Heartworm Medication Part 1: Truths, Omissions and Profits

Written by Jan on May 13, 2009 – 1:00 am

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

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 heartworm medications. Well-known vet, author and columnist Martin Goldstein wrote in his wonderful book The 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 and female 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).”

 Canadians: Please click the link find maps and stats on heartworm in Canada in 2010.  


Read Part 2 of this article: Heartworm Preventative Options Learn how to reduce the number of times you give “preventatives,” about the little-known FDA approved low-dose preventative, and what to do if you don’t want to give meds at all. If you’re considering using the 6-month remedy ProHeart6, please read our previous blog post: Heartworm Protection: Do We Need ProHeart 6? You might also like reading Pesticides & Preventatives Poisoning Pets?

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Please leave us a comment and let us know how your thoughts or questions regarding heartworm prevention.

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Tags: dog, dog heartworm, heart worms, heartworm, heartworm disease, heartworm in dogs, heartworm medication, heartworm medicine, heartworm preventative, heartworm prevention, Heartworms, mosquito, mosquitoes, mosquitos, natural cures, natural preventatives
Posted under Heartworms, Pet Meds | 185 Comments » Email This Post

185 Comments to “Heartworm Medication Part 1: Truths, Omissions and Profits”

  1. Jan Says:

    Edward, it is my understanding, although I could be wrong, that one cold night will just postpone maturity.

  2. Edward Says:

    Thanks Jan

    Will search further as this is a very important point


  3. misticbear Says:

    hi – i have recently moved to the sierra nevada foothills in
    northern ca. from nyc. while in nyc, i never vaccinated or
    gave heartworm to any of my dogs. they didn’t need it and
    all were fine. i have one dog now and when we left nyc i
    gave her the rabies vaccine since we would be living in the
    country with lots of wildlife. i now face the heartworm question…
    as i hear it’s very bad here..i am taking her in for testing today for lyme / heartworm (ticks are terrible here too) .. my decision is do i do the 45 day heartworm regime during the bad months ..or treat her w/ meds if an early detection is made. i will test her every 6 months. i have used a topical like frontline 2x for the ticks…don’t know if i want to keep on because they get on her anyway .. we have been here 7 months now. heading into spring, i am trying to make the best choices .. thanks for any input you have to offer.

  4. Jan Says:

    Misticbear, it sounds like you have a good plan. If it were my dog, I’d do 45 days on the tick meds as well, and do them at separate times weeks apart. There is no great solution.

  5. misticbear Says:

    thank you! headed to the vet now. happy to get your response before i go. much appreciated!

  6. misticbear Says:

    i used sentry fiproguard last time instead of frontline – have you had experience with it? the vet said the mosquitos were around all
    year ..we did have a very mild winter.. does anyone have knowledge
    of this area? i am in nevada city. thanks! all info is much appreciated. it’s a whole new world for us here. we are so happy to be in nature & she loves romping in a big yard but it does come with it’s challenges! 😉

  7. misticbear Says:

    ps – also wondering if anyone has experience with the petguard herbal collar for ticks + fleas? thanks!

  8. Natural/Organic flea and tick protection? - YorkieTalk.com Forums - Yorkshire Terrier Community Says:

    […] friend of mine sent me this article. Thought you may wish to read it. Heartworm Medication: Is Year Round Protection Necessary? | Truth4Dogs Heartworm Medication Part 1: Truths, Omissions and Profits Written by Jan on May 13, 2009 – […]

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  10. Betty Says:

    We have a four year old German Shepherd+Newfoundland mix. We lived in south Louisiana until he was three, and we moved to San Diego a year and a half ago. We recently got him tested for heartworms and he came up positive. We’re assuming that he contracted them in Louisiana.

    The only option the vets here have offered us is the three-month treatment of Immiticide injections. As our dog is very hyperactive, we are wary of giving him a treatment that will essentially require him to have little to no activity for three months. Furthermore, the treatment is cost prohibitive at over a thousand dollars.

    Do you know of any alternative treatment options for when a dog has tested heartworm positive?

  11. Jan Says:

    Betty, I don’t know of any treatments off-hand but you can email my holistic vet’s office and ask. She’s in Solana Beach. I would think it would be less expense and perhaps easier on your dog to get holistic treatment. Also, I believe that there are degrees of infestation. You might want to get a heart ultrasound first. They’re several hundred dollars but might save you money in the long run. Email me and I’ll give you contact info. http://www.dogs4dogs.com/contact.

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  14. Ariana Says:

    I stopped giving my bulldog heartworm meds after reading some disturbing side effects. I live in NYC and up until now I was kind of misinformed about heartworm and how it is transmitted. I didn’t think my dog was at a high risk as he is always indoors for the most part in our apartment. Now that it is fall, we have been leaving our windows open occasionally and my boyfriend noticed 1-2 mosquitoes in our apartment. He really worried me and noted that our guy who is 6.5 has been acting more stubborn on walks. I think this is just a quality of bulldogs but now i’m freaking out and am worried sick that he has heart-worm. Last test he had was a year back and it was negative. He then went on the meds for lets say 6 months and I stopped giving them to him in the winter months. I really don’t want to put him back on these meds but i’m sure the vet will suggest I do. I really hope he doesn’t have it and that there are other preventative options. Any words of advice?

