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 to analyse over time the evolution of <em>E. coli</em> O157:H7 faecal shedding in a dairy herd producing raw milk for direct human consumption. The study was performed between October 2012 and September 2013 in an average size Italian dairy farm where animals are housed inside the barn all over 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 analysed by real time polymerase chain reaction (RT-PCR) and culture. Overall, 16 (5.6%) faecal samples were positive for E. coli O157 by RT-PCR. Cultural examination found 9 (3.1%) samples positive for <em>E. coli</em> O157; all the isolates were positive for stx1, stx 2 and eae genes. One (4.1%) feed sample was positive for <em>E. coli</em> O157 by RT-PCR; none of the water samples was positive for <em>E. coli</em> 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 showed that in an Italian dairy farm housing animals all year, faecal shedding of <em>E. coli</em> O157 followed the same temporal trend reported for other types of farming. The enhanced faecal shedding during warmer months may have a significant impact on environmental contamination and the safety of raw milk and its byproducts.