Heartworm is a spine-chilling word for dog owners. Every veterinary clinic displays gruesome photos of the foot-long spaghetti-like worms in a dog’s heart … and if you don’t want your dog to end up with those long ugly worms, you’d better give her monthly preventive heartworm drugs, preferably year round.

Conventional vets are afraid of heartworm because they’ve seen how difficult and risky heartworm treatment can be. So they want to scare you into buying the preventive drugs … and they’re often successful. Even holistically-minded dog owners who feed raw, minimize vaccines and try to avoid toxic chemicals for their dogs, don’t always take the scary leap of faith to stop giving heartworm medication.

But there are a few facts you need to know before you decide how to protect your dog from heartworm. Here are 10 things you may not know.

1. Who Sponsors The American Heartworm Society

Visit the American Heartworm Society (AHS) website and you’ll find pages and pages of information about heartworm disease (including more of those sickening spaghetti pictures). They provide downloadable brochures, maps, infographics and even children’s games to teach dog owners about the risks of heartworm disease. And there are all kinds of resources that veterinarians can use to improve “compliance.”

It’s all designed to get you to use heartworm medication for your dog. The AHS provides guidelines for diagnosis, prevention and management of heartworm disease. Right now, wherever you live, they recommend you Think 12. That means giving your dog heartworm drugs every month, year round, to protect her … and asking your vet to test her every 12 months for heartworm disease.

But this is hardly unbiased advice. Guess who sponsors the AHS and all these resources?  Yes, it’s the big pharmaceutical companies that make the heartworm drugs. Check out this list.


Does this seem like a conflict of interest? Certainly these companies have a vested interest in giving you as much information as possible to convince you to buy their drugs for your dog. They even state that “No natural or herbal therapies have been shown to be safe and effective prevention or treatment for heartworm disease.” (Many holistic practitioners would disagree with that statement – see number 9 below.

2. Heartworm “Preventives” Are Not Preventive

The so-called heartworm preventive drugs don’t prevent your dog from being infected with heartworms. Instead, they work by killing heartworm larvae that may already be in your dog’s body. So they’re actually treatment drugs, not preventives. And if your dog hasn’t been infected, you’re treating her for something she doesn’t even have.

When you give your dog monthly heartworm “preventives,” what you’re really doing is poisoning the heartworm larvae before they grow into adult heartworms. Keep reading to find out more about that poison …

3. Heartworm Drugs Are Neurotoxic

Heartworm drugs are pesticides. Just the idea of putting pesticides into your dog should make you hesitate. And when you learn how those pesticides work, you’ll really think twice.

The drugs are neurotoxins … and the way they work is by paralyzing the parasites’ nervous systems.

So, do you wonder what effect that might have on your dog? Well, it turns out, if you read the Adverse Drug Experience (ADE) reports on the FDA (Food and Drug Administration) website, you’ll see that many of the side effects reported in dogs are conditions that involve the nervous system … things like:

  • convulsions
  • ataxia (staggering/incoordination)
  • trembling

(Read more details about heartworm drug side effects.)

4. Heartworm Drugs Are “Safe” For Your Dog …

… but not for you!

The warning labels on heartworm medications are very clear: if you or your child should accidentally ingest the drugs, you’d better rush to the doctor immediately. Here’s the warning on the label of Heartgard, one of the most popular heartworm drugs:


Does this make sense to you? According to the manufacturer and your veterinarian, it’s “safe” for your dog to take this drug every single month, year round, but not for you – even once. As far as we know, there have been no long term clinical studies done into the safety of these drugs. And as we saw earlier from the ADEs on the FDA website, the list of common side effects in dogs is long … and includes death!

So, are heartworm meds safe for your dog? I’d say not.

5. It’s Not So Easy For Your Dog To Get Heartworm

Heartworm disease is transmitted by a mosquito bite. But it’s not quite that simple.

For your dog to get infected, here’s what has to happen. First of all:

  • Heartworm is spread only by certain breeds of mosquitoes (not all breeds can transmit heartworm)
  • To be a heartworm carrier, the mosquito must have bitten an infected dog or other animal
  • Dogs can’t catch heartworm from other infected dogs – it can only come from an infected mosquito

Then, there are several other things that have to occur for your dog to get heartworm disease:

Step One

There are several studies showing that the weather must be warm enough for heartworm larvae to develop in the mosquito. The University of Pennsylvania vet school found that “Below a threshold temperature of 14C (57°F), development cannot occur, and the cycle will be halted.”

Step Two

The dog (or other host where the mosquito picks up the heartworm) must already be infected with mature male and female heartworms and they must have produced microfilariae that are alive when the dog is bitten and are at the site of the bite.

Step Three

Now, a female mosquito needs to bite the animal carrying these microfilariae. Only female mosquitoes can act as an incubator for the microfilariae to grow into the mature larvae known as L3. This takes 10 to 14 days and again, it requires the right temperature. 

Step Four

This female mosquito that is actively incubating microfilariae needs to bite your dog to transmit heartworms. And the temperature and humidity need to be high enough for the mature L3 larvae to make it through your dog’s skin. Many studies say this requires a steady daily temperature of at least 64°F, day and night for a month for this to happen, and if, at any time, the temperature drops below 57°F, the maturation cycle will fail and the female mosquito can no longer transmit heartworm to dogs.