  15. Terry D'Souza Says:

    Jan, I love your common sense approach and your candor. Keep up the good work.

    I live in the Cincinnati, OH area and my German Shepherd Sue gets the heartworm etc. pill once a month from May thru’ Nov.
    Thanks for the 57 F guideline.

  16. Newt Says:

    Qts. 1 … If using iverhart max primarily for heartworms , and if
    Skipping winter months per suggestions ….. Would I not lose
    Out on the treatment iverhart max offers for round, hook and
    Tape worms for skipped months ?

    QTs. 2…If not skipping winter months , is iverhart max the only
    Treatment or action I need to take ?

    Qts 3 …not sure I ‘ ve understood how to treat for worms ?

    Qt 4 … Why is a topical product like Advantix 11 needed or justified if I am correct in stating that it does nothing to treat
    Or prevent round and hook worms and only fights fleas which can
    Be licked or eaten and cause tape worms . I guess I’ m trying to
    Determine what is needed for other worms in addition to using iverhart max every 6 weeks in non winter months primarily for heartworms .

  17. Charles Kaufman Says:

    I am reading “scared” and love it. I have been a supporter of the ‘rabies challenge fund’ for years. I also am doing an on-going quiz on my FB rescue page based on your quiz. I love being able to educate people and help dogs. It is a real fight to get people to let go of old beliefs, but well worth it. Thanks
    Charles Kaufman – Atlanta Dog U.com

  18. savannah Says:

    LOVED this posting. I have to say something about the posting from Sam (128), having a degree, even post grad education doesn’t mean you have ARRIVED, if the education you have gotten was biased and tainted. Arrogance runs high in academia, believe me I know, credentials don’t make up for common sense. Sometimes the outcome is the ability to regurgitate the propaganda they have been taught. Lots of PhD’s aren’t as EVOLVED as many of those lacking those initials and the high priced piece of paper.

    Education is important, but it also depends on the SOURCE. Wisdom FAR exceeds most education! Suffice to say I am of the holistic camp. My mom was on her way to being a Dr way back when and she use to say the only thing Drs were good for was setting a bone and reading an xray, everything else- they just practiced, ha.

    She thought dog food was crap way back when and ALL our dogs were fed people food. Had 1 rabies shot, as she said it LASTS, no proof to the contrary was her argument to the Vet. Our dogs died of old age literally-oldest fox hounds ever! I have had shelties and both made 17-why, fed organic people diet, limited vaccines and other “promoted” prevention, use a holistic approach and Vet. I truly believe the food fed to dogs and humans is causing cancer in all of us because of what they AREN’T telling us.

    So please, limit your exposure to stuff…my family all died in their 90’s-lived healthy productive lives w/o disease, never saw Drs., didn’t believe in the medical industry, wouldn’t pop a medical pill, rather use an herb.

    And be a pet advocate, they have no voice. I am not against sound info, but as Jan says-research the source and WHO is backing the advice, statistics, info. Please note that Vet medicine is part of the medical INDUSTRY and it is a FOR profit industry, the focus is NOT what it once was, it is a BUSINESS-bottom line, and revenue has replaced needed care/ with “preventative” care in many cases. Sad reality.

    I could site my “credentials” but I secure enough to not NEED to. You don’t have to be “scared”, opt for being INFORMED on heartworm and other pet concerns, make the best choices for your pet based on SOLID research and not fear!!!!

  19. Jan Says:

    Savanna, thanks so much for posting. For me, information not making a profit for the author is the most trustworthy. I received an email just yesterday saying that my whole website (www.truth4pets.org) was useless because I’m not a vet — even though everything I wrote had links to the information source. The person writing me was slamming one of my best articles on vaccination. http://truth4pets.org/question-before-vaccination/ As my dogs would say: grrrrrrrrrrrrrrrrrrrrrrrrr.

  20. Bee Says:

    I live in the Dallas Ft Worth area…our summer temps range from 80+ in the early morning to 110 in the afternoon. According to your temp range, we fit the mold where mosquitos would be able to not only carry heart worms but produce adults as well as babes. I grew up south of Houston and had a dog that lived outside all the time. He did have heart worms and it did kill him.

    So, my question is what is the best treatment to be sure your animal’s immune system is strong enough to ward them off…especially, if the animal is already dealing with cancer.

