An outbreak of E. coli O157 infection occurred in the Highland Region of Scotland in the
summer of 1999. The source of the outbreak was traced to an untreated private water supply.
All six cases identified arose in visitors to the area, and most had very limited exposure to the
contaminated water. Permanent residents on the same supply were unaffected. The E. coli O157
isolates from the water, sheep faeces collected from around the source and the human stool
samples were indistinguishable using pulsed field gel electrophoresis. Previously reported
outbreaks of E. coli O157 linked to potable water supplies have resulted from structural or
treatment failures, which allowed faecal contamination of source water. Here, contamination of
the water supply and subsequent human infection was due to the use of an untreated,
unprotected private water source in a rural area where animals grazed freely.