs access to a dog already infected with sexually mature male andfemale heartworms that have produced babies.

Step 3: The heartworm babies must be at the L1 stage of development when the mosquito bites the dog and withdraws blood.

Step 4:  Ten to fourteen days later — if the temperature is right –the microfilariae mature inside the mosquito to the infective L3 stage then migrate to the mosquito’s mouth. (Yum!)

Step 5:  Madame mosquito transmits the L3′s to your dog’s skin with a bite. Then, if all conditions are right, the L3′s develop in the skin for three to four months (to the L5 stage) before making their way into your dog’s blood.  But your dog still isn’t doomed.

Step 6:   Only if the dog’s immune system doesn’t rid the dog of these worms do the heartworms develop to adulthood.

Step 7:   It takes approximately six months for the surviving larvae to achieve maturity. At this point, the adult heartworms may produce babies if there are both males and females, but the kiddies will die unless a mosquito carrying L3′s intervenes.  Otherwise, the adults will live several years then die.

In summation, a particular species of mosquito must bite a dog infected with circulating L1 heartworm babies, must carry the babies to stage L3 and then must bite your dog . The adult worms and babies will eventually die off in the dog unless your dog is bitten again!  Oh, and one more thing.

Heartworms Development Requires Sustained Day & Night Weather Above 57˚F

In Step 4 above I wrote that heartworm larvae develop “if the temperature is right.”

The University of Pennsylvania vet school (in a study funded by Merial) found: “Development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27C (80F). Below a threshold temperature of 14C (57F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically.”

Knight and Lok agree: “In regions where average daily temperatures remain at or below about 62˚F (17˚ C) from late fall to early spring, insufficient heat accumulates to allow maturation of infective larvae in the intermediate host [the mosquito], precluding transmission of the parasite.”

The Washington State University vet school reports that laboratory studies show that maturation of the worms requires “the equivalent of a steady 24-hour daily temperature in excess of 64°F (18°C) for approximately one month.”  In other words, it has to be warm day AND night or development is retarded even if the average temperature is sufficiently warm. They add, that at 80° F, “10 to 14 days are required for development of microfilariae to the infective stage.”

Jerold Theis, DVM, PhD, says, “If the mean monthly temperature is only a few degrees above 14 degrees centigrade [57 degrees F] it can take so many days for infective larvae to develop that the likelihood of the female mosquito living that long is remote.”

I have never found this temperature-dependent information on a website promoting “preventatives,” but only in more scholarly works not easily accessed by the public. There is, as far as I can find, only one mention of temperature on the Heartworm Society (on the canine heartworm page) and none in the Merck/Merial Veterinary Manual site or Merial’s heartworm video— even though Merial funded the UPenn study.

The Society also reports, “Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas and presence of animal reservoirs (such as infected dogs or coyotes).”

Read Part 2

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14 Responses to Heartworm Medication Part 1: Truths, Omissions and Profits

  1. heidi April 24, 2013 at 7:07 PM #

    I think it’s important to note a genetic mutation common in collie and collie-related breeds that may influence an owner’s decision on whether or not to treat their dog. My Australian Shepherd is heterozygous for a mutation in her MDR1 gene. The 4bp deletion results in a malfunctioned transporter protein of the blood-brain barrier. This transporter is key in preventing a wide range of chemical from accessing the nervous system. She not only is lacking a functioning pump, but she is also expressing a pump that allows free access to her nervous system. Dogs with this mutation WILL DIE if exposed to high levels of ivermectin and other nervous system toxins. After I had her tested, the results provided a long list of drugs to avoid. Little has been studied on the long term effects of chronic exposure to low doses of ivermectin on MDR1 mutants. Do to this, I avoid all treatment, especially given her development of tremors after several years of “prevention”. I would also like to add that you left out the probability a mosquito would even live so long as to go through all those stages to be able to infect a dog. I think it is more important to feed your dogs real food (not fortified mystery dried pellets) to keep their immune systems strong.

  2. Jeff April 29, 2012 at 1:08 PM #

    I adopted a 6 mo golden retriever from a vet hospital when I lived in Santa Cruz; I was just out of college and not the most responsible person at the time. I moved and I lost my dog’s heartworm meds in the shuffle. At the time, in the mid-80′s, the pills were given every day and you could only get them from the vet. I missed her treatment for 2-3 months. I procrastinated going to the vet…when I finally went, she had contracted heartworm and had to go through the treatment. I felt like the worst person in the world.

