The Truth About Heartworms

Veterinarians and pharmaceutical companies have teamed up in a marketing campaign to frighten pet guardians into giving year-round heartworm preventatives to both dogs and cats. They say they’re doing this to improve protection for individual pets, but the facts say they have other motives.

Except for a the warmest parts of the U.S., heartworms are a completely seasonal problem. There is no reason to give heartworm medicine to most pets year-round (except to make money for those who make and sell it!).

Heartworms are transmitted by mosquitoes. Hearworm larvae, called microfilaria, live in the blood and are sucked up by the bug. Once inside the mosquito, they must further develop before they can infect another dog. For that to occur, outside temperatures must remain above 57 degrees F, day and night, for a certain period of time. The warmer the temperature, the faster the larvae will mature. If the temperature drops below critical level, larval development will stop; but the larvae don’t die—development will re-start at the same point when the weather warms back up. Larvae reach their infective stage in 8 to 30 days (the latter being the entire lifespan of the average mosquito).

In many areas of the country (northern and mountain states, for instance), such warm temperatures simply don’t exist for most of the year, and sustained warm temperatures don’t occur until at least June. In fact, only in Florida and south Texas is year-round heartworm transmission possible. Within 150 miles of the Gulf Coast, heartworm risk exists 9 months out of the year. In the rest of the country, heartworm transmission is possible between 3 and 7 months out of the year. Hawaii and Alaska have each had a few cases of canine heartworm, but the incidence  in those states is very low.

It should be obvious that during seasons where there are no mosquitoes, there is no risk of heartworm. Evidently that little fact escaped the attention of the veterinarian who prescribed heartworm protection—in December–for a puppy living high in the Colorado mountains. At that altitude, temperatures are never warm enough for heartworms!

When an infected mosquito bites a dog or cat, the microfilaria are deposited on the skin, where they crawl into the bite wound and enter the bloodstream. Inside the body, they grow and progress through other larval forms. In dogs, the heartworm’s natural host, larvae migrate to the heart and eventually develop into adult worms, reproduce, fill the blood with microfilaria, and pass it on to the next mosquito.

In cats, full-grown worms can develop, but not reproduce. Adult heartworms are over a foot long, and it doesn’t take but 1 or 2 to fill up a cat’s tiny heart and cause serious problems. In 80% of the time, the cat’s immune system kills the larvae at an earlier stage, and clears the infection. However, microfilaria can cause significant inflammation in the lungs, even in cats who never show any signs of infection. Cases of heartworm have been diagnosed in cats living entirely indoors.

Heartworm preventative drugs do not kill adult heartworms, but they do kill microfilaria up to a certain stage of development. Currently it is believed that larvae under 6 weeks old are affected. This means that in order to prevent heartworms from reaching adulthood, the preventative can be given up to 6 weeks after the mosquito bite and still work. The recommendation is to give the drugs every 30 days, purportedly because once-a-month dosing is easier for most people to remember (and, coincidentally, it also sells more drugs). Preventatives should be given starting 4-6 weeks after the earliest possible infection date and continue 4-6 weeks after the last possible infection date. In most states, protection should be continued through November or December. In southern Texas and Florida, year-round preventatives may be needed. Local conditions may vary from year to year.

The most common preventative drugs for heartworm are ivermectin (Heargard®) and selamectin (Revolution®). While these drugs are generally safe and effective, there are always exceptions. Toxicity associated with ivermectin include depression, ataxia (balance problems or unsteady walk), and blindness, but these are uncommon at the doses used in heartworm preventatives. Selamectin is also used to treat ear mites and some worms; adverse reactions include hair loss at the site of application, diarrhea, vomiting, muscle tremors, anorexia, lethargy, salivation, rapid breathing, and contact allergy.

Update 7/15/2010: The Companion Animal Parasite Council (CAPC) recently reported that mounting evidence suggests that preventatives may be susceptible to a very serious problem: resistance. This is similar to the situation with antibiotics, where massive and unnecessary over-use has caused many bacteria to develop resistance to one or more drugs, creating super-infections, and making many antibiotics useless. The CAPC report states: “There is a growing body of anecdotal reports and experimental evidence that currently available heartworm preventives (macrocyclic lactones) may not be completely efficacious in preventing heartworm infection in dogs. Reports of resistance for dogs in the region [south-central U.S.] have resulted in confusion about how best to prevent infection in veterinary patients.” If ivermectin and related drugs lose their effectiveness, that will be trouble indeed, since these drugs are also used in the treatment of heartworm infections.

