For northern California, the responsible vector is mainly the Western Treehole mosquito (Aedes sierrensis). Other mosquitoes capable of heartworm transmission include Anopheles freeborni, and Aedes vexans. Aedes sierrensis breeds in cavities that develop in trees able to retain rain or irrigation water usually known as treeholes. Often, older trees in city parks and residential yards and dense groupings of oaks in the foothills and mountains provide annual breeding grounds for springtime hatches of these mosquitoes.

Heartworm is a serious and potentially fatal disease affecting dogs. Infections have also been reported in cats, ferrets and other animals. Mosquitoes often pick up the parasite from foxes, coyotes or wolves where the disease can run rampant. Heartworm parasites (Dirofilaria immitis) are then transmitted to an animal through the bite of an infected mosquito. After the heartworm larvae have been transmitted to a dog or other animal, the larvae develop into immature adults and travel to the arteries surrounding the heart. This process takes up to 4 months. The worms reach maturity in about 6 months. Adult worms may reach lengths of 6 - 14 inches and are usually found in the pulmonary artery near the right side of the heart and also in the lungs. Infections of several hundred worms have been reported, but this is very unusual. Infected cats have fewer and smaller worms than dogs. At maturity, the worms may reproduce, releasing offspring called microfilaria. These pathogens can be picked up by another mosquito during a blood meal. Inside the mosquito, the microfilaria develop into larva and can then be transmitted to another animal. This is how the infection cycle is repeated again and again.

Infections with just a few worms can have serious consequences. So treating an infection through your veterinarian is important for re-establishing your pet's good health. Since most pets do not exhibit symptoms in the early stages of infection, annual testing at your vet's office is the best method for insuring early detection. Adult heartworms can be destroyed through a series of injections or in an emergency, removed through surgery. There is no vaccine to prevent infections, but there are methods of prevention that are nearly 100% effective. Only your veterinarian can help you choose the right medication and prevention schedule for your pet.