A large waterborne outbreak of campylobacteriosis in Norway: the need to focus on distribution system safety

Abstract

Background: On 7 May 2007 the medical officer in Røros (population 5600) reported 15 patients with gastroenteritis. Three days later he estimated hundreds being ill. Untreated tap water from a groundwater source was suspected as the vehicle and chlorination was started 11 May. Campylobacter was isolated from patients' stool samples. We conducted an investigation to identify the source and describe the extent of the outbreak.

Methods: We undertook a retrospective cohort study among a random sample of customers of Røros and neighbouring Holtålen waterworks. Holtålen, which has a different water source, was used as a control city. We conducted telephone interviews to gather data on illness from all household members. One randomly selected household member was asked about detailed exposure history. The regional hospital laboratory tested patients' stools for enteropathogens. Campylobacter isolates were typed by AFLP for genetic similarity at the Norwegian Institute of Public Health. Local authorities conducted the environmental investigation.

Results: We identified 105 cases among 340 individuals from Røros and Holtålen (Attack Rate = 31%). Tap water consumption was the only exposure associated with illness. Among randomly selected household members from Røros, a dose-response relationship was observed in daily consumed glasses of tap water (chi2 for trend = 8.1, p = 0.004). Campylobacter with identical AFLP was isolated from 25 out of 26 submitted stool samples. No pathogens were detected in water samples. We identified several events that might have caused pressure fall and influx of contaminated water into the water distribution system. On two occasions, pressure fall was noticed and parts of the distribution system were outdated.

Conclusion: The investigation confirmed a waterborne outbreak of campylobacteriosis in Røros. Although no single event was identified as the cause of contamination, this outbreak illustrates the vulnerability of water distribution systems. Good quality source water alone is not enough to ensure water safety. For a better risk management, more focus should be put on the distribution system security. Waterworks personnel should monitor the pressure regularly; reduce the leakage by upgrading the distribution network and use chlorination when conducting maintenance work.

Figure 1

Map of Røros with details on the waterworks and some events, which may have been relevant to the water contamination. The higher supply zone (RH) of the waterworks is marked with yellow, the lower (RL) with brown. Parts of the waterworks (schematic): underground wells (1), common collecting tank (2) and elevated reservoir (3). Events: maintenance work 30 April (A) with closed valve (B), maintenance work 2 May (C), firemen exercise at the airport 3 and 10 May (D), low pressure observed at slaughterhouse 3 and 10 May (E) and coliform bacteria proven in a tap water sample from dairy 9 May (F).

Figure 2

Cases of gastroenteritis in a sample of Røros and Holtålen household members by date of illness onset (n = 105), from April 30 to May 14, 2007 and the timeline of events, which may be relevant to the water contamination. Gray squares = Røros household members; plum squares = Holtålen household members; One case, marked with a letter R, was exposed to Røros tap water as well. IP = incubation period.

Figure 3

Attack rate according to the amount of tap water consumed (glasses per day) in the first week of May 2007 among 101 selected household members in Røros. AR = attack rate (%); n = total amount of members in this category.