Knowledge of the parasite's life cycle(Dirofilaria immitis) is needed to understand to prevent and treat dogs from heartworm disease. Infection begins when infective larvae in the mouth parts of a mosquito enter the dog's skin at the site of a bite. The larvae burrow beneath the skin and undergo two molts, eventually developing small immature worms. The first molt (L3 to L4) occurs one to 12 days after the mosquito bites the dog. The larvae remain in the L4 stage for 50 to 68 days and then molt into the L5 stage (immature worms).
The L3 stage is 1 to 12 days after the larvae enter the dog's body. Therefore, they are susceptible to the killing effect of diethylcarbamazine. Throughout the L3 and L4 stages, the larvae are susceptible to three other drugs: ivermectin, selamectin, and milbemycin.
Immature worms reach a peripheral vein and are carried to the right ventricle and the pulmonary arteries. Approximately six months after entering the dog's body, they mature into adults. Adults can grow to 4 to 12 inches (10 to 30 cm) long and live up to five years. As many as 250 worms may be found in a heavily-infested dog.
Sexual reproduction occurs if worms of both sexes are present. Females give birth to live young called microfilaria; 5,000 microfilariae can be produced in one day by a single worm. Microfilaria can remain alive in the dog's circulatory system for up to three years.
Before the microfilariae can become infective to another dog, the larvae must go into a secondary host, the mosquito. This occurs when the mosquito bites the dog. The larvae in the mosquito molt to L3 larvae. In warm southern climates, this process takes less than 10 days; in northern climates, it can take up to 17 days. The L3 larvae then move to the mosquito's mouthparts and are ready to infect a new host.