The relative
ratio of the proportion of PT32:proportion of PT21/28 in cattle to the proportion of PT32:proportion of PT21/28 in humans is 2.92 and 10.96 for the SEERAD and IPRAVE surveys respectively, confirming that relative to PT21/28, PT32 is more common in cattle than human cases of E. coli O157. Overall there was a statistically significant difference in the distribution of these PTs between human cases and bovine isolates over the 2 time scales (CMH: 71.07 P < 0.001). There was no significant change in PT21/28, PT32 or 'Other' PTs for humans cases (exact χ2 = 3.73, P = 0.158) whereas there were significant changes across time for bovine isolates (exact χ2 = 12.24, P = 0.002). Figure 3 Distribution of Phage types. Proportion of Phage type (PT) 21/28, PT32 and 'Other' PTs in cattle isolates ICG-001 manufacturer and in culture positive, indigenous selleck inhibitor human E. coli O157 cases with known phage type results reported to HPS, over the time periods equivalent to the SEERAD (March 1998 – May 2000) and IPRAVE (February 2002 – February 2004) surveys. Discussion The surveys examined in this study represent the only reported systematic national surveys of bovine E. coli O157 shedding and present a valuable opportunity to examine changes in patterns of shedding and strain characteristics.
Knowledge of bovine shedding is important as cattle represent a major risk factor both for human E. coli O157 infection, whether from contamination of food or water by bovine faeces, or from direct contact with cattle or their environments, and for transmission to other animals. This is of particular concern in Scotland which has consistently higher rates of human E. coli O157 cases than the rest of the United Kingdom, and other European and North American countries [31–33].
In most instances it is difficult to compare results from different prevalence studies as different study designs, sampling procedures and microbiological methods have been used. The use of similar sampling SB-3CT and identical laboratory methods in the SEERAD and IPRAVE studies allowed direct comparison of E. coli O157 prevalence estimates. Estimates of the prevalence of E. coli O157 from the SEERAD study have been published, but in this study the estimates were recalculated to accommodate differences in sampling design and changes in statistical methodology. The farm-level and pat-level mean prevalence calculated for the SEERAD survey was 0.228 (95%CI: 0.196-0.263) and 0.079 (95%CI: 0.065-0.096) respectively [28]. In this study the same quantities were recalculated to be 0.218 (95%CI: 0.141-0.32) and 0.089 (95%CI: 0.075-0.105). These minor differences are the result of using different statistical models. Pat-level mean prevalence estimates for the IPRAVE study were generated using a bootstrapping technique given the clustered nature of data collection and the zero-inflated nature of the resulting data.