Only Natural Pet HW Protect Herbal Formula is a natural product intended for use as a preventative to be used during mosquito season as part of a comprehensive heartworm control program. The formula was designed with two objectives, using herbs that work together to reduce the likelihood of mosquito bites to lower your pet’s risk of becoming infected, and to help eliminate existing larvae-stage parasites in the bloodstream. This tincture was developed to help prevent heartworm infestation using extracts of herbs well known for their mosquito repelling properties, and others well known for their anti-parasitic properties.


Knight DH, Lok JB. Seasonality of heartworm infections and implications for chemoprophylaxis. Clin Tech Sm An Pract. 1998 May;13(2):77-82.

Atkins C. Feline heartworm disease. NAVC Clinician’s Brief. Accessed 5/20/2009.

Companion Animal Parasite Council, Accessed 7/15/2010.

Pena F, Rosenthal M. Expert shares new protocol to manage heartworm signs. Veterinary Forum. 2008 Aug 1:17-18. Accessed 7/15/2010.

19 thoughts on “The Truth About Heartworms

  1. Ivermectin and selamectin – these are insecticides! Selamectin is listed as a neurotoxin and carcinogen. How can these be considered safe?

    1. Yes, that is exactly why I pointed out the many potential toxic reactions. In reality, adverse reactions are very rare. These products are approved animal drugs that are considered–by the government and veterinary practitioners–to be “safe” for use at the ultra-low doses used to prevent heartworm. There is a risk and benefit to be weighed in every individual case. Here in Colorado heartworm is uncommon, so I don’t use any preventatives on my 4 indoor cats. If I lived in southern Florida, I would have to re-evaluate the risks vs. benefits, which would be very different in that environment.

      1. Considering that Alaska has mosquitoes, I would bet that it gets plenty cold there. We just adopted a young stray dog from East TN and are having to treat him for heartworm, which has already at a young age caused damaged to his heart and lungs and vessels, prevention would have been far better than the treatment and gthe irreperable damage. The treatment is an arsenic based poison and when the worms die there is great risk of embolism, or in very severe cases they have to be surgically removed. Although I do agree there are some very greedy drug pushing slimes out in the world, there is real science behind prevention of diseases in humans and animals, including heartworm. It is how Polio and other diseases have been eradicated and how we hope other diseases such as Ebola will be erradicated, Artificial sugars are insecticides too, and we put billions of gallons of insecticides all over our yards and crops every year, and contaminate our water supply, our food supply…maybe it is a combination of all the chemicals we use that is causing the issues for all of us, human and animal alike. Wolves, foxes and domestic dogs are not the same, breeding by humans has saw to that evolution. We are all being poisoned but the risk of preventing some horrific diseases does outweight the risks and it those things like a nice “looking” lawn we need to worry about the use of insecticides and pesticides.Fear works on both sides and it is the fear of prevention and treatment that led to the almost pandemic Ebola outbreak of recent. It is this fear that is also giving rise to diseases once erradicated. No one knows every thing, and the fact is that people and pets for a variety of reasons are unhealthy at times whether it be because of trauma, aging or other pathogen. I am glad you see the usefullness in evaluating all circumstances instead of pushing on ly one agenda to the exclusion of all others. That is why I have two highly trained veterinarians that I trust.

      2. Very well said, Jeannine! There are risks and benefits to every medical treatment, and what is perfect for one animal will not be appropriate for another. If you live in an endemic area, then preventatives are needed. Here in Colorado, the risk is quite low; but we recommend annual testing to catch microfilaria before they mature to worms. As you said, the treatment for adult heartworms is painful and dangerous.

        But for the American Heartworm Society to recommend year-round prevention for every dog and cat in North America… well, to me that is *not* good medicine.

        BTW I hear from my colleagues that there is no native heartworm in Alaska despite the abundance of mosquitoes; the warm season is too short for it to develop. The only cases they’ve seen have been in dogs that traveled outside the state.

  2. thanks for the info. I had this happen to one of my pets when I was little. You never know.

