Heartworm disease does show up on X-rays, but not in the way you might expect. You won’t see the worms themselves. What a veterinarian sees are the changes the worms cause to the heart, blood vessels, and lungs, and those changes only become visible once the infection has progressed enough to cause real damage. That means X-rays are valuable for assessing how serious a heartworm infection is, but they’re not the right tool for catching it early.
What Vets Actually See on the X-Ray
Adult heartworms live inside the pulmonary arteries and right side of the heart. They’re soft tissue, and they don’t show up as distinct shapes on a radiograph. Instead, the X-ray reveals the structural damage they leave behind: an enlarged heart, swollen blood vessels, and changes in the lung tissue.
The most common finding is enlargement of the right side of the heart. Because the worms physically obstruct blood flow through the pulmonary arteries, the right ventricle has to pump harder and eventually grows larger. On a front-to-back chest X-ray, this creates a characteristic shape where the right border of the heart rounds outward, producing what veterinary radiologists call a “reversed D” shape. In one study of dogs with confirmed heartworm disease, this reversed D shape appeared in nearly 90% of cases.
The pulmonary arteries themselves also change. They become dilated and tortuous, meaning they widen and develop an irregular, twisting course instead of their normal smooth taper. Vets measure the width of specific pulmonary arteries and compare them to the width of nearby ribs as a reference point. In dogs with heartworm-related high blood pressure in the lungs, these artery-to-rib ratios are significantly elevated. The enlargement of the caudal (rear) pulmonary arteries is actually the single finding with the strongest correlation to active infection.
Lung Changes That Point to Heartworm
Beyond the heart and vessels, heartworm disease also leaves marks on the lung tissue itself. The worms trigger inflammation in the surrounding arteries and lung tissue, which shows up as hazy or cloudy patterns on the X-ray. Dogs with heartworm commonly show what’s called an interstitial pattern, a diffuse haziness in the lung fields caused by inflammation spreading through the tissue between the air sacs. In more advanced cases, denser patches called alveolar patterns can appear, indicating fluid or inflammatory material has filled portions of the lungs.
Another telltale sign is a loss of clear margins around the pulmonary blood vessels. In a healthy dog’s chest X-ray, the edges of the major blood vessels in the lungs appear sharp and well-defined. When heartworm-driven inflammation sets in, those edges become blurred and indistinct. This loss of vessel margination showed up in about 84% of infected dogs in radiographic studies.
Why X-Rays Can’t Catch Early Infections
Here’s the important limitation: X-rays only reveal damage that has already happened. In mild or early infections, the heart and lungs may look completely normal on a radiograph. A dog could be carrying adult heartworms for months before the structural changes become obvious enough to spot. Some findings that show up on X-ray, like interstitial lung patterns and mild bronchial markings, also appear in dogs without heartworm, making them unreliable on their own.
This is why the primary screening tool for heartworm is a blood test, not an X-ray. The standard antigen test detects proteins released by adult female heartworms and can identify an infection before any visible damage to the heart or lungs. If your vet suspects heartworm based on symptoms or a positive blood test, the X-ray comes next to answer a different question: how much damage has the infection already caused?
How X-Rays Guide Treatment Decisions
Once a dog tests positive for heartworm on a blood test, chest X-rays become one of the most important next steps. The images help your vet gauge the severity of the disease and plan treatment accordingly. A dog with a normal-looking chest X-ray despite a positive blood test is in a very different situation than one with a massively enlarged right ventricle, bulging pulmonary arteries, and cloudy lung fields.
The degree of change on X-ray correlates directly with how much pressure has built up in the pulmonary arteries. Dogs with significant high blood pressure in the lungs show consistently larger heart measurements and wider pulmonary arteries on radiographs compared to heartworm-positive dogs without that complication. These findings matter because dogs with more severe disease face higher risks during treatment, when dying worms can break apart and lodge deeper in the pulmonary vessels. Knowing the baseline level of damage helps the veterinary team anticipate complications and adjust the treatment timeline.
X-rays are also repeated during and after treatment to track whether the heart and vessels are returning to normal size. In studies following dogs through the full treatment course, radiographic measurements at roughly nine months post-treatment showed whether the pulmonary arteries were shrinking back down or remaining enlarged, a sign that lingering high blood pressure may still be a concern.
Heartworm on X-Ray in Cats and Humans
Cats with heartworm present a different radiographic picture. Because cats are not the worm’s natural host, even one or two worms can cause dramatic lung inflammation. X-rays in cats with heartworm may show enlarged pulmonary arteries and patchy lung infiltrates, but the changes can mimic asthma or other respiratory conditions, making diagnosis tricky.
In the rare cases where heartworm larvae reach human lungs, they die and the body walls them off in small nodules. These show up on chest X-rays as round, well-defined spots called coin lesions. They’re notable mainly because they can look similar to lung tumors on imaging, sometimes leading to unnecessary concern or biopsy before the true cause is identified.