When an infected mosquito bites a dog, it releases young heartworms, called larvae, which mature into bigger worms that lodge in the dog's heart and lungs. (sentinelpet.com)
Additionally, many heartworms in ferrets do not reproduce — for example, an infection with a single male heartworm won't lead to any heartworm babies — so the tests designed to look for young heartworms is likewise often useless. (smallanimalchanel.com)
While young heartworms (microfilariae) can be killed off at home using a monthly prophylaxis, adult heartworms are more stubborn and require hospitalization. (trupanion.com)