What type of parasite can be fatal to dogs if left untreated?

The correct answer is heartworm. Heartworm disease, caused by the parasite Dirofilaria immitis, is transmitted to dogs through mosquito bites. The adult heartworms reside in the heart and pulmonary arteries, leading to severe damage over time. If heartworm infestations are left untreated, they can cause significant cardiovascular issues, pulmonary hypertension, and ultimately can be fatal due to heart failure or other serious complications.

While fleas, roundworms, and hookworms can also cause health problems in dogs, they typically do not lead to fatal outcomes with prompt treatment. Fleas can lead to skin irritation and allergies, roundworms can cause gastrointestinal issues, and hookworms can result in anemia or other health problems, but they are generally manageable with appropriate veterinary care. In contrast, heartworm disease can have a more insidious progression, with severe consequences if not diagnosed and treated in a timely manner. This makes heartworm a particularly dangerous parasite for dogs.