Figure. Case-patients recruited to the study by month of onset and water supply zone. http://www.cdc.gov/ncidod/EID/vol10no6/03-0325_ app.htm). No association was found between consuming bottled water, ice, soft drinks, and pasteurized or unpas- teurized milk. Consuming lettuce, tomatoes, mixed salad, and cream was associated with lower risk (p < 0.05). Any contact with a farm was associated with a twofold increase in risk (OR 2.11, CI 1.4 to 3.2, p < 0.001). Risk was higher for short farm visits (OR 2.56, CI 1.57 to 4.17) and increased with the frequency of farm visits (χ2 test for trend p = 0.003) (online Appendix). Risk was also increased by contact with farm animals (OR 2.23, CI 1.45 to 3.43), eating food within 2 hours of contact with farm animals (OR 3.11, CI 1.79 to 5.38), and stroking farm ani- mals (OR 2.01, CI 1.17 to 3.42). Walking near slurry applied to fields was not associated with increased risk, but contact with slurry showed some evidence of increased risk (OR 2.0, CI 0.99 to 4.02). Contact with pets at home or contact with pets with diarrhea did not increase risk. Risk was increased for feed- ing pets leftovers (OR 3.79, CI 1.0 to 14.69), with margin- al evidence of risk for feeding pets raw vegetables (OR 2.09, CI 0.95 to 4.56) and biscuits (OR 1.76, CI 0.95 to 3.21). Contact with animals other than farm animals and home pets was not associated with infection (OR 0.84, CI 0.54 to 1.30). Risk was increased by having accidentally touched feces from any animal (OR 3.04, CI 1.33 to 6.94). Attendance at a playgroup or nursery and recreational exposure to water was not associated with infection (online Appendix). Multivariable Analysis The usual volume of cold unboiled tap water consumed was independently associated with cryptosporidiosis (OR 1.40, CI 1.14 to 1.71 per pint consumed per day) in the final multivariable model (Table 4). A short visit to farms (OR 2.02, CI 1.04 to 3.9) was also significant. No differ- ence in risk was found between the different water supply zones, irrespective of whether the zones received unfil- tered water from Crummock and Ennerdale Lakes, other public supplies with a variety of conventionally filtered Sporadic Cryptosporidiosis, England, 1996–2000 and unfiltered water, or private water supplies (Tables 3 and 4). Slight evidence was found for increased risk for feeding pets raw vegetables (OR 2.11, CI 0.98 to 4.56) and biscuits (OR 1.77, CI 0.94 to 3.35) but not for age, sex, consuming nonlocally produced cheese, contact with farms without cattle or sheep, or with cattle (Table 4). Subsidiary analysis showed that usual volume of cold unboiled tap water consumed at home was also a signifi- cant risk factor. A further analysis that excluded persons whose house was served by a mixed public water supply found similar results (not shown). Genotyping Results Genotyping was undertaken in 67 of 101 cases from 1998 to 2000. All the smears were confirmed positive, and 56 (83.6%) of tested smears were C. parvum genotype 2 (animal and human) strain, 1 (1.5%) was genotype 1 (human strain), and 10 (14.9%) could not be typed. The primary care patient register was reviewed after patients were recruited for the study. The register con- tained 166,376 names of Allerdale and Copeland residents compared to a population of 162,809 enumerated at the 2001 census (available from www.statistics.gov.uk/cen- sus2001). The computer algorithm used to randomly select potential controls generated 125 tables of registered patients’ names, where registered patients were within the same age category as study case-patients and had the same lead characters of the postal code of residence as study case-patients. The tables contained a median of 496 (range 9–7,800) names. Discussion Drinking cold unboiled tap water from public drinking water supplies was a highly significant risk factor for spo- radic human cryptosporidiosis, regardless of the water source. To our knowledge, this study is the first to show that drinking from public water supplies is an important risk factor for sporadic human Cryptosporidium infection. Most cases were in children, consistent with previous reports from England and Canada (21,22). Many patients required admission to hospital, showing the seriousness of illness. Infection in study patients was also associated with short visits to farms and predominantly with the C. parvum Table 2. Estimated average annual incidence of primary Cryptosporidium cases by water source, 1996–1999a Incidence Water source Population (per 100,000 per y) Crummock 58,295 24 Ennerdale 47,780 22 Crummock and 106,075 23.1 Ennerdale All other sources 59,699 22.6 aPopulation denominators from water company records. Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 6, June 2004 1011