Species (Dirofilaria immitis) of filarial worm that parasitizes mammals, especially dogs. Up to 500 adult heartworms, which can grow to 6–12 in. (15–30 cm) long, live in the dog's heart, and the microfilariae (embryonic larvae) pass into the blood. Mosquitoes transfer infested blood from dog to dog. Both adult and larval heartworms tax the heart and restrict blood flow to the lungs, kidneys, and liver. By the time visible symptoms (chronic cough, labored breathing, listlessness, heart failure) develop, it may be too late for treatment. Preventive medicines and treatments, including surgery, exist.
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