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How Dogs Get Heartworms

By the Dog Health Handbook team · Updated · Fact-checked against current veterinary references (see sources)

Key takeaways

  • Only mosquitoes spread heartworm.
  • Larvae take about 6 months to become adult worms.
  • It isn't contagious between dogs or from dogs to people.
  • People rarely get a lung nodule from mosquito bites.
  • Indoor dogs are at risk too.

Table of Contents

Research Life cycle Contagious? People Cats and wildlife Risk factors FAQ Sources

Heartworm Quicklinks

Heartworm Symptoms How Dogs Get It Testing Treatment Prevention Other Worms

Dogs get heartworms from the bite of an infected mosquito. It is the only way heartworm spreads.1,2,3 When a mosquito bites an infected dog, coyote, fox or wolf, it picks up baby worms called microfilariae. Inside the mosquito they develop into infective larvae over about 10 to 14 days.4,5 When the mosquito bites another dog, the larvae enter through the bite wound. Over about 6 months they travel through the body, reach the heart and lung arteries, and grow into adult worms.4,6,7 Heartworm isn't contagious between dogs, and you can't catch it from your dog.2,8,9 People bitten by infected mosquitoes can very rarely be infected. The worms don't mature, but they can leave a small lung nodule.9,10 Indoor dogs are at risk too, because mosquitoes get inside.11,3

See your vet if your dog:

  • Has missed heartworm prevention doses, especially during mosquito season12,13
  • Was adopted from, or traveled to, a high-risk area such as the southeastern US14,15
  • Has a persistent cough or tires easily4

What the Research Says

  • Mosquito infection rates. AHS cites mosquito surveys with heartworm infection rates from 2.1% to 19.4% in mosquitoes.7
  • Moving north. CAPC reports that two mosquito species that can carry heartworm have "undergone rapid population expansions northward."14
  • Climate risk. A 2026 modeling study projects heartworm risk will increase by 2100 under a high-emissions scenario, especially in the Midwest, Northeast and higher altitudes.16
  • Island elimination. Treating all dogs and protecting them with prevention eliminated heartworm from the dogs of Linosa, an Italian island, by 2022.17

The Heartworm Life Cycle

  1. In an infected dog: adult worms produce microfilariae that circulate in the blood.7,5
  2. In the mosquito: microfilariae develop into infective third-stage larvae (L3) in about 10 to 14 days. Development slows in cool weather and stops below about 57°F.4,6 Microfilariae can't become adult worms without passing through a mosquito.7
  3. In the new dog: larvae molt to the fourth stage (L4) under the skin within days.6,7 About 50 to 70 days after infection they become young adult worms, which reach the lung arteries starting around day 67 to 70.7,10
  4. Adults: worms mature about 120 days after infection. Microfilariae appear in the blood about 6 to 7 months after infection.7,6

Adult worms can live 5 to 7 years in dogs.4,6

Is Heartworm Contagious?

No. VCA explains that "transmission requires the mosquito as an intermediate host, so heartworm disease is not spread directly from dog to dog."2 Dogs can't catch heartworm from playing with, licking or living with an infected dog.3,8 An infected dog in the home does raise risk for other pets, though, because mosquitoes can pick up microfilariae from it.7,18

Can Humans Get Heartworms?

Rarely. The CDC says humans are "accidental hosts that play no role in the transmission." In people the worms never become sexually mature, but they can cause a small lung nodule that shows up as a "coin lesion" on a chest X-ray and can look like cancer.9 Most people have no symptoms, and the nodule is usually removed surgically.9,19 You can't catch heartworm from your dog. It comes only from mosquito bites.9,8

Cats, Ferrets and Wildlife

Coyotes, wolves and foxes are important carriers.4 Cats are atypical hosts. Most worms don't reach adulthood, but even a few can cause serious disease, and there is no approved treatment for cats.4,1 Ferrets are also at risk.4

Risk Factors

  • Location: highest in the Southeast, Gulf Coast and Mississippi River Valley, and rising in western and northern states6,16,20
  • No or irregular prevention: the biggest risk factor21,7
  • Dogs moved from high-risk areas, including rescue transports14
  • Mosquito habitat: warm, humid climates, standing water, irrigation and urban heat islands22,23
  • Indoor dogs are still at risk.11,3

Frequently Asked Questions

How do dogs get heartworms?

From the bite of a mosquito carrying heartworm larvae.

Can my dog get heartworm from another dog?

Not directly. A mosquito has to carry it.

How long does it take for heartworms to grow in a dog?

