Which of the following parasites is a zoonotic agent?

• A. Entamoeba histolytica
• B. Trypanosome brucei
• C. Toxoplasma gondii
• D. All of the above

Answer: (D)

Zoonotic potential means a parasite can move between animals and humans, either through direct contact, environmental contamination, or a shared host in the life cycle. Entamoeba histolytica is often thought of as a human parasite, but similar Entamoeba species and cysts can be found in animals, and environmental contamination from animal sources can contribute to human infection in some settings. Trypanosoma brucei involves animal reservoirs and vectors that connect wildlife to humans, so humans can become infected through those animal–vector–human cycles, with some forms more associated with animal reservoirs than others. Toxoplasma gondii is a classic zoonosis: cats are the definitive hosts, but many warm-blooded animals can harbor tissue cysts, and humans typically acquire infection via undercooked meat or exposure to cat feces. Because each of these parasites can involve animal sources or reservoirs that transmit infection to people, the best answer is all of the above.