Health Stream Literature Summary - Issue 58 - June 2010

A Cryptosporidium hominis outbreak in North-West Wales associated with low oocyst counts in treated drinking water.
Mason BW, Chalmers RM, Carnicer-Pont D and Casemore DP. (2010) Journal of Water & Health, 8(2); 299-310.

This paper reports the results of an epidemiological investigation of an outbreak of cryptosporidiosis in North West Wales during 2005. This region normally has a low rate of notified laboratory-confirmed Cryptosporidium infections (36 cases in total for 2004), and suspicions of an outbreak were raised when 21 notifications were received during September/October 2005. Further cases continued to occur and an Incident Management Team was set up on 7 November. Preliminary investigations suggested a link with the drinking water supply derived from a reservoir (Llyn Cwellyn) which serves a resident population of around 60,000 people as well as numerous tourists. On 18 November, a precautionary reminder for severely immunocompromised patients to boil their water was issued. A general boil water notice was issued on 29 November.

There were 231 laboratory-confirmed cases of cryptosporidiosis notified between 1 October 2005 and 31 January 2006 in the area. Of the 231 cases, 218 were confirmed as Cryptosporidium hominis infection by genotyping at the COWP locus. A case-control study was undertaken to examine whether Cryptosporidium hominis infection was associated with the consumption of unboiled tap water. Cases were those residents aged 16 years or older with diarrhoea (three or more loose stools in 24 hours) with an onset after 1 September. Cases were excluded if they had a history of travel outside the UK or contact with a household member with a history of diarrhoea, during the two weeks before onset. Controls were randomly selected from the same area and were 16 years or older, had no history of travel outside in UK in the two weeks prior to the 5 November, or contact with a household member with new onset of diarrhoea since 1 September. Cases and were interviewed using a structured outbreak questionnaire which collected information about a range of exposures.

At the time of the outbreak, Llyn Cwellyn was classed as a low risk source for Cryptosporidium. Water treatment comprised pressure filtration and chlorination, but there was no requirement to monitor for Cryptosporidium in finished water under drinking water regulations. Continuous monitoring for Cryptosporidium commenced on Cwellyn final water from 2 November as part of the investigation. The attack rate was found to be significantly higher in the population supplied by Cwellyn Water Treatment Works relative to other water sources (relative risk 4.1, 95% CI, 2.8-6.1, P less than 0.0001). There were 45 cases and 37 controls included in the case-control study. Drinking unboiled tap water was found to be significantly associated with disease, odds ratio 6.1, p = 0.002. There was a dose-response relationship found with increasing risk of disease with higher consumption of unboiled tap water. Age and the number of glasses of unboiled tap water drunk per day were the only significant variables in the multivariate analysis.

The average number of oocysts per 10 litres per large volume (greater than or equal to 1,000 l) continuous sample in final treated water at the Cwellyn Water Treatment Works were consistently low and below the regulatory treatment standard (average of less than one oocyst per 10 litres in a continuous sample at 40 litres/hr/about 24 hrs) which is specified for sites identified as at significant risk of Cryptosporidium. However, oocysts were detected in 63% of samples from the plant. The shape of the epidemic curve suggests that it is unlikely that a large number of oocysts entered the distribution system prior to the commencement of continuous monitoring. No failure of water treatment was identified or any adverse event or challenge indicating a plant under strain. There were a number of potential sources of pollution including a sewage treatment works at the head of the lake and at least 13 septic tanks at properties in the catchment. Heavy rain was common in the area with rapid flows into the lake from water courses and intermittent flooding in low-lying areas. There were no effective barriers to Cryptosporidium either naturally occurring in the catchment or man-made in terms of water treatment.

The boil water notice was in place for nine weeks until a UV treatment plant was installed at the Cwellyn WTW. This outbreak led to a major modification of the relevant water quality regulations and also the setting up of a new multiagency liaison group in Wales, to coordinate the approach to any future incidents or outbreaks. This outbreak demonstrated that C. hominis outbreaks can occur in the presence of very low numbers of oocysts detected in treated water. The risk to human health only becomes apparent when this water is supplied to a large susceptible population and an outbreak results.

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