Adult heartworms look like strands of cooked spaghetti, white to off-white in color, living inside the blood vessels and heart of infected animals. Females grow to 10 to 12 inches long, while males are smaller at 4 to 6 inches. They’re thin, smooth, and round-bodied, and in a heavily infected dog, dozens of these worms can be tangled together inside the pulmonary arteries and the right side of the heart.
Size Differences Between Males and Females
Female heartworms are roughly twice the length of males. Fully mature females measure 25 to 30 centimeters (10 to 12 inches), while males reach 15 to 18 centimeters (5 to 6 inches). Both are quite thin relative to their length, roughly the diameter of a piece of angel hair pasta. The worms reach these full dimensions about six and a half months after an infected mosquito first delivers larvae into the animal’s skin.
If you’ve seen photos of heartworms removed during surgery or necropsy, the tangled mass can look alarming. Dogs can harbor 30 or more adult worms, and in severe cases, several hundred. Cats are a different story. Because cats are not the parasite’s natural host, most larvae don’t survive to adulthood. Cats with heartworm disease typically carry just one to three adult worms, though even that small number can cause serious illness.
What Heartworm Larvae Look Like
The baby heartworms, called microfilariae, are invisible to the naked eye. They circulate in an infected animal’s bloodstream and can only be seen under a microscope, where they appear as tiny, wriggling threads in a drop of fresh blood. A vet examining a blood sample can sometimes spot them by the rippling movement they create among the surrounding blood cells, even before focusing on the larvae themselves.
Under magnification, heartworm microfilariae have a pointed nose and a straight tail. These details matter because other, harmless parasites can also show up in a blood sample and look similar. The harmless variety has a flat nose and a curvy tail. Distinguishing between the two requires a trained eye and sometimes a special concentration technique that makes the larvae easier to identify.
How Heartworms Look on Ultrasound
Most pet owners will never see a heartworm with their own eyes, but you might see one on an ultrasound screen at the vet’s office. On echocardiography, heartworms show up as bright, parallel lines that resemble equal signs or tiny railroad tracks. These lines are created by the worm’s outer coating, which reflects sound waves strongly. A cluster of worms inside the heart chambers appears as a nest of these bright parallel marks, sometimes visibly shifting with each heartbeat.
Ultrasound is particularly useful in advanced infections where worms have migrated from the pulmonary arteries into the right atrium and ventricle of the heart. This condition, called caval syndrome, is a medical emergency. Seeing worms directly on imaging confirms the severity and helps guide treatment decisions.
Where the Worms Live Inside the Body
Heartworms don’t actually spend most of their time sitting in the heart itself. Their primary home is the pulmonary arteries, the large blood vessels that carry blood from the heart to the lungs. The worms cause inflammation and damage to the inner lining of these vessels, which over time leads to thickening of the artery walls and restricted blood flow. As the worm population grows, they back up into the right side of the heart.
In dogs with heavy infections, the physical mass of worms becomes part of the problem. A dense tangle of foot-long parasites inside blood vessels creates a mechanical obstruction that forces the heart to work harder. The combination of physical blockage and ongoing vascular inflammation is what makes heartworm disease progressively dangerous the longer it goes untreated.
How Heartworm Infections Are Detected
Because heartworms live deep inside the cardiovascular system, you won’t spot them by looking at your pet. The American Heartworm Society recommends annual screening for all dogs over seven months of age using two tests: an antigen test and a microfilariae test.
The antigen test detects proteins released by adult female heartworms. Current versions of this test are nearly 100% specific, meaning a positive result is very reliable. However, a negative result doesn’t guarantee the dog is worm-free. Infections with only male worms, very young worms, or low numbers of females can slip past the test. That’s why the microfilariae test matters too. It involves examining a blood sample under a microscope to look for those tiny, swimming larvae. About 20% of heartworm-positive dogs in high-prevalence areas don’t have detectable microfilariae, so using both tests together gives the most complete picture.
Any positive antigen result should be confirmed with a second, different type of test before treatment begins. This step avoids unnecessary treatment based on a rare false positive and ensures the diagnosis is solid.