What is heartworm transmitted by?

The story behind the answer

Mosquitoes transmit heartworm (Dirofilaria immitis). When a mosquito bites an infected dog, it ingests microfilariae (baby worms). Inside the mosquito, these develop into infective L3 larvae, which are injected into the next dog the mosquito bites. The larvae migrate and mature in the heart and pulmonary arteries, leading to serious, sometimes fatal, cardiopulmonary disease.

Remember: Heartworm hitches a ride with hungry Mosquitoes—M to M (Mosquito to Mammal). Ticks spread Lyme and ehrlichiosis; fleas spread tapeworms and Bartonella; sandflies spread leishmaniasis—not heartworm. Because just one bite can transmit it, year-round prevention and regular testing are essential, even for mostly indoor dogs.