Health Command-Pacific

With adult male and female worms present, mating begins to occur. Heartworms do not lay eggs like other worm parasites; instead they give live birth and the baby worms are called microfilariae. Microfilariae are released into the circulatory system in hope that they will be slurped up by a mosquito taking a blood meal and carried to a new host. Microfilariae may live, up to 2 years within the host dog in whom they were born; if after this period a mosquito has not picked them up, they die of old age. Microfilariae may also be transmitted across the placental barrier to unborn puppies if the mother dog is infected with heartworm. It is important to realize that such puppies will not develop adult heartworms or heartworm disease from these microfilariae; in order for a heartworm to reach adulthood, it must be passed through a mosquito.