Sporadic Cr yptosporidiosis, Nor th Cumbria, England, 1996–2000 Stella Goh,* Mark Reacher,† David P. Casemore,‡ Neville Q. Verlander,† Rachel Chalmers,§ Margaret Knowles,¶ Joy Williams,* Keith Osborn,# and Sarah Richards** Risk factors for sporadic cryptosporidiosis were deter- mined in 152 patients and 466 unmatched controls who resided in two local government districts in North Cumbria, North West England, from March 1, 1996, to February 29, 2000. Risk was associated with the usual daily volume of cold unboiled tap water drunk (odds ratio [OR] 1.40, 95% confidence intervals [CI] 1.14 to 1.71 per pint consumed per day [p = 0.001]) and short visits to farms (OR 2.02, 95% CI 1.04 to 3.90, p = 0.04). Fifty-six (84%) of 67 fecal spec- imens from patients obtained from January 1, 1998, and February 29, 2000, were Cryptosporidium parvum geno- type 2 (animal and human strain). Livestock fecal pollution of water sources appears to be the leading cause of human sporadic cryptosporidiosis in this population and shows the need for better protection of water catchments from live- stock and improved drinking water treatment in this area of England. Tleading cause of infectious diarrhea in humans and he protozoan parasite Cryptosporidium parvum is a livestock with fecal-oral transmission by ingestion of oocysts (1,2). Infection is generally self-limiting, followed by variable protective immunity involving humoral and cell-mediated responses, except in the immune-sup- pressed, when infection may be prolonged and fatal (2,3). Cryptosporidium oocysts remain viable in water and damp soils for prolonged periods and are resistant to disinfec- tants at concentrations usually used in water treatment (4,5). Although sound, conventional water treatment is believed to substantially reduce the risk of viable oocysts passing into treated water, the possibility of low-level intermittent contamination has been recognized; whether such contamination affects public health is uncertain (4,6). Outbreak investigations have shown diverse modes of transmission, including contact with livestock (7,8); per- *Carlisle and District Primary Care Trust, Rosehill, Carlisle, United Kingdom; †Communicable Disease Surveillance Centre, London, United Kingdom; ‡University of Wales, Aberystwyth, Ceredigion, Wales, United Kingdom; §Singleton Hospital, Swansea, Wales, United Kingdom; ¶Cumberland Infirmary, Carlisle, United Kingdom; #United Utilities, Great Sankey, Warrington, United Kingdom; and **West Cumberland Hospital, Whitehaven, United Kingdom son-to-person transmission in households and care settings (9); consumption of contaminated foods and drinks, including milk (1,10); water from private supplies (11); and recreational water exposure (12). Infection may also be associated with travel to countries with higher incidence of cryptosporidiosis (13). In 1992, a community outbreak of cryptosporidiosis occurred in residents of Allerdale and Copeland local gov- ernment districts in North Cumbria, North West England, which compose part of the Lake District National Park; these areas have a predominantly agricultural and tourism- based economy and a population of approximately 160,000. The lakes have livestock farms and open grazing land abutting them. Approximately one third of the popu- lation received public water supplies from Ennerdale Lake, one third from Crummock Lake, and one third from a number of smaller sources. Water from Ennerdale and Crummock Lakes was disinfected with chlorine but unfil- tered because the low level of particulate matter in these sources precluded chemically assisted flocculation. The smaller sources used for public supplies received a variety of conventional treatments, including coagulation, filtra- tion, and chlorination, and chlorination alone. A small number of households had private supplies. A case-control study conducted during the outbreak showed a significant association between cryptosporidiosis and consuming cold unboiled mains tap water for persons served by water from Ennerdale Lake but no such association for those served by water from Crummock Lake and other water sources (North Cumbria Health Authority, Carlisle, 1992, unpub. data). After the outbreak, rates of laboratory-confirmed cryptosporidiosis from 1993 to 1995 were 31.2–44.2 per 100,000 in Allerdale and Copeland compared to 19.8 to 23.9 per 100,000 in the neighboring local government dis- tricts of Carlisle and Eden. Cases were not obviously clus- tered in time and could not be linked. A prospective case-control study was therefore undertaken to test the hypothesis that no dose-response relationship existed between consuming unboiled tap water from public water supplies and risk for sporadic Cryptosporidium infection in Allerdale and Copeland. Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 6, June 2004 1007