Adult heartworms are long, pale, spaghetti-like worms that live inside a dog’s heart and pulmonary arteries. Females can reach 10 to 12 inches long, while males are smaller at 5 to 6 inches. They have a grayish-white color, a cylindrical body, and a ridged outer surface visible under magnification.
Adult Heartworm Appearance
An adult heartworm looks strikingly like a strand of cooked spaghetti. The body is cylindrical, elongated, and grayish-white with a finely ridged outer layer running in rings along its length. The head end is rounded and symmetrical, while the rest of the body tapers slightly.
The size difference between males and females is significant. Females measure roughly 10 to 12 inches (up to about 30 cm), making them nearly twice as long as males, which top out around 5 to 6 inches (about 18 cm). Males are also slightly thinner. One easy way to tell them apart: female heartworms have straight, rounded tails, while male tails are distinctly coiled, curling inward like a hook. When a mass of heartworms is extracted from an infected animal, the tangled cluster is mostly female worms, but the coiled tail of a male is usually visible somewhere in the middle.
Where They Sit Inside the Body
Heartworms don’t float freely through the bloodstream. Adults anchor themselves primarily in the pulmonary arteries (the large vessels carrying blood from the heart to the lungs) and in the right side of the heart itself. In heavy infections, worms can physically crowd the heart’s tricuspid valve, interfering with blood flow. This is called caval syndrome and causes sudden weakness and dark-colored urine as red blood cells are destroyed. A veterinarian performing an echocardiogram or opening the heart during a necropsy would see the worms draped and tangled within these spaces, sometimes dozens packed together.
What Heartworm Larvae Look Like
Before heartworms become the large adult worms most people picture, they spend months as microscopic larvae called microfilariae circulating in an infected animal’s blood. You cannot see microfilariae with the naked eye. Under a microscope, they appear as tiny, thread-like organisms roughly 300 to 322 micrometers long and only about 7 micrometers wide. For perspective, that’s about one-third of a millimeter long and thinner than a human red blood cell’s diameter.
Microfilariae have a straight body and tail with a tapered head. They sit relatively still on a blood smear rather than thrashing around. This matters because veterinarians use these features to distinguish heartworm larvae from a harmless look-alike species that can also show up in a dog’s blood. That other species has a blunt head and curved tail and is slightly shorter (250 to 288 micrometers). A specialized blood test called a modified Knott test lets vets see these details clearly enough to make the right call.
Differences Between Dogs and Cats
Heartworms look the same regardless of which animal they infect, but the scale of infection differs dramatically. Dogs are the natural host for heartworms, so infections can involve dozens of adult worms packed into the heart and pulmonary arteries. Cats, on the other hand, are not an ideal host. Their immune system fights the infection more aggressively, which means cats typically harbor only a few adult worms, sometimes just one or two. This makes heartworm harder to detect in cats through standard testing, since the tests rely on proteins produced by female worms, and fewer worms means a weaker signal.
Despite the lower worm count, even one or two heartworms in a cat can cause serious damage because a cat’s heart and blood vessels are so much smaller than a dog’s.
How Heartworms Change as They Grow
The full journey from larva to adult takes about six to seven months. A mosquito deposits microscopic larvae onto the skin during a bite. Those larvae burrow into the tissue and spend several weeks migrating through muscle and connective tissue, growing but remaining invisible to the naked eye. Over the following months, they travel toward the heart and lungs, gradually increasing in size. By the time they reach the pulmonary arteries and settle in, they’ve grown from microscopic threads into the pale, foot-long worms that show up on X-rays as abnormal shadows or on ultrasound as bright, parallel lines inside the heart.
Adult heartworms can live five to seven years in dogs. During that time, they continue to produce microfilariae that circulate in the bloodstream, ready to be picked up by the next mosquito and start the cycle again.