Shigellosis, an acute intestinal infection of humans, can be caused by any of the different species-serotypes of the genus Shigella (family Enterobacteriaceae). The presence of Shigella in water has been a concern for more than 100 years. Shigella invades the intestinal mucosa, producing dysentery (shigellosis), which is characterized by abdominal pain, tenesmus, and bloody diarrhea. The infectious dose for Shigella spp. is low; only a few cells may be sufficient. The low infective dose often results in person-to-person transmission and laboratory-acquired infections. When outbreaks occur, they usually are associated with fecal contamination of foods, but waterborne transmission is also important. Shigella accounted for 34 of 291 (11.7%) drinking water-related outbreaks that were reported to the CDC in the United States during 1972 through 1994. (For subsequent Shigella surveillance reports, see https://www.cdc.gov/shigella/surveillance.html.)
The genus Shigella has 4 named species, 3 of which can be further divided into O antigen groups (serotypes): Shigella dysenteriae (serological group A, O antigen groups 1–15), Shigella flexneri (group B, O groups 1–6), Shigella boydii (group C, O groups 1–20), and Shigella sonnei (group D; with a single O group). The latter is biochemically different from the 3 other Shigella species (see Table 9276:1). Shigella dysenteriae O1 is an extremely important pathogen in developing countries and frequently causes waterborne outbreaks; S. sonnei and S. flexneri predominate in most developed countries. Shigellosis is most common among children. Outbreaks with person-to-person transmission have been reported in schools, childcare centers, and institutions providing custodial care. Waterborne outbreaks are associated with the following situations: fecal contamination of private or noncommunity water supplies in which chlorination is inadequate; cross connections between wastewater and potable water lines; and recreational waters contaminated with feces from cases of shigellosis.