  3. I am a veterinary assitant and have been for the past 6 years. Although there are some things that I do not agree with in the veterninary field, heartworm prevention is one that I strongly agree with. I live in the state of Tennessee and I can understand someone in the mountains of colorado having an issue giving year round heartworm prevention. On the other hand, in tennessee and warmer states this is unfortunately not the case. Mosquitos when alive have the potential to carry and pass the microfilaria(baby heartworms). The prevention of heartworm disease is like giving candy to a baby compared to heartworm treatment. A product called immiticide is used. This is a poison that is injected in the back of a dog twice. Dogs can not even live through tthe treatment! Not only is the treatment sometimes fatal to dogs, it is very expensive and the care that has to be done afterward is very time consuming. On the side of heartworm prevention: most heartworm preventions back track. This means that they kill anything that the pet has come into contact with in the last 30 days. A common misconcepton in pet owners is that these pills are helping their pets to not get things for 30 days after it is given. That is why it is important to have year round use. When you give a pill in November it is treating what happened in October. I know that some people feel that vets try and promote things just for the sake of making money and that may be true for some, but all the vets that I have worked for have been genuine and wonderful people who only have the best interests of pets at heart. If it is recommended in your area for dogs to have year round heartworm prevention there is probably a reason. If you have any questions about this just visit This is the organization that studies heartworm disease. It may clear up some of the misconceptions that were mentioned above. Just as in any job there will be some who just want money, there will also be those who want whats best for the people, things, or pets that they are serving.

    1. And also on the herbal route. I take mucho ammounts of herbs daily. If there is an herbal remedy out there for the prevention of heartworm disease that would be great considering any prescription medication has side affects.

  4. Hi Christa, thanks for your comments!

    You are correct that the treatment for heartworm disease is potentially dangerous, even deadly. You definitely do not want your pet to get heartworms!

    However, according to the research I have done, year-round heartworm prevention is clearly unnecessary in most of the country. I am not by any means saying don’t give your pet *any* heartworm preventatives, but simply to give it only when necessary and appropriate. I agree that you can trust most veterinarians to do what’s right for the individual animal, but I also know that most veterinarians are extremely busy and simply take the materials provided by their drug reps and follow those recommendations. My point is that we should not rely solely on the arguments presented by folks who have a financial stake in heartworm medication.

    This “debate” just appeared in an April journal article (Ask the Expert: Year-Round Heartworm Prevention: Two Viewpoints, by By Dwight Bowman and James Lok, published in Clinician’s Brief, the official publication of the North American Veterinary Conference, 2009/04/01). Both authors are university professors in parasitology.

    The argument presented by Dr. Bowman *for* year-round heartworm medication focused on just two points: (1) the speculation that “scenarios can arise where transmission may occur in cooler climates in the “off season;” and (2) the completely unrelated issue of prevention of internal parasites by additional drugs added to the heartworm preventative.

    Arguing on the other side, Dr. Lok lays out the case for appropriate seasonal control, and concludes, “Besides incurring unnecessary costs for the client, indiscriminate application of broad-spectrum medications can engender further confusion about the primary imperative for these medications–heartworm prevention–and when they are most crucial–during the season of heartworm transmission.”

    You might find it interesting that in Dr. Lok’s original research (Seasonality of heartworm infection and implications for chemoprophylaxis. Knight DH, Lok JB. Clin Tech Small Anim Pract. 1998 May;13(2):77-82.), the recommendation for Tennessee is to give the monthly preventative from June (to cover infections that may have begun in May) to December (to cover the last possible transmission date in November). I checked with and these dates seem quite reasonable given the historical temperatures in Memphis. However if–anywhere–the weather is unseasonably warm for long enough, exceptions to those recommendations should be made. I find it even more interesting that this study was funded by a grant from the American Heartworm Society, but its results have never been publicized, least of all to veterinarians.

    When looking at a site such as the American Heartworm Society, it’s important to investigate whether that their agenda is truly unbiased. On the Society’s board are representatives from Ft. Dodge and Eli Lilly, and their Vice President works for Merial. These drug companies have a strong financial interest in selling more heartworm products and have been aggressively promoting year-round prevention for all pets, period.

    Having looked at both sides of the issue, I have to agree with those who suggest that giving year-round treatment to animals in states where year-round transmission does not occur is doing an injustice to both the animals being given drugs they don’t need, as well as the pocketbooks of their guardians. This argument is rarely presented since the drug companies have the resources to express *their* views in a wider and louder fashion. Since I am neither in practice nor employed by the drug companies, I feel obliged to present that “other side” to our community. Lots of vets and others will disagree with me, but I think there is tremendous value in just having the discussion!

