In a cluster of pediatric cases with watery diarrhea linked to a water park, which organism is the most likely cause?

• A. Norovirus
• B. Cryptosporidium spp.
• C. Giardia lamblia
• D. Escherichia coli O157:H7

Answer: (B)

Recreational water outbreaks are often driven by organisms that can survive normal pool disinfection and spread through fecal-oral contact in swimmers. Cryptosporidium is a parasite that forms oocysts highly resistant to chlorine, so it can persist in swimming pools and water parks even when the water seems clean. In children, it typically causes watery diarrhea that can be prolonged, and transmission occurs when an infected person contaminates the pool water.

This makes Cryptosporidium the best answer for a cluster of pediatric watery diarrhea linked to a water park. Norovirus can cause outbreaks of gastroenteritis and is very contagious, but the linkage to a water park specifically points toward the chlorine-resistant parasite in this setting. Giardia can cause greasy, malabsorptive diarrhea and is more commonly linked to untreated water sources, while E. coli O157:H7 usually presents with bloody diarrhea and is not the classic cause of a water-park–related watery-diarrhea outbreak.