Can Humans Get Heartworms?

All About Heartworms and How They Are Spread

A woman with a dog
smerikal/flickr CC 2.0

Most dog owners have heard of heartworms. If you have a dog, your vet likely has asked you to give your dog a preventive heartworm medication every month. So, what's the big deal about heartworms? Well, like they sound, they are large worms that live in the hearts of dogs and cats. 

Heartworms can also be found in other species, including ferrets, foxes, wolves, sea lions, and horses—and rarely, humans. Dogs are the most common host for this parasite. If left untreated, your dog can die from this parasitic infection, which may eventually cause congestive heart failure.

Humans Can Get Heartworms?

Humans are not natural hosts for heartworms, but there have been a few rare reports of human cases. In humans, the heartworm is usually found as a single worm actually present in the lung. 

Diagnosis in humans is unusual and is often only made after an infected person has a chest X-ray. A nodule may be large enough to resemble lung cancer or tuberculosis on an X-ray and then require an open-chest biopsy to rule out a cancerous tumor or TB lesion. The risks associated with having an invasive procedure to rule out cancer and TB are much greater than living with the parasitic infection itself.

What Exactly Are They?

Heartworm is also known as Dirofilaria immitis. It is a long, roundworm that can be anywhere from 6 to 14 inches in length. They look a lot like strands of spaghetti. Female worms are longer than males. Heartworms have been found in every state in the U.S. The highest rates of incidence occur from Texas to New Jersey and along the Mississippi River.

How Do They Spread?

Heartworms cannot be spread directly from animal-to-animal. Heartworms need a mosquito to complete their life cycle.

A mosquito bites a heartworm-infected animal. The mosquito is then carrying microscopic versions of the heartworm, called microfilariae. When the mosquito bites another dog or cat, that animal is now infected with the heartworm microfilariae.

Within 70 to 90 days, the microfilariae have made it through the tissues to the animal's heart, where they reproduce (providing both male and female worms are present) and live for several years. If both sexes of worms are present, they will be producing their own little microfilariae within 6 to 7 months after that mosquito bite. The heartworm lifecycle continues.

Best Defense

The best defense against heartworms is prevention. If you live in an area that has heartworms, a once-a-month chewable tablet will protect your pet from heartworm and other parasites. You should see your veterinarian and discuss what heartworm preventive option is best for your pet.


Early signs that your dog might have heartworm disease can include a cough, especially when running, and getting exhausted easily. In the more advanced cases where many adult worms have built up in the heart without treatment, your dog can experience severe weight loss, fainting, coughing up blood, and heart failure.

To rid your dog of heartworms, your vet will likely prescribe medications or give your dogs shots. Treating heartworm disease involves killing the adult worms that live in the heart and the lung arteries, as well as the larval microfilariae that are in the bloodstream of the dog.

Additional Resource

The American Heartworm Society is a resource for pet owners, veterinarians, and the general public to understand more about heartworm disease. The society currently invests hundreds of thousands of dollars a year in heartworm research. And, with peer support, the society has established the current guidelines for canine and feline prevention, diagnosis, and treatment of heartworm disease.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.