    1. Dr. Jean,

      Wow, that’s very informative.

      Just so I’m clear:
      1-The temp needs to stay above 57 degrees for 8 to 30 days.
      2-A mosquito has to bite a dog or cat that already has heartworms and/or microfilaria in it’s bloodstream.
      3-That mosquito has to then bite my dog 8-30 days later.
      4-I must give my dog heartworm preventative meds within 6 weeks to kill the microfilaria in his bloodstream to prevent my dog from getting heartworms.

      Is that correct?
      Do you know how prevalent heartworms are in Denver?

      1. Hi Donna, that’s a fantastic and accurate summary–Thanks!

        In Denver, the prevalence of heartworm is less than 3%. Heartworms have been slowly moving up the Platte and Arkansas River valleys, but the overall prevalence hasn’t changed dramatically. I’ve seen Colorado rated high for heartworm by various organizations, but we have a fairy mobile population; such studies do not take into account where the dog came from or its travel history. The natural transmission rate is very low.

  5. Please help! I have a rescue dog that I love and just found out that she has heartworms. She shows no signs of coughing or loss of appetite. The vets won’t treat her because she has a heart murmmer. The won’t even give her heart guard as a slow kill. I resorted to an herbal program to rid her of the worms. I can’t afford to send her to a heart doctor which is what two vets have told me to do. No one will help me! Can You?

  6. Hi Stacey, I am sorry to hear that your vets have taken such an attitude; it is illogical and contradicted by advice from the experts. A heart murmur is simply an indication of turbulence in the heart, and is to be expected if heartworms are present.

    First, please double-check to make sure your dog actually has heartworms; different tests mean different things. The most common test evaluates antibodies, which only means the dog has been exposed to heartworms, not that there are still heartworms present.

    I am separately emailing you with an article on heartworm management, but if others would like their vets to see it, it was published in the journal “Veterinary Forum” in August 2008. The original is here:
    /ArticleID/3289/Default.aspx (any vet can sign up for access).

    The basic protocol is as follows:

    1. If the dog is symptomatic, attempt to stabilize it with furosemide, angiotensin-converting enzyme inhibitors and, if ascitic, abdominal drainage.

    2. Put the dog on a heartworm preventative at standard doses for 2 to 3 months and continue throughout treatment. If using milbemycin, pre-treat with diphenhydramine and dexamethasone for the first dose.

    3. Start the dog on doxycycline at 5 to 10 mg/kg q12h for 4 weeks. Studies also have shown that doxycycline-treated heartworms become unthrifty and cease producing microfilariae; the female heatworms become smaller. This reduction in total heartworm mass, along with control of Wolbachia organisms, reduces the morbidity associated with thromboembolisms.

    4. Start all dogs on prednisone – if the dog is symptomatic, before giving melarsomine or if the dog is asymptomatic, at the time of melarsomine administration. This is also thought to reduce the lethal complications caused by pulmonary thromboembolisms when you traett with melarsomine. The suggested dose of prednisone is 1 mg/kg/day divided bid and tapered weekly.

    5. After giving a heartworm preventative for 3 months, giving doxycycline for 1 month, and starting prednisone if the dog is symptomatic, give one injection of melarsomine. This will kill 30% to 50% of the adult worms. If there are any complications associated with step 5, delay step 6 for another month.

    6. One month later, give two injections of melarsomine 24 hours apart. This will kill the remaining worms.

    7. Enforce strict exercise restriction during the entire period, not just after giving melarsomine. Any dog with complications should be crated.

    — Thomas Nelson, DVM

    * Treatment involves off-label use of melarsomine.

    Good luck!
    Dr. Jean

    1. The American Heartworm Society states that 6-25% of the clinics that report to them have seen heartworm positive dogs, but will not disclose how many clinics that is, or what the actual prevalence is. (See AHS’s map here: From those figures I would guess that the rate is about 10-15%. However, a colleague on Kauai tells me he almost never sees heartworm in pet dogs; but it is more common in dogs who spend most of their time outside, and in feral dogs.

  7. Thank you for blog. My vet recommended Trifexis and Parastar to pretreat my 14-mo old Aussie and I am baffled as I have read Trifexis may cause seizures in Australian Shepherds. Thanks to this thread, I understand that I can treat my dog seasonally which should help to reduce any affects of overmedicating (poisoning him) and the overuse of one product or another. Man, isn’t there a natural way to prevent fleas, ticks and heartworms? All these meds seem ridiculous.

