Outbreak of cryptosporidiosis in North West Wales, 2005 Report of the Outbreak Control Team (Anglesey County Council, Anglesey Local Health Board, Centre for Research into Environment and Health, Dwr Cymru, Environment Agency Wales, Gwynedd County Council, Gwynedd Local Health Board, National Public Health Service for Wales) Outbreak Control Team Report on Outbreak of Cryptosporidiosis in North West Wales, 2005 Contents Summary…………………………………………………………………………. 6 The outbreak ………………………………………………………… 6 Observations and lessons learned …………………………………... 7 Recommendations ………………………………………………….. 9 Glossary of terms and abbreviations ……………………………………………. 10 1. Background ………………………………………………………………. 17 2. Chronology of the outbreak and public health response …………………. 20 2.1 Background ………………………………………………………… 20 2.2 Early Cases ……………………………………………………….... 20 2.3 Establishment of incident management team ……………………… 20 2.4 Incident management team meeting, 18th November ……………… 21 2.5 Progress of outbreak, 18th November – 29th November …………… 23 2.6 Outbreak declared, 29th November ………………………………... 24 2.7 Decline and end of outbreak, 15th December 2005 - 30th January 2006. ………………………………………………... 26 2.8 Aftermath ………………………………………………………….. 30 3. Microbiology and epidemiology ………………………………………… 31 3.1 Microbiology ……………………………………………………… 31 3.2 Descriptive epidemiology based on the first 218 confirmed cases …………………………………………………... 31 3.3 Case control study ………………………………………………… 33 3.4 Discussion of epidemiological studies ……………………………. 36 3.5 Conclusion ………………………………………………………… 37 4. The water supply ………………………………………………………… 38 4.1 History of Cwellyn water treatment works ……………………….. 38 4.2 Development of the water treatment works ………………………. 38 4.3 Turbidity event 2004 ……………………………………………… 39 4.4 Cwellyn regulatory improvement programme ……………………. 40 4.5 Drinking Water Inspectorate audit, September 2005 …………….. 40 Version: 6 Date: 28th Status: Approved for publication November 2006 Author: Chris Lines, NPHS Wales Page: 2 of 184 Outbreak Control Team Report on Outbreak of Cryptosporidiosis in North West Wales, 2005 4.6 Cryptosporidium monitoring at Cwellyn water treatment works …. 40 4.7 Installation of UV plant …………………………………………... 42 4.8 Topographical description of catchment …………………………... 43 4.9 Hydrogeology ……………………………………………………… 43 4.10 Hydrodynamic survey ……………………………………………... 44 4.11 Land use in the catchment …………………………………………. 44 4.12 Climatic conditions ………………………………………………… 45 4.13 Sewage treatment …………………………………………………... 46 4.14 Investigation of potential sewage treatment failure ………………… 47 4.15 Results of environmental sampling ………………………………… 48 4.16 Conclusions of environmental sampling ……………………………. 51 5. Communications ………………………………………………………….. 52 5.1 Communications with cases, their families and the public ………… 52 5.2 Communications to health professionals …………………………… 56 5.3 Communications to Government, politicians and public officials …. 57 6. General Discussion ………………………………………………………... 58 6.1 Introduction …………………………………….. ………………….. 58 6.2 The catchment ………………………………………………………. 58 6.3 Likely impact of sewage effluents on the lake ……………………… 59 6.4 General considerations: fate of oocysts in larger bodies of water such as Cwellyn …………………………………………………….. 61 6.5 Aspects of water treatment at Cwellyn …………………………….. 62 6.6 Oocyst monitoring …………………………………………………. 64 6.7 Infectivity of Cryptosporidium species …………………………… 66 6.8 Other factors in outbreak dynamics ……………………………….. 66 7. Lessons and recommendations ……………………..……………………. 70 7.1 Observations and lessons learned ………………………………... 70 7.2 Recommendations …………………………………………………. 72 8. References ………………………………………………………………... 73 Version: 6 Date: 28th Status: Approved for publication November 2006 Author: Chris Lines, NPHS Wales Page: 3 of 184 Outbreak Control Team Report on Outbreak of Cryptosporidiosis in North West Wales, 2005 Appendix A – Membership of outbreak control team …………………………. 78 Appendix B – Outbreak control team meeting minutes ……………………….. 80 Appendix C – Selection of press briefings …………………………………….. 117 Appendix D – Model questions and answers: Briefing for those dealing with enquiries from the public ……………………………………………………….. 134 Appendix E – Questionnaire for interviews of cases …………………………… 139 Appendix F – The Water Supply (Water Quality) Regulations 2001 ………….. 148 Appendix G – Correspondence …………………………………………………. 153 Letter from Drinking Water Inspectorate to water suppliers advising revision of risk assessments …………………………………………………….. 153 Letter to Dwr Cymru from Water Consumer Council for Wales ……………….. 158 Letter to GPs suggesting investigation of diarrhoeal illness …………………….. 162 Letter to clinicians caring for patients with severely impaired immune systems…. 163 Letter to Oncology Unit, Ysbyty Gwynedd ………………………………………. 