A field survey of Escherichia coli O157 ecology on a cattle farm in Italy.

A field survey was performed in a heifer raising operation in Northern Italy to study the introduction, maintenance and dissemination of Escherichia coli O157 in the herd and to identify possible control measures at the farm level. Rectal swabs from two different groups of animals (surveys 1 and 2) were tested for E. coli O157 by an immunomagnetic separation technique. In survey 1, a group of female calves (341 animals initially) introduced from 30 dairy herds during April 1996 to March 1997 were tested for E. coli O157 on arrival from the original herd when housed in individual hutches, 2-3 days after completion of weaning (which was associated with grouping) and 2 months after weaning. No statistically significant difference between excretion rates (3.8%, 4.2%, 4.4%, respectively) was found. Calves from which E. coli O157 was isolated on arrival came from 6 of the 30 dairy herds. Strains isolated during survey 1 belonged to seven different pulsed field gel electrophoresis (PFGE) profiles. In survey 2, a group of young animals aged, at the beginning of the study, between 2 1/2 and 7 1/2 months (median = 124 days) was tested monthly for E. coli O157 for 11-15 months from May 1996 to July 1997. The group included 92 animals for 11 months and then gradually decreased to 59 animals. Overall, E. coli O157, belonging to six different PFGE profiles, were isolated from 138 (10.7%) of 1293 rectal swabs. Monthly excretion rates ranged from 2.7% to 23.7%, with summer peaks in both years. Fifty-nine (64.1%) of the 92 heifers were positive at least once: of these 59 animals, 22 (37.3%) were positive on only one occasion, 23 (39%) were positive on two occasions and 14 (23.7%) were positive on three or more occasions. From two heifers positive on 9 out of the 15 sampling visits, strains with the same PFGE profile were isolated, respectively, on seven and eight occasions while strains with only one band difference were isolated on the remaining occasions. E. coli O157 was also isolated from 6 of 16 samples of bedding, two of two samples of slurry and one of five samples from water troughs collected during survey 2.