In an outbreak of diarrheal disease traced to a municipal water supply, which is the most likely causative agent?

The most likely causative agent of a diarrheal disease outbreak traced to a municipal water supply is Cryptosporidium spp. This organism is a protozoan parasite that is known to be resistant to chlorine treatment, which is commonly used in water treatment facilities. When the water supply becomes contaminated with Cryptosporidium, typically through runoff or improper sanitation, it can lead to widespread outbreaks, especially in environments where water is the primary vehicle for transmission.

Cryptosporidium is characterized by its small, hardy oocysts that can survive in the environment for extended periods and are infectious at very low doses. Symptoms of infection typically include watery diarrhea, abdominal cramps, and nausea, which align with the symptoms that commonly arise in diarrheal disease outbreaks. The ease of contamination of water sources by this pathogen makes it a significant public health concern, particularly in municipal systems that serve large populations.

The other organisms listed are generally not associated with outbreaks traced back to municipal water supplies. Cystoisospora belli, for example, is typically linked to foodborne transmission. Entamoeba histolytica, while capable of causing dysentery, is more commonly transmitted through fecal-oral routes involving contaminated food or water but is less frequently associated with widespread water