Which parasite produces environmentally resistant thick-walled oocysts and is a common cause of waterborne outbreaks?

• A. Cryptosporidium parvum (and hominis)
• B. Giardia lamblia
• C. Entamoeba histolytica
• D. Cyclospora cayetanensis

Answer: (A)

The key idea is that some intestinal parasites form oocysts that are unusually hardy in the environment, especially in water. Cryptosporidium produces thick-walled oocysts that resist chlorine and other disinfectants, so these infectious forms can survive water treatment and persist in drinking supplies. This makes Cryptosporidium a leading cause of waterborne outbreaks, with species like Cryptosporidium parvum and Cryptosporidium hominis commonly implicated in human infections.

Giardia forms cysts that are also environmentally robust, but they are not oocysts, and the emphasis on thick-walled oocysts that withstand disinfection points to Cryptosporidium. Entamoeba histolytica also forms cysts, yet its waterborne outbreaks are not as characteristically tied to chlorine-resistant oocysts as Cryptosporidium. Cyclospora cayetanensis creates thick-walled oocysts too, but its notable outbreaks are more often linked to contaminated produce than general water supplies.