Roundworms are the most prevalent endoparasite in pets. Others include hookworms, whipworms, and tapeworms. Pets are typically infected with these parasites through accidental ingestion of parasite eggs (which are microscopic) from areas that have fecal contamination from other infected animals. Alternatively, some parasites are acquired through ingestion of intermediate hosts such as rodents (Taenia tapeworm species; Toxocara roundworm species) or fleas (Dipyllidium tapeworm species). These parasites are also a health risk to humans and are considered zoonotic – meaning they can be transmitted from animals to people. For example, if a person accidentally ingests roundworm eggs, the larvae can migrate in the body and cause organ damage and potentially blindness. Hookworm larvae in the soil and grass can infect bare skin and cause a condition in people known as cutaneous larval migrans.
Heartworm is another important endoparasite, but one which is not zoonotic. Heartworm infections result from pets being bitten by infected mosquitos. The larval form of the heartworm travels through the bloodstream to the heart where it develops into an adult. The adult heartworms live on the right side of the heart and if left untreated, result in progressive heart failure and death. In the initial stages of heartworm disease, pets may be asymptomatic. As the condition progresses, symptoms may evolve including a cough and exercise intolerance in dogs, and vomiting/coughing in cats. Treatment of heartworm disease can be very risky for the pet, and very costly.