What life-stage forms are primarily responsible for transmission of Cryptosporidium in water?

• A. Oocysts
• B. Trophozoites
• C. Sporozoites
• D. Cysts

Answer: (A)

Oocysts are the form responsible for spreading Cryptosporidium through water. These thick-walled oocysts are shed in feces and can survive for long periods in the environment, making them hardy enough to persist in contaminated water and resist standard disinfection. When someone drinks water containing these oocysts, they are ingested and then release sporozoites in the small intestine to establish infection. The other stages—trophozoites and sporozoites—are inside the host after infection (trophozoites are the feeding form inside intestinal cells; sporozoites are released inside the gut to invade cells), so they aren’t the forms that are transmitted via water. The term cyst isn’t the precise environmental form for Cryptosporidium in this context, whereas oocysts specifically describe the environmentally resistant, water-transmitting stage.