Heartworm disease is a serious condition that can result in hundreds of worms in a dog’s body and be fatal if left untreated. Heartworms are parasitic roundworms that live and reproduce in the heart and blood vessels of dogs, says Amy Attas, VMD, founder of mobile veterinary practice City Pets: The House Call Vets in New York City. These worms are transmitted when certain mosquito species carry an immature form of the parasite Dirofilaria immitis from feeding on one dog’s blood and bite another dog. Once bitten, the dog becomes infected with the parasite larvae, which eventually enter their bloodstream, Dr. Attas says. When the larvae reach the heart and the surrounding blood vessels, they can mature into adult worms and reproduce, with the resulting microfilariae (microscopic worms) circulating in the bloodstream. These long, slender worms, which resemble cooked spaghetti, are usually a foot in length and live in the pulmonary arteries and the heart of infected animals, causing substantial damage to the lungs and heart. “The life cycle of this parasite requires the dog species, as dogs are the definitive hosts for this parasite,” Dr. Attas says. “Although heartworms can be found in other mammalian species, they cannot complete their life cycles without a dog.” Dogs, cats, ferrets, and non-domestic canids (e.g., coyotes, foxes, wolves) can get heartworm disease. Heartworm disease has been diagnosed in all 50 states and can occur all over the world. However, Dr. Attas says, its prevalence can vary depending on: Climate: The risk of heartworm transmission is higher in warmer climates with large mosquito populations, such as tropical and subtropical areas. Seasonality: Because seasonality will also affect mosquito populations, the potential for infection is greater in the warmer months of the year in certain places. It’s also hypothesized that dogs relocated from subtropical locations carrying microfilariae may be responsible for bringing the parasite to new climates, she notes. Because there are plenty of common myths about heartworm disease, Dr. Attas shares a few facts about heartworms to inform pet parents about this serious disease. Heartworms live in the blood vessels, not in the intestinal tract. Dogs cannot clear heartworm infections without treatment. Heartworm infections occur in all 50 U.S. states, not just southern states. Garlic and other herbal methods are insufficient mosquito repellents.,Jump to section What Is Heartworm Disease? Causes of Heartworms in Dogs Signs of Heartworms in Dogs How To Treat Heartworms in Dogs Heartworm Prevention for Dogs FAQs About Heartworms in Dogs