You can’t see the actual heartworms on a standard X-ray, but you can see the damage they cause. Adult heartworms are soft-tissue organisms living inside blood vessels and heart chambers, so they don’t show up as distinct shapes on a radiograph. What veterinarians look for instead are telltale changes to the heart, lungs, and pulmonary arteries that signal an active infection.
What an X-Ray Actually Shows
Rather than revealing the worms themselves, chest X-rays reveal the consequences of heartworm disease. The most reliable sign is enlargement of the caudal lobar pulmonary arteries, the large blood vessels at the back of the lungs. In a study of 34 heartworm-positive dogs, 65% had visibly enlarged peripheral pulmonary arteries on their radiographs. These arteries can appear swollen, twisted, or blunted at the tips, a pattern sometimes described as “tortuous and pruned.”
Other common findings include a bulging main pulmonary artery (seen in about 24% of cases) and lung tissue changes like hazy or patchy densities spread across the lung fields. In that same group, 41% showed visible lung damage on their X-rays. Heartworm-positive dogs consistently show an interstitial pattern in the lungs, a cloudy or web-like texture that reflects inflammation and scarring caused by the parasites and the immune response they trigger.
The “Reverse D” Heart Shape
In more advanced cases, heartworms cause the right side of the heart to work much harder than normal, because the worms physically obstruct blood flow through the pulmonary arteries. Over time this leads to right-sided heart enlargement. On an X-ray, this shows up as a distinctive “reverse D” silhouette: the right side of the heart bulges outward, giving the heart an abnormal rounded shape instead of its usual profile. When a vet sees this combined with enlarged, twisted pulmonary arteries and dense lung fields, heartworm disease moves high on the list of likely diagnoses.
X-Rays Can Miss Early Infections
A normal-looking X-ray does not rule out heartworms. In roughly 24% of dogs with confirmed heartworm infections (verified by ultrasound), chest radiographs appeared completely unremarkable. This is especially true in early infections, when worm numbers are low and the heart and lungs haven’t yet sustained enough damage to show visible changes. Dogs with mild infections or small body frames may carry worms for months before any radiographic signs develop.
This is why vets don’t rely on X-rays alone to diagnose heartworm. A blood test that detects proteins released by adult female heartworms (the antigen test) is the primary screening tool. X-rays play a different role: they help determine how much damage has already occurred, which directly affects treatment decisions and risk.
When X-Rays Are Most Useful
X-rays become especially valuable once a dog has already tested positive on a blood test. At that point, the vet needs to know how severe the infection is. The size of the pulmonary arteries, whether the heart is enlarged, and how much lung tissue is affected all help classify the disease into stages. A dog with clean-looking lungs and a normal heart silhouette is in a very different situation than one with a reverse D heart, swollen arteries, and patchy lung damage.
Your vet will typically take at least two views: a side view (lateral) and a view from the front or back (ventrodorsal or dorsoventral). The side view is useful for assessing overall heart size and lung patterns, while the front-to-back view helps evaluate individual pulmonary arteries and detect asymmetric changes.
Ultrasound Can Show the Worms Directly
If you want to literally see heartworms, echocardiography (ultrasound of the heart) is the tool that makes it possible. On an echocardiogram, adult heartworms appear as bright, parallel lines moving inside the heart chambers or pulmonary arteries. This is the preferred method for confirming worms are physically present inside the heart, which matters most in severe cases where worms have migrated backward from the pulmonary arteries into the right side of the heart itself.
Echocardiography also helps estimate worm burden by visualizing how many worms are clustered in the pulmonary arteries and heart. Before ultrasound was widely available, veterinarians had to rely on indirect signs like labored breathing, anemia, and dark-colored urine to recognize the most dangerous stage of heartworm disease. Now, a quick ultrasound can confirm exactly where the worms are sitting and how many are visible, information that determines whether a dog needs emergency worm extraction or can proceed with standard treatment.
What About Humans?
Heartworm occasionally affects people, though it’s rare and plays out differently than in dogs. When a heartworm larva reaches the human lung, it typically dies in a small blood vessel, creating a small, round spot called a “coin lesion” on a chest X-ray. These spots are often discovered incidentally during imaging for something else and can initially be mistaken for a lung nodule or even a small tumor. Biopsy usually confirms the cause. Unlike in dogs, heartworms don’t survive to adulthood or reproduce in humans, so the concern is limited to these isolated lung spots rather than progressive heart disease.