Thirty-six dairy herds in Idaho, Oregon, and Washington were selected on the basis of cattle housing and manure-handling practices. Approximately 60 fecal samples from heifers were collected monthly in each herd for 6 months and cultured for Escherichia coli O157. One hundred seventy-nine of 12,664 (1.41%) individual fecal samples from 27 of the 36 herds (75%) were culture positive for E. coli O157. Within-herd prevalence ranged from 0% to 5.5% with a strong clustering toward the lower end of this range. A tendency was observed for herds to maintain either a relatively low or high prevalence of E. coli O157. Prevalence of E. coli O157 was similar in herds which housed heifers in dry lots and on pasture with and without application of manure. Also, application of manure to cattle forage crops was not associated with the prevalence of E. coli O157 in dairy herds.

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