Quantitative risk assessment of Cryptosporidium in tap water in Ireland E. Cummins a, ⁎, R. Kennedy a , M. Cormican b a UCD School of Agriculture, Food Science and Veterinary Medicine, College of Life Sciences, Agriculture and Food Science Centre, Belfield, Dublin 4, Ireland b Department of Bacteriology, National University of Ireland, Galway, Ireland abstract article info Article history: Received 20 May 2009 Received in revised form 2 November 2009 Accepted 6 November 2009 Available online 27 November 2009 Keywords: Cryptosporidium Risk assessment Water Cryptosporidium species are protozoan parasites associated with gastro-intestinal illness. Following a number of high profile outbreaks worldwide, it has emerged as a parasite of major public health concern. A quantitative Monte Carlo simulation model was developed to evaluate the annual risk of infection from Cryptosporidium in tap water in Ireland. The assessment considers the potential initial contamination levels in raw water, oocyst removal and decontamination events following various process stages, including coagulation/flocculation, sedimentation, filtration and disinfection. A number of scenarios were analysed to represent potential risks from public water supplies, group water schemes and private wells. Where surface water is used additional physical and chemical water treatment is important in terms of reducing the risk to consumers. The simulated annual risk of illness for immunocompetent individuals was below 1 × 10 -4 per year (as set by the US EPA) except under extreme contamination events. The risk for immunocompromised individuals was 2–3 orders of magnitude greater for the scenarios analysed. The model indicates a reduced risk of infection from tap water that has undergone microfiltration, as this treatment is more robust in the event of high contamination loads. The sensitivity analysis highlighted the importance of watershed protection and the importance of adequate coagulation/flocculation in conventional treatment. The frequency of failure of the treatment process is the most important parameter influencing human risk in conventional treatment. The model developed in this study may be useful for local authorities, government agencies and other stakeholders to evaluate the likely risk of infection given some basic input data on source water and treatment processes used. © 2009 Elsevier B.V. All rights reserved. 1. Introduction One of the Millennium Development Goals is to halve the number of people without safe drinking water by 2015 (WHO, 2009; Anon, 2009). In many of the least developed countries the infrastructure to provide safe drinking water is fragmentary or absent and there are high endemic levels of waterborne disease. In more developed countries systems and infrastructure may be in place and generally effective, however there is very significant burden of waterborne infection related to catchment management (Coffey et al., 2010) and systems failures resulting in outbreaks (Coffey et al., 2007). Cryptosporidium is a protozoan parasite with spherical oocysts that are 3–7 μm in diameter. The species that comprise the genus are morphologically indistinguishable; however they are genetically distinct and differ in host range and potential to cause human infection. Cryptosporidium hominis is a human pathogen whereas Cryptosporidium parvum has a broad host range that includes humans and a number of domestic and wild animals. Exposure to Cryptos- poridium is by ingestion of the oocysts (in water, food or by direct contact) and infection may follow. The principle clinical feature is diarrhoea which persists for up 2 weeks in immunocompetent people and may persist indefinitely and is associated with significant mortality in those with impaired immune function. In patients with acute infection faeces may contain up to 1 × 10 7 oocysts per gram (Chappell et al., 1999; Angus, 1987). Outbreaks of infection with Cryptosporidium species involving from hundreds to many thousands of people have been reported in many developed countries including the United States, United Kingdom and recently in Ireland. A number of outbreaks in the republic of Ireland (Jennings and Rhatigan, 2002a,b; O'Toole et al., 2004) and Northern Ireland (Glaberman et al., 2002; Smyth, 2001) served to heighten concerns regarding cryptosporidiosis. In 2004, under the Infectious Diseases (Amendment) (No 3) Regulations 2003 (S.I. 707 of 2003), cryptosporidiosis became a notifiable disease in Ireland in all age groups (Garvey and McKeown, 2004). This has resulted in better monitoring of cryptosporidiosis. Multiple treatment processes are used in most surface water treatment plants in order to prevent exposure to waterborne pathogens. Many researchers have studied the removal of Cryptos- poridium by water treatment processes (Plummer et al., 1995; Hamann et al., 1990; Edzwald and Kelley, 1998; McTigue, 1998). Oocysts are not totally removed by conventional water treatment Science of the Total Environment 408 (2010) 740–753 ⁎ Corresponding author. Tel.: + 353 1 7167476. E-mail address: Enda.Cummins@ucd.ie (E. Cummins). 0048-9697/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2009.11.008 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv