Outbreak of norovirus illness associated with a swimming pool

Abstract

On 3 February 2004, the Vermont Department of Health received reports of acute gastroenteritis in persons who had recently visited a swimming facility. A retrospective cohort study was conducted among persons attending the facility between 30 January and 2 February. Fifty-three of 189 (28%) persons interviewed developed vomiting or diarrhoea within 72 h after visiting the facility. Five specimens tested positive for norovirus and three specimen sequences were identical. Entering the smaller of the two pools at the facility was significantly associated with illness (RR 5.67, 95% CI 1.5-22.0, P=0.012). The investigation identified several maintenance system failures: chlorine equipment failure, poorly trained operators, inadequate maintenance checks, failure to alert management, and insufficient record keeping. This study demonstrates the vulnerability of recreational water to norovirus contamination, even in the absence of any obvious vomiting or faecal accident. Our findings also suggest that norovirus is not as resistant to chlorine as previously reported in experimental studies. Appropriate regulations and enforcement, with adequate staff training, are necessary to ensure recreational water safety.

Fig. 1

Flow chart of study participants.

Fig. 2

(a) Cases of illness associated with attendance at a private swimming facility by date of onset, Vermont, 30 January to 11 February 2004. ■, Primary cases (n=53); □, secondary cases (n=16). (b) Incubation period between first exposure to the activity pool and onset of symptoms for primary cases of illness (n=53).

Fig. 3

Number of well (□) and ill (■) persons and attack rates () of acute gastroenteritis among persons attending events at the programme pool, by exposure group and day; Vermont, 30 January to 2 February 2004. Six employees (3 ill, 3 well; 50% attack rate) not depicted because they had multiple exposures to the activity pool over the period of interest.