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

The most likely causative agent in an outbreak of diarrheal disease linked to a municipal water supply is Cryptosporidium spp. This protozoan parasite is notorious for causing waterborne outbreaks, particularly in areas where municipal water supplies may become contaminated with fecal matter. Cryptosporidium can withstand chlorine disinfection used in municipal water treatment processes, making it a resilient pathogen in water systems.

Traditionally, cryptosporidiosis presents with watery diarrhea, abdominal cramps, and vomiting. The mode of transmission typically involves ingesting oocysts through contaminated water, which aligns well with the scenario of a municipal water outbreak. In addition to its environmental resilience, the prevalence of Cryptosporidium in surface waters from agricultural runoff or sewage can further contribute to its role as a major waterborne pathogen.

In contrast, other parasitic agents such as Cystoisospora belli, Entamoeba histolytica, and Dientamoeba fragilis, while also capable of causing gastrointestinal symptoms, are less commonly associated with large-scale outbreaks linked to municipal water supplies. For instance, Cystoisospora belli is primarily transmitted via contaminated food rather than water, and Entamoeba histolytica has a different transmission route involving fecal-