Pets that exhibit weight loss, lethargy, and vomiting might be infested with which type of parasite?

Pets that exhibit symptoms such as weight loss, lethargy, and vomiting are often associated with a parasitic infestation, particularly roundworms. Roundworms are known to cause significant gastrointestinal distress. They feed off the nutrients in the host's intestines, which can lead to malnutrition and subsequent weight loss. Moreover, the presence of these worms can trigger a host's immune response, causing lethargy due to the body’s effort to fight the infection. Vomiting can occur as a result of irritation in the gastrointestinal tract caused by the presence of the worms.

While heartworms primarily affect the cardiovascular system and can lead to lethargy and weight loss, they do not typically cause vomiting in the early stages of infection. Fleas are external parasites that can lead to discomfort, skin issues, and stress in pets, but they generally do not cause significant vomiting or weight loss on their own unless accompanied by other medical issues. Hookworms also cause weight loss and lethargy, but they are more likely to lead to anemia and diarrhea rather than vomiting. Thus, roundworms are the most likely culprit for the combination of symptoms described.