Heartworms are transmitted between animals through the bites of infected mosquitoes. When a mosquito feeds on an animal carrying heartworms, it picks up microfilariae, which are the larval stage of the worms, from the host’s blood. These larvae develop inside the mosquito and are then passed to other animals when the mosquito feeds again. This transmission method means that heartworms do not spread by direct contact between animals but through these mosquito intermediaries. Regular preventive treatments are essential to protect animals from this dangerous parasite.