sis. As all case-patients had been in contact with goats, cal- culating an OR for “any contact” was not possible. Goat contact was therefore stratified into high- and low-contact categories. Attendance on Bank Holiday Monday, eating ice cream, eating cotton candy (i.e., “candy floss”), any con- tact with cows or goats, and high goat contact were all associated with increased risk (p<0.10). All case-patients had eaten either cotton candy or ice cream. No link between the risk for illness and duration of contact with cows or goats was found. The main barn was the only area visited by all patients. Cotton candy was only available on Bank Holiday Monday, a special festival day on which visitors were also more likely to have contact with cows (OR 5.56, p=0.03 for those attending on May 31 compared with other days). For these reasons, Bank Holiday Monday was not an inde- pendent variable and so was excluded from the multivari- able analysis. The results of multivariable analysis for the other four variables are shown in Table 2. The association between illness and eating ice cream or cotton candy increased and remained significant; the magnitude of effect for cow and goat contact was similar to univariate analysis, although neither factor was statistically signifi- cant. To check whether Bank Holiday Monday was unique, the analysis was repeated for the 24 Bank Holiday Monday attenders only; results are shown in Table 3. No OR reached statistical significance, reflecting the smaller dataset; however, the magnitude of the independent effect of these variables (as evidenced by ORs) is similar to that for the whole study population, suggesting that the risks were similar on the Bank Holiday Monday to the whole study period. Discussion This outbreak is the largest caused by VTEC O157 in visitors to an open farm and the first case-control study of risk factors for infection on an open farm in the United Kingdom. Our study has demonstrated a strong association with eating either ice cream or cotton candy and an increase in risk associated with goat and cow contact. A case-control study among farm visitors in the United States in 2000 showed an association between E. coli O157 infection and contact with cattle, nail biting, and food pur- chase (12). Handwashing was protective in that study. Considering sources of bias, particularly in the selec- tion of controls or in gathering information, is important. Callers to a helpline are likely to differ in some ways from the other visitors, perhaps being better informed and more anxious. However, their ice cream or cotton candy eating habits are unlikely to differ. Information on known risk factors, such as handwashing and food history, may be sus- ceptible to “rumination bias,” that is, a tendency for those RESEARCH Table 2. Results of multivariable analysis of significant animal contact and food consumed and risk for illness (49 observations)a OR p value 95% CI Cows 7.19 0.07 0.86 to 59.81 Goat (high contact) 4.85 0.16 0.54 to 44.03 Ate ice cream 11.99 0.046 1.04 to 137.76 Ate cotton candy 51.90 0.008 2.77 to 970.67 aOR, odds ratio; CI, confidence interval. Boldface type indicates variables with increased OR statistically significant at 95% level. who have been ill to systematically bias the reporting of exposure. This bias would explain the apparent lack of pro- tective effect of handwashing. Patients may have been more likely to recall eating ice cream, as this was one of the foods widely reported in the media as a possible source of infection in the early stages of the investigation. However, the association with cotton candy was unexpect- ed, and there is no reason to think that patients were more likely to recall this than the controls. The association between illness produced by VTEC O157 and contact with cows and goats reflects previous experience of direct transmission to humans (4,5,7). Cattle are regarded as the most important reservoir for VTEC O157 (13). However, the strong association with cotton candy and ice cream merits further discussion. Ice cream was supplied by the same local manufacturer to 65 other outlets in North Wales. Cotton candy was manufactured on site on the May bank holiday by a vendor using a process repeated at different fairs throughout North Wales. Illness associated with the ice cream or cotton candy was not reported elsewhere. However, both foods appeared to be strongly associated with the risk for illness. Both are par- ticularly sticky, messy foods, and it is possible to envisage two mechanisms by which eating them makes the inges- tion of E. coli O157 more likely. First, after eating one of these foods, sticky hands may be more prone to pick up contaminated organic matter from the environment or directly from animal coats by stroking. Secondly, to clean sticky hands, small children are likely to lick their fingers. Our investigation reinforces existing advice (9,14) on handwashing, specifically that handwashing facilities should be positioned immediately adjacent to exit areas where animal contact is encouraged, and that one-way sys- tems and adequate supervision can facilitate effective handwashing. Advice concerning the importance of super- Table 3. Results of multivariable analysis of significant animal contact and food consumed for Bank Holiday attenders only and risk for illness (24 observations)a OR p value 95% CI Cows 13.65 0.14 0.42 to 438.71 Goat (high contact) 3.58 0.38 0.21 to 61.98 Ate ice cream 10.35 0.10 0.63 to 169.31 Ate cotton candy 38.44 0.07 0.71 to 2,092.60 aOR, odds ratio; CI, confidence interval. Emerging Infectious Diseases • Vol. 9, No. 5, May 2003 529