  21. Jan Says:

    Your dog has a lot of issues and needs a really knowledgeable vet. I can recommend:
    Shawn Messonnier DVM Plano, TX “a vet who does very minimal vaccines and uses multiple holistic modalities.” Dr. Messonier is author of the award-winning The Natural Health Bible for Dogs & Cats and The Natural Vet’s Guide to Preventing and Treating Cancer in Dogs. There are other vet referrals here: http://truth4pets.org/vets/

  22. Esmer Says:

    I will share a personal experience about my dog and heartworms: I adopted a dog that tested light for heartworms and was told by the vet that she needed a costly treatment, but after lots of research I found that since she wasn’t infested with them, I could use the heartworm meds as a treatment because they are in fact poison that will kill heartworms. The vet advised against this, but I followed my own research and wallet. A year later she was heartworm free. I would not advise this if they tested high on the heartworm scale because they can die. I live in San Antonio and only give my dogs heartworm meds 4 x a year and no of them have tested positive.

  23. maria goines Says:

    i live in northern michigan how often should i give heart gaurd plus if at all i have a 12 lb sheet zoo witha kidney problem she is now on a raw diet for kidneys , but i also have a 11year old mixed shepherd i rescued 8 years ago who had heart worm when i adopted her and it freaks me out that she may get it again as the vet keeps telling me he is seaing more cases of heart worm up here she cost me almost 2 thousand dollars to save her life the first time and i lost my 11year old black lab to cancer of the spleen july7th 2015 have stopped giving all shots no flea topicals using natural flea pills iworry that all the shots and flea meds are poisoning my dogs

  24. Jan Says:

    Maria, find another vet. http://www.truth4pets.org/vets

  25. Kate Says:

    Hi, I just adopted lambic puppy 6 months old. Today I first time visited a vet, and she recommended to us to do vaccine against heartwarm every month. Do you think is it necessary in NYC? We really do not want give a lot of medication to our dog with no reason. Thank you

  26. Jean Meier Says:

    I have a healthy 7 year old shih tsu. she has been tested each year for heartworms and has been negative. She has been on preventative all her life. Several years ago after moving from MN to CO i tried the injectable. She became very lethargic and would not eat for 3-4 days each month. Stopped that treatment. Now on trihart plus 0-25 lbs. she weighs about 13 lbs. for. The past several months, since I restarted the preventative this spring she stops eating for 4-6 days. I have a call in to the vet right now, i think the dosage should be reduced or I am thinking of eliminating the preventative all together. She is an indoor dog, only outside for short periods and we live in Denver. The map by the American Heartworm Society show 0-5 cases per reporting clinic. So this is not a heavily infected area. Thank you for the information. It was helpful in helping me make a decision.

  27. Jan Says:

    Jean, you might consult with a local holistic vet about safer heartworm protection in your area. http://www.truth4pets.org/vets

  28. Jan Says:

    Jean, your dog is trying to tell you that the medication is harming her. Contact a holistic vet and see what they recommend for your area. One thing some people do is to test for heartworms twice yearly so you can treat any problem early. A vet will have other ideas as well. http://www.truth4vets.org/vets

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  30. Betty Bell Says:

    Most vets test yearly for heartworm disease because they test for the protein in female adult heartworms which become adults in about months time. The prevention prevents the larvae from becoming adults. one they are in the heart you can’t treat with a “dewormer.” Eventually, if you continue to give heartworm medicine the adults will eventually die of old age because it will kill of the larvae. It’s called a slow kill method. Vets test yearly to catch and treat adult heartworms early because they are in the heart growing, having sex, and can possibly cause an embolism in the lungs or heart because they live in the heart which filters blood through the lungs. Not to mention that heartworms in the heart are causing damage to the heart and lungs that is irreversible. Did you know that in rare cases people can get heartworms -they travel into the vessels into the lungs and usually die-but sometimes look like other diseases like lung cancer on xrays. I worked in the city and we treated as many as 3 heartworm positive dogs a month. I think that if it is a case of immunosuppressed systems, then by that amount alone that is a lot of immunosuppressed dogs which interestingly enough—some of these cases don’t really show immunosuppression on the ability to create tithers from vaccines or in bloodwork leading up to an infection. I have an immunosupression-my body does not create tithers from vaccines.

  31. Donna Says:

    As my grandmother always said, “Everything in Moderation”.
    This year I decided not to give heartworm meds for the sake of raising an immune healthy dog with the ability to fight off any foreigners. I had been a long time believer in Big Pharma , but not anymore. The heartworm scare sickens me and at least three of the vets I go to try to sell me this story and scare me to death. I really think they are good vets but just really don’t know the truth. In NC a vet told me to give year round and he didn’t even know about the temperature at which mosquitoes develop. Just give year round and don’t ask any questions. $$$$$ When dogs are dying at a very young age you need to look at diet, vaccinations, and exposure. Thank you for this wonderful article and it’s ability to keep me sane when the Heartworm Scare is upon us by Big Pharma.

  32. Jan Says:

    Donna, I’m so glad the article was helpful. But since you live in one of the heartworm endemic areas, I urge you to learn about healthier options to combat them and test regularly.

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  34. Sandra Paslaski Says:

    Has anyone tried using diatomaceous earth FOOD GRADE in the dogs food to prevent heart-worm. I read a article about it and am considering using it this summer with my furbabies. I have 10 total so any incite would be welcome.

  35. Derrick christian Says:

    Thanks for sharing the article. It help me to gain more knowledge which would help to take more care about pet.

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