How heartworms are spread All dogs are at risk for potentially deadly heartworm disease. Heartworms live in the heart and blood vessels of the lungs of dogs, cats and other mammals like wolves, foxes and coyotes. Heartworms cannot be spread directly from animal to animal without a mosquito as an intermediary. Heartworms are spread when a mosquito bites an infected dog and picks up tiny larvae called microfilariae from the dog’s bloodstream. Then that mosquito bites another dog infecting it with the heartworm larvae. Over the next several months the heartworm larvae grow and migrate to the heart and lungs.