Heartworm disease is a serious and potentially fatal disease in dogs and cats. It is caused by a mosquito-borne parasite, called Dirofilaria Immitis. Adult heartworms cause disease by invading blood vessels of the heart and lungs, causing mechanical obstruction and inflammation. The female heartworm can produce millions of offspring, called microfilariae, in the bloodstream of the dog or cat. These immature heartworms cannot complete their life cycle in the animal; they must develop and mature in the mosquito.