Which of the following best describes heartworm disease in dogs?

• A. It is a bacterium causing skin infections
• B. It is a protozoan parasite that mainly affects the liver
• C. It is a nematode that commonly lodges in the lungs
• D. It is a nematode (heartworm) that blocks blood flow and can affect the heart

Answer: (D)

Heartworm disease is caused by a parasitic nematode that lives in the heart and nearby vessels of dogs. It is transmitted by mosquitoes, which spread larvae from one dog to another. As the worms mature, they block blood flow, mainly affecting the heart and pulmonary arteries. This can lead to coughing, exercise intolerance, and eventually heart failure if not treated. The description that fits best is a nematode that blocks blood flow and can affect the heart. The other options mischaracterize heartworm: a bacterium would cause skin infections, a protozoan parasite would typically target the liver, and while heartworms involve the lungs, they are not solely a lung-lodging parasite—the heart and blood vessels are central to the disease.