What body organ of a dog is most adversely affected when the dog is infected by Dirofilaria immitis?

• A. Brain
• B. Liver
• C. Lungs
• D. Heart

Answer: (D)

Heartworms reside in the right side of the heart and the major arteries leading from it. When a dog is infected with Dirofilaria immitis, the adult worms physically occupy and obstruct parts of the heart and its connected vessels, causing inflammation, damage to the heart valves, and reduced pumping efficiency. This directly injures the heart and can lead to heart failure, making the heart the most adversely affected organ.

The lungs can be affected secondarily because the worms and resulting inflammation also involve the pulmonary arteries, leading to coughing and pulmonary hypertension, but they are not the primary site where the worms live. The brain and liver are not typically the main targets of heartworm disease.