Heartworms, Dirofilaria immitis, belong to the same class of worms as roundworms. In fact, they look a bit like roundworms, but that is where the similarity ends. Heartworms spend their adult life in the right side of the heart and the large blood vessels connecting the heart to the lungs. Heartworms are found in dogs, cats, and ferrets. They also occur in wild animals such as California sea lions, foxes and wolves. They have rarely been found in people.

Adult heartworms in the heart lay very tiny larvea called microfilariae, which then live in the bloodstream. These microfilariae enter a mosquito when it sucks blood from an infected animal. In 2-3 weeks, the microfilariae develop into larger larvae in the mosquito and migrate to the mosquito’s mouth. When the mosquito bites another animal, the larvae enter the animal’s skin. The larvae grow and after about three months finish their migration to the heart, where they grow into adults, sometimes reaching a length of 14 inches. The time from when an animal was bitten by an infected mosquito until adult heartworms develop, mate, and lay microfilariae is about 6-7 months in dogs and 8 months in cats.

It is much easier to prevent heartworm infection than it is to treat it which is why at Sitara Animal Hospital heartworm prevention is part of our regular deworming program and is recommended once monthly from May until October of each year.