    Santa Cruz,CA is not known for its hot humid weather and/or mosquitoes. Now I live in Austin, TX and there is no way I would skip it! I understand why people are concerned about the toxicity but as stated by others, the benefits far outweigh the risks.

    I also have reservations about topical flea meds. When I do use it, I only give my dog half a dose and only during the flea season. It seems to be enough.

  3. Denise March 27, 2012 at 1:16 PM #

    Having fostered a couple of dogs that were sick due to heartworms, I’ll keep my dogs on it year round. It’s a case where I think the benefit outweighs the risk. Heartworm treatment is painful for the dog and requires 6-8 weeks of rest. Not fun for anyone.

  4. Dawn March 22, 2012 at 11:19 AM #

    I am curious to know at which of the 7 steps would a dog test positive for heartworms?

  5. PM March 15, 2012 at 8:37 PM #

    Ms. Howell,

    I’m glad you have “Omissions” as part of your title, since you’ve omitted several facts about heartworm disease and the actual risks of the preventatives that would undermine your reasoning.

    For one; the vast majority of toxicities are caused by massive overdoses of the medication. We’re talking 100 to 1000 times the recommended dose in most cases. Next; research has also shown that just like the adult mosquito can lay dormant through the coldest times of the year, so can the infective stage of the heartworm within that mosquito. This means that one day warm enough to bring an adult mosquito out of dormancy is also a day your dog could be infected. Also you have omitted the fact that heartworm preventatives also protect dogs from other intestinal parasites, including roundworms and hookworms, which can be passed to humans at ANY time of the year.

    Do you really believe that since heartworm infestation is not an “immutable death sentence” that you should forego preventative measures?

    Clearly you have never seen a dog die of hearworm disease. I have.

    Clearly your human child has never undergone treatment for a condition called, ocular larval migrans (look it up). I did.

    Clearly your hatred (or fear) of big pharma is keeping you from giving the readers ALL of the information they need to make a TRULY informed choice.

    So, based on ALL of the information, I will continue to give a safe monthly heartworm preventative year round for the sake of my pets and the family who loves them.

    • Lori July 6, 2012 at 2:19 PM #

      I used to groom 2 Scotties belonging to a physician who worked for the then makers of a popular heartworm preventitive starting with an H………………….;)
      He said he would NEVER give it to his dogs after witnessing the toxicity of it. This was not a Holistic man by any stretch of the imagination. He had merely seen with his own eyes the results of the testing and was horrified.
      That’s all I needed to hear.
      I honestly believe you and probably even most vets, truly believe in it’s safety because that is what the Pharmaceutical companies are telling us.
      SO not true……………

  6. Debbie and Coco March 15, 2012 at 5:16 PM #

    While agree that much of the marketing is hype and that we over vaccinate and medicate our animals, I must say that year round heartworm preventatives are a must here in Florida where the mosquitoes are year round.

    Additionally, our rescue continually takes in dogs who test positive for heartworms. The treatment is painful and expensive.

    I guess, long story short, what you do in this regard should be determined by where you live.

  7. Sarah Smith March 15, 2012 at 11:41 AM #

    Heart worm medication is recommended by most vets year-round here in Minnesota and has been for as long as I can remember. I’m a big proponent of doing things naturally, but I think there are two other factors you need to consider when determining whether to treat your dog year round, part of the year, or not at all for heart worm. One is how prevalent mosquitoes are in your area. Due to the sheer number of mosquitoes here during the summer months, I don’t doubt that we have the right type and carriers of the disease flying around. But in dryer parts of the country, this may not be true. You also have to consider the recovery process if a dog DOES get heart worm. Yes, there is medication available to treat it so it’s not a death sentence, but the recovery can mean several weeks on “cage rest” (difficult for both dog and owner) and takes its toll on a dog’s body. Our dogs aren’t outside dogs and we live in an urban area. Your dog’s individual lifestyle is also important to consider: If my dog spent an appreciable amount of time outside or had a lifestyle that exposed him to more mosquitoes (i.e., a field or hunting dog), if we lived in the country or had a cabin up North in the woods where we spend a lot of time, this would also influence my decision.

    We treat part of the year: April through October or from around the time that things thaw out to the time they freeze again.

    • Paula March 6, 2013 at 1:15 PM #

      I like the answer from Laura.


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