Causes of Heartworms in Dogs, Prevention, and the Symptoms

Heartworm prevention is a year-round task. That's the first thing you should know if you're researching this life-threatening disease. As a pet parent, your dog depends on you to protect them from these fatal parasites that could lead to lung disease, heart failure, and other organ damage. So, before heading out for a game of fetch, take a few minutes to learn what causes heartworms in dogs and why prevention—rather than treatment—is essential.

What are Heartworms?

A heartworm (Dirofilaria immitis) is a variety of parasitic roundworms. These worms mature into adults, mate, and produce offspring all while living inside your dog. During this time they infest muscles, blood vessels, the pulmonary artery, and heart. Adult heartworms, which look like cooked spaghetti, can live inside your dog for 5 to 7 years, leading to a wide range of health problems. They vary in length with female worms reaching up to 12 inches and male worms reaching up to 9 inches.

Once you understand the heartworm life cycle it becomes easier to see why heartworm prevention for dogs is crucial.

How Do Dogs Get Heartworms?

Dogs get heartworms through the bite of an infected mosquito. Infected dogs cannot pass heartworms to other dogs since these parasites need a mosquito to develop to the “infective stage.” Mosquitoes serve as the intermediate host where larvae mature as well as the vector that transports parasites to dogs, cats, ferrets, and other mammals.

Heartworms go through a long series of larval (immature) stages before becoming adults. The first larval stage exists in already infected dogs who have microfilaria (baby worms) in their bloodstream. When a mosquito feeds on an infected dog, first-stage larvae migrate to the mosquito where they complete two more larval stages over a period of 10-14 days. At this point, larvae are at the “infective stage.”

When an infected mosquito bites a dog, they leave a bite wound, creating a path for infective larvae to enter their new host. Once passed on to a dog, the larvae continue their progression into adulthood over 6 months, infecting areas in and around the dog's heart and lungs. Breeding-age adult heartworms then produce a new set of first-stage larvae and the cycle begins again.

In the United States, the warm southern regions have historically seen a larger occurrence of heartworms, but all 50 states have reported the disease. Humans see mosquitos as a nuisance, but when thinking about what causes heartworms in dogs, they can be life-threatening to pets.