The occurrence of Campylobacter spp., Listeria monocytogenes and Shiga toxin-producing Escherichia coli in Norwegian dairy cattle farms; a comparison between free stall and tie stall housing systems.

Abstract Aims: This study explores how different dairy farm operating systems influence the occurrence of zoonotic bacteria in raw milk. Methods and Results: Samples from bulk tank milk, milk filters, feces, feed, teats and teat milk were collected from eleven farms with loose housing and seven with tie-stall housing every second month over a period of 11 months and analyzed for the presence of Campylobacter spp., L. monocytogenes and STEC. Campylobacter spp., L. monocytogenes and STEC were abundant in samples from the farm environment and were also detected in 4%, 13% and 7% of the milk filters, respectively, and in 3%, 0% and 1% of bulk tank milk samples. Four STEC isolates carried the eae gene, which is linked to the capacity to cause more severe human disease. Conclusion: The results indicate a higher prevalence of L. monocytogenes and Campylobacter spp. in samples collected from loose housed herds compared to tie-stalled herds suggesting that the operating system can influence the food safety of raw milk. Significance and Impact of the study: This study highlights that zoonotic bacteria can be present in raw milk independent of hygienic conditions at the farm and what hosing system is used. Altogether, this study provides an important knowledge base for evaluating the risk of drinking unpasteurized milk.