Dog heartworms are long, slender, white worms that look similar to cooked spaghetti noodles. Adult females can reach 10 to 12 inches in length, while males are smaller at 5 to 6 inches. They live inside the heart and the blood vessels leading to the lungs, sometimes tangled together in clusters.
Adult Heartworm Appearance
An adult heartworm has a smooth, cylindrical body covered in a tough outer coating made largely of collagen. The worm is white to off-white in color and roughly the diameter of a thin piece of string. Their bodies taper slightly at both ends, giving them a streamlined shape well suited to living inside blood vessels.
Female heartworms are noticeably larger than males. A female can grow 10 to 12 inches long, while a male typically tops out at 5 to 6 inches. In photos taken during heartworm removal surgery, the worms often appear as a pale, tangled mass pulled from the heart chambers or pulmonary arteries. When separated, each individual worm looks like a single strand of thin white pasta.
How Many Worms Are Typically Inside a Dog
The average infected dog carries about 15 adult heartworms, but the actual number can range anywhere from a single worm to 250. At the higher end, that means a large, writhing bundle of foot-long parasites occupying the heart and surrounding vessels. Even a modest worm burden of 15 creates a visible mass when the worms are clustered together, and that cluster physically obstructs blood flow through the heart and lungs.
What Heartworms Look Like on Ultrasound
Most dog owners will never see heartworms directly. Instead, the first visual confirmation usually comes from an echocardiogram (heart ultrasound). On the screen, heartworms appear as bright parallel lines, often described by veterinarians as looking like long, glowing equal signs. This distinctive pattern comes from the worm’s tough outer coating, which reflects ultrasound waves strongly. A cluster of worms shows up as multiple sets of these parallel lines tangled within the right side of the heart.
What Larval Heartworms Look Like
Before heartworms reach the large, spaghetti-like adult stage, they spend months as microscopic larvae migrating through a dog’s body. A mosquito delivers the initial larvae into the skin during a bite. These early-stage larvae are invisible to the naked eye.
Over roughly two and a half months, the larvae develop while migrating through tissue. By four to six months, the developing worms enter the bloodstream and travel toward the heart and lungs. Full maturation to the adult stage takes six to eight months total from the initial mosquito bite. During this entire growth period, the worms are too small and too deep within the body to see without specialized testing.
Microfilariae Under the Microscope
Once adult heartworms mate inside the heart, the females release tiny offspring called microfilariae into the dog’s bloodstream. These are the stage that a mosquito picks up during a blood meal, continuing the cycle. Under a microscope, heartworm microfilariae are 284 to 325 micrometers long (roughly a third of a millimeter) and appear as slender, tapered threads that tend to stay relatively stationary on a blood smear rather than wriggling around.
Veterinarians sometimes need to distinguish heartworm microfilariae from a harmless lookalike species that can also circulate in dog blood. The harmless variety is shorter (237 to 288 micrometers), has a blunt head instead of a tapered one, and its tail curves into a small hook. On a blood smear, these lookalikes move progressively across the slide, while true heartworm microfilariae mostly stay in place. A specialized blood test called a modified Knott’s procedure lets the lab tell them apart definitively.
Why You Might See Heartworms in Photos
Images of heartworms circulate widely online, and they typically come from one of three situations: necropsy (post-mortem examination) of an infected dog’s heart, surgical extraction of worms during treatment, or preserved specimens in veterinary teaching collections. In all of these, the worms appear as a pale, intertwined mass filling the heart chambers or spilling out of the pulmonary arteries. The sheer volume of worms in severe cases can be striking, sometimes filling the entire right ventricle.
In a living dog, there is no way to see heartworms from the outside. The infection produces no visible skin signs, no worms in the stool, and no worms that emerge from the body on their own. Detection relies on blood tests that identify proteins released by adult female heartworms, or on ultrasound imaging that reveals the characteristic bright parallel lines inside the heart.