RESEARCH Results Overview of Cases A total of 17 primary cases (1 adult, 16 children) and 7 secondary cases in household contacts (2 adults, 5 chil- dren) were ascertained. Ten patients (1 adult, 9 children) required hospital admission, including 3 children with hemolytic uremic syndrome. Ten of the primary case- patients had visited the farm on May 31, Bank Holiday Monday; the remaining seven had visited during the fol- lowing 15 days. No patient had visited before May 31. Isolates from all the cases except one, where the sample was not submitted for typing, were characterized as phage type (PT) 2, verocytotoxin type (VT) 2, and were resistant to streptomycin, sulphonamides, and tetracycline (SSuT). Secondary transmission was not evident in any of the schools or nurseries screened. Microbiology Eleven of 46 (mainly floor) fecal samples taken by vet- erinarians were positive for E. coli O157. Ten strains were PT2, VT2, SSuT, and one strain was PT4, VT2, and sensi- tive to antimicrobial agents. These 11 strains were from pens or paddocks containing calves, goats, pigs, sheep, and a pony. Rabbit, fowl, and donkey samples were negative. The results of PFGE showed that the human and animal strains of E. coli O157 PT2 VT2 were indistinguishable. Case-Control Study Sixteen children met the case definition. Questionnaires were completed for 13 of these 16 case-patients and 36 controls. Controls had a mean age of 4.5 years (SD 2.7) compared with the patients’ mean age of 5.4 years (SD 3.1); this difference was not significant (p=0.35). The pro- portion of girls was similar in patients (55.6%) and con- trols (61.5%). Table 1 shows results of univariable analy- Table 1. Results of univariable analysis of risks for illness caused by Escherichia coli O157 in visitors to an open farm, Wales Cases Controls Univariable analysis Exposure Exposed/ not exposed Exposed/ not exposed Odds ratio p value 95% CI Contact with animals Cows 11/2 16/20 6.88 0.01 1.15 to 52.69 Goats (any contact) 13/0 27/9 Undef 0.09 Undef Goat (high contact) 10/3 17/19 3.72 0.06 0.75 to 20.70 Rabbit 9/4 20/16 1.80 0.39 0.40 to 8.62 Sheep 10/3 27/9 1.11 1.00 0.21 to 6.48 Pigs 5/7 11/24 1.56 0.72 0.33 to 7.32 Pony 4/9 20/16 0.36 0.12 0.07 to 1.61 Shire horse 4/9 13/23 0.79 1.00 0.16 to 3.64 Areas of farm visited Play area 5/7 27/9 0.24 0.07 0.05 to 1.13 Paddock 6/7 28/8 0.24 0.08 0.05 to 1.13 Pony ride 4/9 15/21 0.62 0.49 0.13 to 2.84 Tractor ride 9/4 28/8 0.64 0.71 0.13 to 3.31 Main barn 13/0 35/1 Undef 1.00 Undef Food consumption and personal behaviors Sucks thumb 1/11 6/30 0.45 0.66 0.02 to 4.74 Bites nails 2/10 6/30 1.00 1.00 0.12 to 7.08 Ate any food while on farm 13/0 33/3 Undef 0.56 Undef Ate immediately after barn 6/6 11/23 2.09 0.31 0.45 to 9.81 Ate food bought on farm 8/5 14/21 2.40 0.18 0.55 to 10.85 Ate ice cream 9/4 14/22 3.54 0.06 0.77 to 17.19 Ate cotton candy 7/6 4/32 9.33 0.004 1.69 to 57.10 Bought animal feed 10/3 32/4 0.42 0.36 0.06 to 2.88 Picked up animal feed from floor 5/8 6/30 3.13 0.13 0.61 to 16.29 Ate animal feed 1/12 0/36 Undef 0.26 Undef Clung to animal barriers 6/3 25/7 0.56 0.66 0.09 to 3.76 Fell over while on farm 1/12 7/29 0.35 0.66 0.01 to 3.43 Washed hands at all 10/3 24/9 1.12 1.00 0.23 to 7.35 Environmental observations Wet underfoot 3/10 6/30 1.50 0.68 0.24 to 8.85 Dirty hands, shoes, or clothes 0/13 9/27 0.00 0.09 0.00 to 1.46 Noticed queue for toilets 2/10 3/32 2.13 0.59 0.21 to 19.56 Type of visit Family visit 12/1 33/3 0.92 1.00 0.08 to 30.07 Bank Holiday Monday 11/2 13/20 9.73 0.003 1.38 to 66.32 aCI, confidence interval; undef, undefined (when one of the cells contains a zero, defining a confidence interval is not possible). Bold typeface highlights variables with increased odds ratio statistically significant at 90% level. 528 Emerging Infectious Diseases • Vol. 9, No. 5, May 2003