Health Stream Article - Issue 40 - December 2005

Cryptosporidium Outbreak In Wales

Over 70,000 people in 40 towns in the north western region of Wales, UK have been told to boil their drinking water due to an outbreak of crypto-sporidiosis that is suspected to be waterborne. An increase in the number of cases of cryptosporidiosis in the area was noted by public health officials during late October but initial investigations failed to reveal an obvious source of infection.

As the number of people affected continued to grow, a decision was made to alert local doctors to the outbreak, and on 23 November advice was issued to remind people with severely compromised immune systems to boil tap water before drinking as a precautionary measure (the British Chief Medical Officer advises such groups to always boil tap and bottled water before consumption or making ice for consumption). A boil water notice was issued to the general public on 30 November after a case-control study implicated tap water as a strong risk factor (see below). A total of 203 laboratory-confirmed cases had been identified by 15 December and, according to press reports, several people had been hospitalised with severe gastroenteritis.

The affected towns are supplied from the Llyn Cwellyn reservoir, which is located in the Snowdonia National Park. The reservoir's catchment has a mixture of land uses including some sheep and cattle grazing. The catchment also contains the village of Rhyd-Ddu which discharges treated sewage to the reservoir, and a number of houses with septic tank systems. Water from the reservoir is treated by microstraining, pressurised sand filtration and chlorination before distribution to consumers. The geographic distribution of cases appears to be consistent with this water source as the incidence rate of cryptosporidiosis is about 7-fold higher in areas supplied by the Llyn Cwellyn reservoir than in the remainder of north western Wales.

A case-control study was carried out by public health investigators who interviewed 45 cases and 37 unmatched controls about possible sources of exposure. The results of the investigation showed that people with cryptosporidiosis were 9.5 times more likely to drink tap water than unaffected controls, and a dose-response relationship was also seen with the volume of tap water consumed. According to the water supply company Dwr Cymru (Welsh Water), the water treatment plant has been operating normally with no evidence of problems. Under the risk assessment framework employed by the UK Drinking Water Inspectorate, the water supply was not considered to be at risk for Cryptosporidium contamination, and therefore was not required to carry out continuous monitoring under DWI regulations. However, by chance the equipment needed for continuous monitoring had recently been installed at the treatment plant in preparation for an operational monitoring program to investigate the performance of the treatment process. Dwr Cymru was therefore able to rapidly initiate continuous monitoring for Cryptosporidium on 2 November after the company was notified by health authorities of the increasing number of cryptosporidiosis cases in the region. Since then, oocysts have been detected in both raw and treated water from the Llyn Cwellyn reservoir, but the levels in treated water have been below the DWI regulatory standard of 1 oocyst per 10 litres. Supply of drinking water which exceeds this limit may be subject to prosecution under UK regulations.

Genotyping of Cryptosporidium oocysts from patients has shown the infections were due to C. hominis, a species which is harboured only by humans. Investigations are therefore focusing on sources of human waste in the catchment. The area experienced heavy rains in October, leading to speculation that waste from the sewage treatment plant or from septic tanks may have been washed into the reservoir. Dwr Cymru has stated that the sewage treatment plant was not flooded during the rains, and there is no evidence of problems with the operation of the plant. However as a precautionary measure, effluent from the plant is currently being trucked out of the catchment for disposal at another site. Other sewage plants and septic tanks in the catchment are also being inspected to determine whether they may be the source of contamination.

On 9 December the boil water notice was lifted for 9,000 residents after water from alternative sources was diverted to some towns. However due to the geography of the area and the limited capacity of other water sources, it is not possible to remove the Llyn Cwellyn reservoir from service and still provide an adequate water supply to the remaining affected areas. Residents in these areas have been advised they may need to continue boiling water for drinking until early January. Public health authorities have been criticised for not issuing the boil water notice earlier, however until the results of the case-control study became available, the outbreak investigation team considered the evidence implicating the water supply was considered insufficient to justify action of this nature.