165 Letter to patients with severely impaired immune systems ………………………. 167 Letter to water customers advising boiling of water, etc …………………………. 169 Letter to GP’s ………………………………………………………………………170 Letter to Welsh Assembly Government from Drinking Water Inspectorate ……… 172 Appendix H – Environment Agency discharge consent for Rhyd Ddu sewage treatment plant ……………………………………………………………………. 176 Appendix I – Map of Cwellyn water treatment catchment ……………………… 184 Tables 1. Attack rates by water source ………………………………………… 32 2. Univariate analysis of risk factors …………………………………... 34 3. Stratified analysis of drinking unboiled tap water at home by age group ……………………………………………….. 35 4. Dose response number of glasses of unboiled tap water at home ….. 35 5. Summary of the monitoring results from the Filta-MaxTM sampling... 42 6. Cryptosporidium oocysts detected at frequently sampled sites……… 48 Version: 6 Date: 28th Status: Approved for publication November 2006 Author: Chris Lines, NPHS Wales Page: 4 of 184 Outbreak Control Team Report on Outbreak of Cryptosporidiosis in North West Wales, 2005 7. Cryptosporidium oocysts detected at occasionally sampled sites …… 49 8. Samples in which Cryptosporidium hominis was detected…………... 50 Figures 1. Dates of notification for confirmed cases Cryptosporidium hominis …. 30 2. Age distribution of confirmed cases Cryptosporidium hominis ………. 31 3. Dates of onset for confirmed cases Cryptosporidium hominis ………... 32 4. Calls answered by the Helpline ……………………………………….. 54 5. Cryptosporidium in water: dynamics of outbreaks …………………… 69 Version: 6 Date: 28th Status: Approved for publication November 2006 Author: Chris Lines, NPHS Wales Page: 5 of 184 Outbreak Control Team Report on Outbreak of Cryptosporidiosis in North West Wales, 2005 Summary The outbreak An outbreak of cryptosporidiosis occurred in North West Wales in the last quarter of 2005. The number of confirmed cases associated with the outbreak was 231. Cryptosporidiosis is a diarrhoeal illness caused by a parasite called cryptosporidium. Symptoms may not appear until five to ten days after becoming infected. Several lines of investigation produced evidence that the outbreak was waterborne and linked to the mains water supply from Llyn Cwellyn: • A high proportion of cases were resident in the area supplied with water from Llyn Cwellyn; • A significant number of people falling ill were adults indicating that the cause of the outbreak was a common, low level, community source (the contamination was an unusual event); • The dates on which people developed symptoms of illness pointed to the cause being a continuing, low level, common source, not a single point source in time; • People with cryptosporidiosis were more likely than people who were not ill to have drunk unboiled mains tap water; the more mains tap water drunk, the greater the probability of being infected; • Microbiological investigations demonstrated the same strain of Cryptosporidium hominis in pre- and post-treatment water samples as had been identified in cases; No other plausible hypotheses have been identified that would explain the findings. Environmental investigations, confirmed by microbiological testing, identified several routes by which Cryptosporidium hominis from sewage treatment systems in the Cwellyn catchment could have entered the reservoir. Sewage effluent, even after standard treatment, cannot be guaranteed to be free from cryptosporidium. It would seem likely that one or more people in the catchment area of Llyn Cwellyn had cryptosporidiosis before the main outbreak. Although they would have been ill with diarrhoea, they would probably not have known that this was due to Cryptosporidium hominis. There are no recorded cases of cryptosporidiosis in the area at the time so either people did not submit samples to the laboratory or they may have been visitors to the area. Cryptosporidium hominis entered Llyn Cwellyn through one or more of the routes identified from sewage treatment systems in the catchment area. There were no effective barriers to prevent it then reaching the mains water supply. The Cwellyn water treatment works appears to have been operated by Dwr Cymru in compliance with regulatory requirements and its risk assessment. However, the water treatment process in place at Llyn Cwellyn at the time was not designed to remove or kill Version: 6 Date: 28th Status: Approved for publication November 2006 Author: Chris Lines, NPHS Wales Page: 6 of 184 Outbreak Control Team Report on Outbreak of Cryptosporidiosis in North West Wales, 2005 cryptosporidium. The risk assessment had assumed that sewage effluent would be diluted significantly in the lake, but this may not be the case under certain conditions. Examination of water quality data