Veterinary Technician Practice Exam 2025 - Free Veterinary Technician Practice Questions and Study Guide

The organism that causes heartworm is Dirofilaria immitis, which is a parasitic roundworm. This nematode is transmitted to dogs and other animals through the bite of an infected mosquito. When a mosquito feeds on a host that is infected with the larvae of Dirofilaria immitis, the larvae develop inside the mosquito and can then be transmitted to a new host during subsequent feedings.

Once inside the new host, the larvae migrate to the bloodstream and eventually settle in the pulmonary arteries, where they mature into adult worms. The presence of these adult heartworms can lead to serious health issues, including congestive heart failure and damage to the lungs and surrounding blood vessels.

The other options represent different organisms: Toxoplasma gondii is a protozoan that causes toxoplasmosis, Ancyclostoma caninum is a type of hookworm that primarily affects dogs, and Ctenocephalides felis is the cat flea, which is an ectoparasite but not a causative agent of heartworm disease. Thus, Dirofilaria immitis is correctly identified as the organism responsible for heartworm.