OCCURRENCE OF VEROCYTOTOXIN-PRODUCING ESCHERICHIA-COLI O157 ON DUTCH DAILY FARMS

During the period from September 1996 through November 1996, 10 Dutch dairy farms were visited to collect fecal samples from all cattle pres ent, The samples were examined for the presence of verocytotoxin (VT)p roducing Escherichia roll (VTEC) of serogroup 0157 (0157 VTEC) by immu nomagnetic separation following selective enrichment, Cattle on 7 of t he 10 dairy farms tested positive for 0157 VTEC, with the proportion o f cattle infected varying from 0.8 to 22.4%. On the seven farms positi ve for 0157 VTEC, the excretion rate was highest in calves ages 4 to 1 2 months (21.2%). In a follow-up study, two 0157 VTEC-positive farms a nd two 0157 VTEC-negative farms identified in the prevalence study wer e revisited five times at intervals of approximately 3 months, Cattle on each farm tested positive at least once. The proportion of cattle i nfected varied from 0 to 61.0%. Excretion rates peaked in summer and w ere lowest in winter. Again, the highest prevalence was observed in ca lves ages 3 to 12 months (11.8%). 0157 VTEC strains were also isolated from fecal samples from horses, ponies, and sheep and from milk filte rs and stable flies. 0157 VTEC isolates were characterized by VT produ ction and type, the presence of the E. coli attaching-and-effacing gen e, phage type, and pulsed-field gel electrophoretic genotype, No overl apping strain types were identified among isolates from different farm s except one, The predominance of a single type at each sampling sugge sts that horizontal transmission is an important factor in disseminati on of 0157 VTEC within a farm. The presence of more than one strain ty pe, both simultaneously and over time, suggests that there was more th an one source of 0157 VTEC on the farms. Furthermore, this study demon strated that the 0157 VTEC status of a farm cannot be ascertained from a single visit testing a small number of cattle.