In developed countries the most common bacterial cause of gastroenteritis is campylobacter infection. Most of these infections are sporadic and known risk factors include drinking unpasteurised milk or untreated water, eating chicken, barbecuing, and living or working on a farm. Several community outbreaks of campylobacter gastroenteritis arising from contamination of the water supply have been described but the mechanism of water contamination has usually been unknown. Now an outbreak in a rural community in southern Finland has been traced to a non-chlorinated well water supply.

In August 2000 there were 463 known cases of gastroenteritis among a population of 8600 people. The median age of those affected was 41 years (range 1–96 years) and 62% were female. The outbreak began around 27 July and ended around 24 August, the peak incidence being on 7 August. Seventy four stool samples were submitted from patients and Campylobacter jejuni was isolated from 24. No other pathogens were isolated. One sample of tap water contained C jejuni. Samples from other parts of the water supply were negative.

A case-control study included 113 patients and 241 controls. Ninety four per cent of patients and 58% of controls had drunk unboiled tap water during the previous two weeks (matched odds ratio (MOR) 21). For 88% of cases and 57% of controls the drinking water in the home was unboiled tap water and the risk of illness increased with the amount of unboiled tap water drunk. Drinking water from private wells, bottled water, or boiled water was protective. Consumption of eggs was associated with a decreased risk. Only drinking unboiled tap water remained significant after adjustment for other factors.

About two thirds of this population received their water from two water stores supplied by two wells. The water was not chlorinated but it was tested monthly for coliforms. No contamination had been detected before the outbreak. Some holiday homes near one of the wells were not connected to the municipal sewage system and there was a dry toilet and a compost heap about 15 metres from this well. The wells were unfenced and people and animals had free access. The rainfall in July 2000 was high.

Sixteen C jejuni isolates from patients were serotyped and all 16 had the same serotype (Penner 12). Eight of these isolates were subtyped by pulsed field gel electrophoresis (PFGE) and all but one showed the same pattern. The isolate from tap water had the same serotype and on PFGE was identical to the seven patient strains. Prior to the outbreak 17 strains of C jejuni had been identified in the area, one of which was of the same serotype as the outbreak strain.

The evidence points to this outbreak having arisen from a defective water supply. People were advised to boil their drinking water on 11 August and the water supply was chlorinated from 12 August.