Abstract Background In July 2017, a Shiga toxin-producing Escherichia coli (STEC) outbreak was reported among 18 individuals with exposure to untreated recreational water at a California lake. Ten cases were hospitalized and five (ages 1–5 years) developed hemolytic-uremic syndrome. Methods An environmental investigation was conducted to confirm STEC transmission via the lake and determine the outbreak strain contamination source. Large-volume water and sediment samples were collected in August 2017 and July 2018 and tested for STEC and molecular microbial source tracking markers. Results The STEC O157:H7 outbreak strain was recovered from the suspected exposure location and from goose scat, and goose genetic material was detected in every 2017 environmental sample, confirming lake water as the transmission vehicle and implicating the lake’s resident Canada goose population in disseminating the pathogen. An unrelated STEC O157:H7 strain and ruminant fecal contamination were detected the following year, suggesting repeated STEC introductions to the lake environment. Subsequent large-scale outbreaks of the same highly virulent STEC O157:H7 strain were associated with romaine lettuce and ground beef, resulting in its designation as the reoccurring, emerging, or persisting (REP) strain REPEXH01. Conclusions The severe illnesses associated with outbreaks of virulent pathogen strains like REPEXH01 are more likely to be detected. Conducting environmental investigations in response to these reoccurring outbreaks allows for the collection of incremental evidence about the patterns of transmission and sources of consequential pathogens and strains. Such information can provide critical insights for mitigation strategies to interrupt ongoing transmission and prevent future outbreaks.
Holcomb et al. (Thu,) studied this question.