  8. Pingback: Heartworm Debate?
  9. I adopted my dog when he was 6 or so months old. Gave him hist first HW prevention about that time. Tested him for HW a month after the first pill, he was negative. They replenished more Trifexis for up to a yr. The dates and timing are not exact, I don’t have my records with me atm. Never missed a beat on Trifexis, spray my yard with mosquito spray (organic), spray my dog with a safe repellent for our 40 mins or so walk and while we were walking too. Use DE (Food Grade Diatomaceous Earth) for my plants and I bought a $400+ mosquito trap that runs 24 hours a day 7 days a week during the summer in SW Florida. My dog tested HW positive not even 12 months after his initial test. He is going thru HW treatment now and Elanco (the makers of Trifexis) is paying for 50% of the treatment. I am not an isolated case. When I drop off my dog to get his HW treatment shot (Immiticide I think it is?) there are at least 6 other dogs coming in for the same. They take 6 per time 4 days a week. The shot is every 30 days so that makes 24 different dogs per week to more then 100 dogs per month at up to $600 (some areas are more expensive) per dog for the treatment. That’s $60,000 for that alone. Even if the laboratory company pays for 50% of EVERY treatment the Veterinary office still gets their $ AND the owner is “happy” because they didn’t have to pay the whole treatment. The customer will continue to come back to the veterinary clinic. That’s a good partnership the laboratory company and the vet’s have. And this is only one veterinary office in SW Florida. Every pet owner at that office (that I asked) has the same story, they all used HW prevention, some Trifexis, some HeartGuard. When I selected to start using preventative HW “poison” I did think about the pros and cons. I want him safe from HW and a little bit of this “poison” maybe not be bad if it helps him not go thru the treatment nor suffer damages from the possible HW infection. I made the wrong choice, I will look for a holistic approach when I’m done with this horrible treatment. I will use Amber Technologies products, change his diet probably to raw or semi raw food. At least that way, if my dog does get infected again he didn’t have to take an ineffective monthly tablet that didn’t do any good for his immune system. This is my experience and my point of view now is that HW prevention medicine doesn’t work in Florida. I told the vet: So I guess all this prevention doesn’t work? So she said “All you need is one infected mosquito”.
    Seriously? Is that all you have to say? – That’s why he is taking his prevention medicine, to fight against THAT infected mosquito. So what you are telling me is that is about “luck” and not prevention. He was unlucky to get an infected mosquito and unlucky that he was taking the monthly medicine that was doing nothing for his immune system to battle the infection.

    1. Wow, so sorry that happened to you! What a terrible ordeal!

      There are two problems in Florida that make these experiences all too common. It isn’t so much that you happened to encounter that “one” mosquito”–it’s that the mosquito population is so heavily infected that the chances of even “one” bite being from an infected mosquito is extremely high because so many of them carry it.

      The other problem is that, due to misuse and overuse of heartworm drugs, the heartworm larvae themselves are becoming immune. So now you not only have a mosquito with heartworms, you have a mosquito with heartworms that are already immune to the drugs. So the drug is useless, and continuing to use it makes the whole situation worse by spreading resistance to more and more mosquitos.

      What is really needed is 100% protection from *all* mosquitos, which of course is unrealistic short of dressing your dog in head-to-toe netting. That’s why you have to bring in the whole arsenal, including natural repellents, mosquito control, and the diet and protocols you mentioned.

      The Gulf Coast really has been given a raw deal and the drug companies have made it worse. Resistance has been known for years, but vets aren’t disclosing it, and the drug companies aren’t advertising it. Why would they? As you say, they make so much on the drugs that $50,000 here and there is nothing to them. 🙁

      Wish I could give you a silver bullet, but it doesn’t exist. Thanks for your extraordinary efforts on behalf of your dog!

      1. Thank you for your reply. Yes I read afterwards on one of the preventative packages that in the southern states the mosquitoes have a resistance to the medicine. When talking to one of the representatives at Amber Technologies he said the same thing, the whole mosquito population needs to be treated at the state level and the city needs to spray for that. Which they do here, but darn mosquitoes…. Unfortunately, the current HW prevention medicine is not effective in our area.

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