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6 Surprising Facts About Heartworms & Heartworm Medication

Cedarcide blog post image, 6 Surprising Facts about Heartworms and Heartworm Medication

Heartworms are one of the leading causes of death in both dogs and cats in the United States. Heartworm disease occurs in all 50 states and, especially in southern regions, is a year-round threat. Once they find their way into your cat or dog, these sometimes foot-long parasites mature and then navigate to your pets’ lungs and heart. If left untreated, these worms will wreak havoc on your pet’s health, causing severe organ damage and possibly death. Heartworms can live inside your animal for up to seven years—it’s not unheard of for a single dog to have upwards of two-hundred heartworms. Whether you’re currently dealing with a heartworm situation or just doing some research, here’s six surprising facts about heartworms and heartworm medication.


Heartworms Are Only Spread by Mosquitoes Bites

The only way your pet can get heartworms is through the bite of an infected mosquito. Mosquitoes are both hosts and vectors for these tiny parasitic roundworms. Heartworms cannot spread from animal to animal, even if a mosquito bites one and then the other. In order to contract heartworms, your pet must be bitten by a mosquito that is itself already infected with heartworms (these heartworms will have already undergone a two-week maturation process inside the body of the mosquito). The process goes like this:

  • First, a mosquito picks up heartworm larvae by biting a dog, cat or wild animal infected with heartworms.
  • Next, the heartworms begin to mature within the mosquito. This process takes about two weeks.
  • Then, the mosquito passes on the heartworms by taking a blood meal from a dog, cat, or wild animal.
  • The heartworms continue to mature, becoming fully developed roughly six to seven months after entering your pet’s bloodstream. This process can cause serious, often irreversible organ damage.
  • Soon after maturation, the heartworms begin to multiply and the new larvae are released into your pet’s bloodstream, where they can remain for up to two years.
  • Lastly, another mosquito bites your pet, infecting itself and starting the whole process over again.

People Can Get Heartworms, Too!

Although rare and almost always harmless, humans can also contract heartworms. Unlike dogs, humans are not productive hosts to these parasites, meaning heartworms cannot reproduce within our bodies. The risks associated with human infection are so minimal that treatment is almost never recommended.

If they’re able to survive long enough, heartworms sometimes find their way into the lungs of their human host. While this poses virtually zero threat to the infected person, there’s concern that the inflammation associated with the heartworms’ presence in the lungs could be confused for more serious situations—like cancer or tuberculosis. In rare instances, humans have undergone invasive surgery to inspect what was believed to be cancer, but turned out only to be a relatively harmless heartworm.

The Only FDA Approved Heartworm Medication is Arsenic-Based

While preventative medications do not contain arsenic, the medication used to treat dogs already infected with heartworms does (there is currently no such treatment for cats; the medication is simply too toxic for their immune systems). Called Immiticide, this arsenic-based treatment is the only FDA approved method for ridding your dog of adult heartworms.

The medication is administered to your pet through a series of two to three injections. Unfortunately, these injections are not only expensive but harmful to your pet. In fact, the medication is so risky that your pet must first be evaluated to confirm they’re capable of surviving the treatment. Merial, the company that manufacturers Immiticide, describe the drug as having “a low margin of safety.” Side effects include lethargy, vomiting, fever, coughing, even death. During and after treatment, your pet will need to remain inactive for weeks, possibly even months. This is because exertion can cause dead worms to become lodged in the lungs, which can lead to death.

Heartworms Are Becoming Resistant to Medication

Not only are heartworm medications toxic, but they’re slowly becoming less effective, too. “We now have proof there is resistance,” says Dr. Byron Blagburn, a parasitologist at Auburn University. “It’s accepted by the entire industry now.” In warmer regions like in the Southern U.S., where mosquitoes and therefore heartworms are much more common, many people choose to treat adult heartworms with preventative medication instead of the prescribed Immiticide. It’s cheaper and comparatively less dangerous, but it’s also far less effective. It’s because of this approach—and the subsequent missed monthly doses that inevitably occur—that mosquitoes have started to build resistance to heartworm medication.

You Shouldn’t Believe Everything the Pharmaceutical Companies Tell You

The American Heartworm Society and pharmaceutical manufacturers advocate year-round preventative medication for the life of your dog or cat. But is this really the best, healthiest option for your pet? As veterinarian Dr. Karen Becker points out “The American Heartworm Society has three ‘platinum’ sponsors and five ‘bronze’ sponsors. All eight are major pharmaceutical manufacturers.” It’s this conflict of interest that Dr. Becker argues should make us think twice about the AHS’ recommendations. Dr. Becker recommends consulting with a holistic veterinarian before resorting to toxic, chemical-based heartworm treatments (a directory of these vets can be found at

Dr Karen concludes, “This information is not intended to minimize the need to protect your dog, but only to point out the actual potential for heartworm disease is less than you’ve been led to believe by financially-motivated marketing campaigns designed to scare pet owners into buying 12 doses of preventive, year in and year out, regardless of where you live.”

There Are Natural Alternatives

Under the guidance of a holistic veterinarian—along with monthly or bi-monthly heartworm screenings—there are in fact natural ways to combat heartworms. Considering the potential damages of traditional heartworm medications, you owe it to your furry friend to at least research and explore alternative methods for controlling heartworms. However, because natural preventatives and treatments are not always one-hundred percent effective on all pets, please consult a vet before choosing this approach. For more information on natural alternatives and how they work, read here.

The most obvious natural method for avoiding heartworms is to regularly apply a non-toxic insect repellent to your pet. Because mosquito bites are the only source of heartworm infection, consistently protecting your pets from such bites is essential. Cedarcide recommends applying a naturally sourced nsect repellent to your pet before and after going outdoors. Whether hiking, going to the dog park, or simply playing in the backyard, keeping your pet shielded from harmful insects should be a top priority.

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  1. More discounts I’m on a budget. Please!!

    1. Whitney at CedarCide

      Hi Celia!

      Have you subscribed to our newsletter? If not, you’ll receive 10% off your next purchase when you do. Also, we always announce special discounts and savings in there! You can sign up here:

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