A peer-reviewed article published in March 2026, in the Open Forum Infectious Diseases provides an overview of the first reported outbreak of persistent Shiga Toxin-producing Escherichia coli O157:H7 strain REPEXH01 associated with untreated recreational water. The publication is authored by scientists from the Centers for Disease Control and Prevention (CDC), the California Department of Public Health (CDPH), the Nevada County Public Health Department (NCPHD) and the Nevada County Environmental Health Department (NCEHD).  

The publication delves into a 2017 investigation of a STEC outbreak involving 18 patients, mostly children. Of these, 11 were confirmed by laboratory tests, 10 were hospitalized and 5 developed hemolytic uremic syndrome (HUS). Outbreak questionnaires from the CDPH identified untreated recreational water in a northern California lake during a birthday party as the common exposure among 16 of the 18 patients; the remaining 2 were in close household contact with lab-confirmed patients.  

The CDPH, in collaboration with the CDC, conducted environmental investigations at the lake in August 2017. The investigations included a characterization of the surrounding land, wildlife observations and the collection of environmental samples. Microbial analyses of water and sediment samples included quantification of generic E. coli and coliforms, microbial source-tracking analysis (e.g., human Bacteroides and mitochondrial DNA from geese and deer) and isolation and subtyping of STEC O157. Additionally, goose and deer scat were collected when available. 

A year later, after a water sample tested positive for STEC O157:H7, a follow-up environmental assessment was conducted to determine whether the outbreak strain remained present and to identify potential environmental reservoirs. For this follow-up, additional sample sites and MST markers were collected, including creeks (e.g., upstream and outlet) and MST markers (e.g., ruminant and bovine). STEC isolates from both environmental assessments were subjected to Whole-genome sequencing (WGS). The initial environmental assessment in 2017, with a follow-up in 2018, highlighted the key findings outlined below. 

• Generic E. coli exceeded the US recreational water quality criteria statistical threshold value (410 MPN/100 mL), reaching 861 MPN/100 mL in 2017, and ranging from 770 to over 2420 MPN/100 mL in 2018 samples. 

• Water samples from 2017 taken from two collection sites, Beach A and Beach B, showed that only Beach B had STEC O157 detected, along with positive goose MST markers and negative human and deer MST markers. Similar findings were reported for sediment samples. Interestingly, even though water samples from Beach A were negative, sediments from Beach A tested positive for STEC O157. 

• In the 2018 follow-up assessment, the goose marker was only found at beach B, which also had the highest fecal indicator bacteria (FIB) levels. Human and ruminant MST markers were detected only in the water from the creek outlet, where FIB levels were also high. However, no bovine-specific marker was identified. 

• STEC O157 was isolated in 2017 from sediment and water samples. Pulsed-field gel electrophoresis (PFGE) patterns matched STEC O157 isolated from patients and from goose scat. WGS of the different samples were positive for H7 and carried the shiga toxin gene stx2a, the intimin gene (eae), and the enterohemolysin gene (ehxA). The isolates were also resistant to various antimicrobial compounds, including, but not limited to, phenicols, sulfonamides and quaternary ammonium compounds. 

• Positive STEC O157 isolates from the 2018 investigation were unique in the PulseNet database and unrelated to the 2017 lake outbreak strain. Although virulence markers, including eae and ehxA, were identified, the isolates did not exhibit antimicrobial resistance markers.  

After the outbreak, local authorities’ public health response involved issuing a no-swim advisory and conducting weekly beach water quality tests, which led to the closure of all swimming beaches around the lake. Conversely, after co-detection of ruminant and human MST markers, along with STEC O157, in a single sampled creek, it was determined that parcels along the creek were used for grazing, with cattle having unrestricted access to the creek banks and water. However, no further MST investigation was conducted. Instead, a non-lethal deterrence method was implemented after the organization obtained a federal permit, resulting in the culling of 69 resident Canada geese. Later in 2019, the organization reported a reduction in E. coli levels in the beach water, which they attributed to efforts to control the geese population.  

The authors suggest that, although the outbreak strain was recovered from most geese samples during the two environmental assessments, it was unclear whether the geese were the STEC source or had been colonized by STEC after being introduced to the lake environment by another source. The authors further explain that the 2018 finding suggests that cattle could have repeatedly introduced STEC into the environment, and that the geese may have acquired the pathogen and spread it in the water body. 

Since the 2017 STEC lake-related outbreak, 9 additional outbreaks have involved the same O157:H7 strain. Including the largest romaine lettuce outbreak of 2018, where the pathogen was isolated from canal water adjacent to lettuce fields and a concentrated animal feeding operation (CAFO). 

Why this matters: 

• Although past outbreaks of E. coli O157:H7 linked to recreational water have been documented, this is the first outbreak associated with the REP strain. Identifying this strain helped recognize it in foodborne outbreaks that happened in later years. 

• This investigation highlights the importance of monitoring environmental waters and recognizing that ecological systems are dynamic.  As highlighted in the publication, adjacent land and wildlife patterns (e.g., bird migration) influence how pathogens emerge and spread in the environment. 

• The use of microbial source tracking in environmental samples can be particularly valuable in complex systems, where isolating foodborne pathogens is challenging, as it helps narrow down potential sources rather than searching for a needle in a haystack.