About 6 months from the mosquito bite to adult worms.

Can I get heartworms from my dog?

No. People can rarely be infected by mosquito bites, but not by contact with dogs.

Can indoor dogs get heartworm?

Yes. Mosquitoes can get inside.

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Sources and further reading

This article was rewritten and fact-checked on October 24, 2026 against current veterinary references and specialist guidelines:

  1. US Food and Drug Administration. An Ounce of Prevention is Worth a Pound of Cure: Protect Your Pet from Heartworms Year-Round. FDA Consumer Update, 2023.
  2. Barnes C, Williams K, Weir M, Barnette C, Ward E. Heartworm Disease in Dogs. VCA Animal Hospitals, updated June 2026.
  3. American Veterinary Medical Association. Heartworm disease. AVMA.
  4. American Heartworm Society. Heartworm Basics. American Heartworm Society.
  5. Montoya-Alonso JA. Dirofilariosis: Clinical Update of an Emergent Disease. South European Veterinary Conference, Barcelona, 2014 (IVIS).
  6. Companion Animal Parasite Council. CAPC Guidelines: Heartworm. CAPC, updated January 2026.
  7. Nelson CT, McCall JW, Moorhead A, Starkey L, Ames M. Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. American Heartworm Society, revised 2024.
  8. Ontario SPCA and Humane Society. Commonly asked questions about heartworm in dogs. Ontario SPCA.
  9. Centers for Disease Control and Prevention. About Dirofilariasis. CDC, 2024.
  10. Ames MK. Heartworm Disease in Dogs, Cats, and Ferrets. Merck Veterinary Manual, updated August 2026.
  11. Atkins CE. Heartworm Disease in Dogs. Merck Veterinary Manual, Pet Owner Version, updated September 2024.
  12. American Heartworm Society. Canine Heartworm Guidelines: Highlights. American Heartworm Society, 2024.
  13. Moorhead A. Practical Advice About Heartworm Preventive Lapses. Today's Veterinary Practice, 2019.
  14. Companion Animal Parasite Council. 2025 Annual Pet Parasite Forecasts. CAPC, 2025.
  15. Traversa D, Diakou A, Colombo M, Kumar S, Long T, Chaintoutis SC, et al. First case of macrocyclic lactone-resistant Dirofilaria immitis in Europe: cause for concern. International Journal for Parasitology: Drugs and Drug Resistance, 2024.
  16. Morchon R, Rodriguez-Escolar I, Collado-Cuadrado M, Infante Gonzalez-Mohino E, Montoya-Alonso JA, Carreton E, et al. Potential risk of heartworm infection in the United States of America and its projected incidence by 2100. Parasites & Vectors, 2026.
  17. Brianti E, Napoli E, De Benedetto G, Venco L, Mendoza-Roldan JA, Basile A, Bezerra-Santos MA, Drake J, Schaper R, Otranto D. Elimination of Dirofilaria immitis Infection in Dogs, Linosa Island, Italy, 2020 to 2022. Emerging Infectious Diseases, 2023.
  18. McCall JW, Mansour A, DiCosty U, Fricks C, McCall S, Carson B, et al. Blocking Dirofilaria immitis transmission by treatment of heartworm microfilaremic dogs with a tetracycline-class antibiotic. Parasites & Vectors, 2026.
  19. Shah MK. Human Pulmonary Dirofilariasis: Review of the Literature. Southern Medical Journal.
  20. Castillo T. Heartworm hotspots, expanding risk areas identified in latest report. Veterinary Practice News, April 2026.
  21. Mwacalimba K, Morrison JA, Ly P, Spofford N, Yang M, Saito E, Sheehy J, Adolph C, Poulsen Nautrup B, Brennan C. Prevention and long-term outcomes of naturally occurring canine heartworm infection in primary care settings. Frontiers in Veterinary Science, 2024.
  22. Morchon R, Montoya-Alonso JA, Rodriguez-Escolar I, Carreton E. What Has Happened to Heartworm Disease in Europe in the Last 10 Years? Pathogens, 2022.
  23. Couper LI, Mordecai EA. Ecological drivers of dog heartworm transmission in California. Parasites & Vectors, 2022.

About this article

Last updated : written using the 2024 American Heartworm Society canine guidelines, 2026 Companion Animal Parasite Council recommendations, the CDC, the Merck Veterinary Manual, IVIS proceedings and 2022 to 2026 research, plus International Veterinary Information Service (IVIS) proceedings and research published 2022 to 2026.

This page is for general information and is not a substitute for an exam by your veterinarian.