Dirofilaria immitis is a species of parasitic worm better known by pet owners as heartworms.

Heartworm larvae can grow into adult worms in your dog’s blood and block major blood vessels. If untreated, your dog may experience organ conditions that can cause major harm or death.

Heartworms are rarely transmitted from dogs to humans. In fact, only 81 cases of heartworm in humans were reported from 1941 to 2005. But it’s best to seek treatment for heartworms if you notice any symptoms in your pet or yourself.

Both dogs and humans can get heartworm infections. But your dog can’t give it to you through their bodily fluids. Heartworms get into the bloodstream of both humans and dogs through mosquito bites.

Heartworms in the blood of an infected animal reproduce in the gut of the mosquito after a blood meal. Then, they get carried over to another host by the mosquito and are passed during feeding.

Heartworms first enter the bloodstream as undeveloped heartworms known as microfilariae, or heartworm larvae.

What happens next varies by species.

  • In animals, the larvae eventually mature into adult heartworms. They can then cause dirofilariasis, a full-blown infection that can cause blockage of major arteries or organ infections.
  • In humans, heartworm larvae never fully mature. As young heartworms die, your body reacts to their tissue with inflammation as it tries to destroy the heartworms. This condition is known as pulmonary dirofilariasis.

The symptoms of heartworm infections in animals and humans differ because of how they develop in the bloodstream. You may not always experience any symptoms because the heartworms will die before maturity in a human host.

Symptoms and signs of heartworm infections in humans can include:

  • abnormal cough
  • coughing up blood
  • pain in your chest
  • wheezing
  • chills
  • fever
  • buildup of fluid around your lungs (pleural effusion)
  • round lesions that show up on chest X-rays (“coin” lesions)

Seek immediate medical attention if you notice these symptoms, whether you’ve been bit by a mosquito or not. (Mosquito bites appear as red, itchy bumps with dots in the middle.) This is especially important because these symptoms can also indicate other, more serious conditions.

You may not realize you’ve had an infection until your doctor sees a coin lesion on an X-ray.

These lesions show up on X-rays or computed tomography (CT) imaging tests as darkened spots. They mostly appear near the edge of the lungs. A lesion may also be called a granuloma. These result from inflammation and buildups of immune cells known as histiocytes that fight the heartworm infection.

You doctor may want to take a tissue sample (biopsy) from the lung to test for heartworm infection if they spot one of these lesions on an X-ray. Your doctor may also test lung tissue to rule out other possible causes. In rare cases, a coin lesion can indicate a bacterial infection, tuberculosis, or lung cancer.

Heartworms don’t live long in human blood, so you won’t need to have heartworms removed through either medication or surgery. Treatment for heartworms addresses any granulomas that appear on an imaging test that may have resulted from dead heartworm tissue buildup in your arteries.

If a granuloma isn’t causing any symptoms or blockage in your arteries, you probably won’t need any further treatment.

If your doctor suspects that a granuloma may be cancerous or the result of another, more serious condition, they’ll likely take a tissue sample (biopsy).

To take a tissue sample, your doctor may use one of these methods:

  • Lung needle biopsy. Your doctor inserts a thin needle through your chest tissues in your lungs.
  • Bronchoscopy. Your doctor inserts a lighted scope through your mouth into your lungs.
  • Mediastinoscopy. Your doctor inserts a lighted scope through a small cut in your skin into the mediastinum, an area between the lungs.

If your doctor finds that a granuloma isn’t cancerous or a result of another condition, you may not need any further treatment.

If your doctor believes that granulomas need to be removed, you may need surgery to remove the granuloma. This will prevent any further symptoms.

If the granuloma is found to have cancerous tissue, your doctor will likely refer you to an oncologist to further examine your bodily tissues for the presence of cancer.

You can’t get heartworms from your dogs, cats, or other pets — only from mosquitos that carry the infection.

Most heartworm microfilariae die on their way through the skin. Even if they do get into your blood somehow, heartworms can’t mature and will eventually die off.

In most cases, heartworms in humans aren’t a serious problem unless they cause pain, discomfort, and other noticeable symptoms.

Heartworms are serious business for dogs; without treatment, your dog can experience severe symptoms and even die from an infection.

Ask your veterinarian for heartworm prevention medications for your dog. This is especially important if you live where there are lots of mosquitos or plan to travel somewhere with mosquitos. (Think outdoor walks, camping trips, or vacations in humid areas.)

If you notice any heartworm infection symptoms, take your dog to the vet right away to get tested. If necessary, have them treated for heartworms as soon as possible.