Longitudinal monitoring of Shiga toxin-producing Escherichia coli (STEC) concentrations in surface waters and groundwater supplies within an Irish catchment.

Shiga toxin-producing Escherichia coli (STEC) are zoonotic agents causing human gastroenteritis, with symptoms ranging from asymptomatic/mild to bloody diarrhoea and in rare cases haemolytic uraemic syndrome (HUS) and death. STEC serogroups O157 and O26 are most commonly linked to infection. Waterborne transmission represents a key exposure route and occurs through contamination of drinking water sources with human or mammalian faeces. Ireland has consistently reported the highest STEC infection rates across the EU. This study sought to investigate STEC concentrations in surface water bodies and groundwater wells located in the Corrib catchment in western Ireland over the peak human infection periods.

From late May to early December 2023, 19 sites comprising river (n=5) and private groundwater wells (n=14) were sampled on a fortnightly basis in the Black River sub-catchment of the Corrib catchment. Colilert-18® and quantitative PCR were employed to monitor the presence of total coliforms, E. coli and STEC serogroups targets O157 (rfbE) and O26 (wzx). Measured physico-chemical parameters included pH, temperature, and dissolved oxygen content, in addition to river discharge and groundwater table. These were amalgamated with publicly available data, including groundwater vulnerability and rainfall data from Geological Survey Ireland (GSI) and Met Éireann, and continuous river level and discharge data from the Office of Public Works (OPW).

Overall, 265 samples were collected comprising 75 river samples and 190 groundwater samples. E. coli was detected in 178 samples (67%), including all 75 river samples and 103 (54%) of groundwater samples.

STEC was detected in 225 samples (85%) overall and in 119 (67%) of E. coli positive samples. A total of 168 samples (64%) tested positive for STEC O157, comprising 34 rivers (45%) and 134 groundwaters (71%). STEC O26 was detected in 56 samples overall (21%) and was more prevalent in rivers (n=22, 30%) than groundwater (n=34, 18%). Of 103 E. coli positive groundwaters, 86 were STEC positive, giving a STEC to generic E. coli detection ratio of 83.5%. STEC O157 was detected in 82 (80%) and STEC O26 was detected in 24 (23%) of E. coli positive groundwaters.

Rainfall peaked in July (224.1mm), leading to increases in river discharge (2.137 m³/s mean peaking at 6.058 m³/s in mid-July) and groundwater level (10.11m mean peaking at 9.77m two weeks after heavy rainfall). STEC O157 concentrations peaked in the river samples during August (80% positive, mean concentration of 1.3x10⁶ copies), and later in groundwaters (64% positive, mean concentration of 3.1x10⁵ copies) during September. STEC O26 concentrations peaked earlier in late July for river samples (10% positive samples, mean concentration of 3.58x10⁵copies) and early July for groundwater (22% positive, mean concentration of 2.9x10⁶ copies).

We describe the dynamics of STEC in surface and groundwaters of a catchment in western Ireland. The peaks for STEC serogroups O157 and O26 in groundwaters correlate with “traditional” human peaks of infection for these serogroups in Ireland and are preceded by peaks in surface water concentrations. Findings will be useful in designing strategies for source protection and risk management of drinking water supplies.