So, to summarize, it’s not just being bitten by a mosquito that can give your dog heartworms. It has to be the right kind of mosquito – and a female; one that’s bitten another animal that was carrying microfilariae, and then bites your dog … and the temperature has to be warm enough.

6. Heartworm Risk Isn’t Everywhere

The AHS recommends year-round medication for dogs, no matter where you live. Their rationale for this is that there have been heartworm cases in all 50 US states.

But have a look at this AHS map of heartworm incidence in 2016.


Those white parts experienced less than one case per clinic. And the pale pink areas only have 1 to 5 cases per clinic. So I’d say that means that in significantly more than half the country, the chances of your dog getting heartworm are really low. Is that really enough risk to justify giving your dog a poisonous pesticide every month – one that could cause him to become seriously ill or even die?

7. Heartworm Drugs Are Becoming Less Effective

It’s known that heartworms are becoming resistant to heartworm drugs. Even the AHS admits it, reporting in December 2013 that, in certain areas of the United States, most notably the Mississippi Delta region, an inordinate number of dogs have become infected with heartworms while reportedly taking preventive medication.

They’re not really sure why this is happening, but they still recommend year round heartworm meds, no matter where you live. I can’t help wondering … if a drug doesn’t work, why react by giving more of it?

And going back again to those ADEs on the FDA website, you’ll see that many of the ADEs list the most common adverse effect as “Ineffect HW Larvae” – in thousands of cases!

And this reduced effectiveness leads into the next point …

8. Drug Manufacturers “Guarantee” Their Products …

… but good luck collecting. They don’t make it easy.

One company, Zoetis, which makes Revolution, says, “If any dog determined by a licensed veterinarian to be free at the onset of treatment with Revolution develops heartworm disease, we will provide reimbursement (up to $750 and the acquisition cost of melarsomine dihydrochloride) associated with the diagnosis and treatment of heartworm disease and provide a year’s supply of Revolution.”

(Oh good, a year’s supply of the product that didn’t work!)

But in order to pay the guarantee, they require:

heartworm disease guidelines revolution

These are very stringent requirements and other heartworm med manufacturers have similar standards.

How do you prove the first point, that you gave Revolution at all times according to label directions? Your dog is your witness!

So, if you use these drugs and your dog gets heartworms, don’t hold your breath that you’ll be able to collect under the guarantee.

9. Immiticide Isn’t The Only Treatment Option

One of the reasons vets urge pet owners to use heartworm medications to protect their dogs is because they’ve witnessed the very difficult, dangerous treatment that’s used in conventional medicine.

The treatment involves a series of injections with Immiticide (melarsomine) over a period of three months. During this time, your dog has to be on crate rest and restricted exercise. This is because of the risk of dead worms getting into your dog’s bloodstream, which can be fatal.

And the treatment isn’t always effective either. In fact, being ineffective is the first of many side effects reported. Here’s the first page of the ADE for merlarsomine from the FDA website:


But there are several natural treatment options that are much gentler on your dog and don’t require restricted activity. These include:

  • Commercially available herbal blends
  • A herbal formula developed by Steve Marsden DVM, published in the Manual of Natural Veterinary Medicine and used successfully by many holistic veterinarians (consult your holistic vet for this one)
  • Homeopathic treatment through your homeopathic veterinarian
  • Black walnut and wormwood: there are various protocols using these herbs. They are very powerful and can be toxic if used improperly, so use them only with guidance from your holistic veterinarian or herbalist.

Here are more details about these natural treatment options.

10. Wild Dogs Don’t Get Sick From Heartworm

It’s true. Wild canines like wolves, coyotes and foxes don’t seem to get sick or die from heartworm disease.

In a Wisconsin study from 1991 to 1996, researchers captured and tested adult wolves for disease.  Only 2% of those wolves had any trace of heartworm.

Another study in Illinois tested 920 coyotes caught by hunters from 1995 to 1997. Only 16% had any incidence of heartworm, and even the ones who had it, weren’t sick … their body weights and fat reserves were unaffected.

These wild animals are outdoors 24/7, so they’re exposed to a much higher risk of being bitten by mosquitoes than our domestic dogs who spend much of their time indoors. So why are our dogs more susceptible to heartworm disease?

The key is the immune system. The difference lies in what we do to our dogs to weaken their immune systems … things like:

  • Vaccination or over-vaccination
  • Poor nutrition
  • Exposure to toxins like drugs, pesticides, fertilizers, household cleaners, and other chemicals

Even the AHS admits on its website that “host immune responses affect the presence of circulating microfilariae” … in other words, your dog’s immune system can kill the microfilariae or prevent them from reproducing.

What Can You Do For Your Dog …

Again, the immune system is key to helping your dog prevent heartworm. Here are some things you can do to help strengthen your dog’s immune system:

  • Feed a fresh, whole food diet
  • Minimize vaccines
  • Avoid prescription drugs like antibiotics or non-steroidal anti-inflammatories (NSAIDs)
  • Avoid chemical pest control products (like spot-on products) and dewormers (and heartworm meds!)
  • Don’t use pesticides or herbicides on your lawn
  • Use natural cleaning products in your home

Take these 10 facts into consideration before deciding how to protect your dog from heartworm. Learn more about heartworm disease in this article.