Introduction 1 2 Cryptosporidium C. hominis C. parvum 3 C. parvum C. hominis 2 4 5 C. hominis C. parvum Background C. hominis C. parvum 6 7 C. parvum 6 2 C. hominis 8 9 C. hominis 10 2 Cryptosporidium 11 12 13 Cryptosporidium 14 Methods Cryptosporidium Cryptosporidium 15 However, the main reason for the removal of cases were records which, had incomplete postcodes (59%) meaning that it was impossible to identify an accurate residential location. It is important to consider whether this omission may be correlated with any of our independent variables and hence introduce bias into the results. One source of missing postcodes is transcription omissions at individual laboratories but because these are likely to affect samples in a random manner they are unlikely to be correlated with any independent variables. The second major source is omissions by general practitioners due to them having inaccurate patient records or failing to include postcodes when submitting samples to laboratories for analysis. There is no evidence as to whether this varies systematically in a manner, which would bias the results of this research. 16 2 2  2 17 18 19 20 Cryptosporidium 2 21 22 C. parvum C. hominis A range of socioeconomic variables were obtained by identifying the 2001 Output Area within which each postcode was located. Output areas are the smallest area for which census data are released and contain approximately 125 individuals. Within each Output Area the percentage of people in each of the eight socio-economic status bands were identified. These ranged from higher managerial and professional occupations to those in routine occupations. Additionally, the proportion of the population aged 0–4 years and the percentage employed in agriculture was obtained. Finally a measure of health care accessibility was derived by calculating the travel time from each postcode to the nearest GP. Cryptosporidium Cryptosporidium C. hominis C. parvum Cryptosporidium Results Cryptosporidium 1 Cryptosporidium P P P P P P Table 1 Cryptosporidium Variable OR 95% CI P Cryptosporidium 1.084 1.012–1.163 0.022 Standardised proportion of population in ages 0–4 1.145 1.090–1.203 <0.001 a 1.203 1.140–1.270 <0.001 b 0.770 0.679–0.874 <0.001  Standardised proportion of water supply that is groundwater 0.821 0.729–0.925 0.001 Standardised proportion of water supply from non superiorly treated surface water 0.869 0.772–0.977 0.019  R 2 a b C. parvum C. hominis C. parvum 2 P Cryptosporidium P P P P P Cryptosporidium P Cryptosporidium P Table 2 Cryptosporidium parvu m Variable OR 95% CI P Standardised urban area 0.852 0.779–0.932 <0.001 Cryptosporidium 1.167 1.047–1.230 0.005 Standardised proportion of population in ages 0–4 1.094  1.015–1.179 0.018 a 1.109  1.025–1.200 0.010  b 0.738  0.646–0.842 <0.001  Standardised proportion of water supply that is groundwater 0.679  0.554–0.833 <0.001 C. parvum 1.289  1.088–1.527 <0.001 C. parvum 0.846  0.746–0.959 0.009  R 2 a b C. hominis 3 P P P Table 3 Cryptosporidium hominis Variables OR 95% CI P Standardised urban area 1.261 1.154–1.378 <0.001 a 1.297 1.201–1.401 <0.001 Standardised proportion of population in ages 0–4 1.190 1.112–1.274 <0.001 R 2 a Discussion Cryptosporidium Cryptosporidium Cryptosporidium C. hominis C. parvum Cryptosporidium C. parvum C. hominis Cryptosporidium C. hominis C. parvum C. parvum 9 8 C. hominis 4 2 9 10 2 Cryptosporidium C. parvum C. hominis C. hominis C. parvum C. parvum C. hominis 2 C. hominis C. parvum C. hominis C. parvum C. hominis C. parvum P C. parvum C. hominis P C. hominis Cryptosporidium C. parvum C. hominis C. parvum C. hominis C. parvum C. parvum Cryptosporidium Cryptosporidium Cryptosporidium 19 Cryptosporidium Cryptosporidium 23 23 C. parvum Cryptosporidium Cryptosporidium C. parvum Cryptosporidium 23 24 C. parvum Cryptosporidium C. parvum C. parvum C. hominis 25 Cryptosporidium C. parvum C. hominis C. parvum C. hominis C. parvum C. hominis In this study no associations were found between cryptosporidiosis and the proxy variable for biofilm build-up, accessibility to healthcare or the proportion of the population employed in agriculture. This suggests that these variables are not of major importance to cryptosporidiosis aetiology or that our methodology was unable to create effective measures of them. Cryptosporidium C. hominis C. parvum C. hominis C. parvum Cryptosporidium C. hominis Cryptosporidium C. parvum C. parvum C. hominis Cryptosporidium C. parvum C. parvum C. parvum