Temporal variation of faecal shedding of Escherichia coli O157:H7 in a dairy herd producing raw milk for direct human consumption.

The objective of this study was a longitudinal analysis of the evolution over time of E. coli O157:H7 faecal shedding in a dairy herd selling raw milk directly to the consumer. The study was performed between October 2012 and September 2013 on a average size Italian dairy farm in which animals are housed inside the barn for all the year. The farm housed about 140 animals during the study, 70 cows and 70 calves and heifers. Twenty-six animals were randomly selected from both the cows and young animals group, and faecal sampling was performed rectally six times two months apart in each animal. Eleven animals were culled during the study and a total of 285 faecal samples were collected. At each faecal sampling, three trough water samples and two trough feed samples were also collected for a total of 36 water samples and 24 feed samples. Samples were analyzed by real time PCR and culture. Overall 16 (5.6%) faecal samples were positive for E. coli O157 by real time PCR. Cultural examination found nine (3.1%) samples positive for E. coli O157; all the isolates were positive for stx1, stx 2 and eae genes. One (4.1%) feed sample was positive for E. coli O157 by real time PCR; none of the water samples was positive for E. coli O157. The model highlighted a general significant reduction of the number of positive samples observed during the study from the first to the sixth sampling (p = 0.000) and a positive relation between the presence of positive samples and average environmental temperature (p = 0.003). The results of the study show that on an Italian dairy farm housing animals all year, faecal shedding of E. coli O157 followed the same temporal trend reported for other types of farming. The enhanced faecal shedding during the warmer months may have a significant impact on environmental contamination and the safety of food produced by dairy cows, in particular raw milk.