Canine heartworm is a serious and potentially fatal disease of dogs. It is caused by parasitic nematodes (roundworms) called Dirofilaria immitis. Heartworm infections have also been reported in cats, ferrets, foxes, coyotes, wolves, seals and sea lions. Mosquitoes pick up heartworm parasites when they feed on animals that have heartworm infections. Infected mosquitoes then bite other animals or dogs, which causes the disease to spread. The disease does not spread directly from dog to dog.
Adult heartworms living in a dog’s heart produce immature heartworms called microfilaria and release them into a dog’s bloodstream. These microfilaria can be picked up by a mosquito as it bites a dog. In the mosquito, the microfilaria develop into infective stage larva in 10 – 14 days. When the mosquito feeds on another dog, these infective stage larva move out of the mosquito’s body and enter the dog’s body through the hole in the dog’s skin made by the mosquito as it fed. The worms then move slowly through the dog’s bloodstream until they reach the heart. This can take up to 6 months. By the time they reach the heart they have become adult worms, and can be 6 – 14 inches in length. They are usually found in the pulmonary artery near the right side of the heart and also in the lungs. Adult heartworms can live for 5 to 7 years. Infections of several hundred worms have been reported, but this is very unusual. Infected cats have fewer and smaller worms than dogs.
In California, the responsible vector is primarily the Western Treehole Mosquito (Aedes sierrensis). Aedes sierrensis breeds in cavities in trees that can hold rain or irrigation water. Older trees in city parks and residential yards and dense groupings of oak trees in the foothills and mountains can provide annual breeding grounds for springtime hatches of these mosquitoes. Control of treehole mosquitoes can be difficult, especially in areas with many trees.
Infections with just a few worms can have serious consequences.  Veterinary care is important to re-establish your pet’s good health. Since most dogs 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. Dog heartworm medications can control and kill immature and adult heartworms. There is no vaccine to prevent infections, but there are highly effective medications. Your veterinarian can help you choose the right medication and prevention schedule for your pet.