Which structure is most likely to be enlarged in chest films of a dog positive for heartworm disease?

In cases of heartworm disease, the structures most likely observed to be enlarged in chest films are the pulmonary arteries. This enlargement occurs due to the presence of adult heartworms residing in the pulmonary arteries, leading to increased pressure and volume overload as the heart attempts to pump blood through these obstructed vessels. The inflammatory response and vascular changes stemming from the presence of the heartworms result in dilation and other visible alterations in the pulmonary arterial tree, making them prominent on radiographic imaging.

Additionally, the pulmonary arteries are responsible for carrying blood from the heart to the lungs, and when heartworms cause increased resistance or obstruction within these vessels, they respond by enlarging. This radiographic finding is significant in diagnosing heartworm disease and assessing its severity.

While other structures such as the left atrium can become enlarged due to heartworm-related conditions like pulmonary hypertension or right heart failure, the most direct and consistent enlargement due to heartworm infection occurs in the pulmonary arteries.