The incidence of Listeria monocytogenes in raw milk from farm bulk tanks in north-east Scotland

Over 180 farm bulk milk tanks were tested for the presence of Listeria monocytogenes on three separate occasions which included periods when cows were grazing and confined inside on a silage diet. The incidence of L. monocytogenes contamination was low, ranging from 3.8% in the summer samples to 1.0% in October. The level of contamination was estimated to be lower than one L. monocytogenes bacterium per ml in positive samples, as most required cold enrichment of 10-20 ml volumes before recovery. The distribution was sporadic; only one farm gave positive isolations on all three sampling occasions, one other on two, and all others were from different farms. No correlation between the presence of L. monocytogenes and hygiene standards or the feeding of silage was found.