Milking routine: improper hygienic practices during the milking may facilitate the introduction of bacteria from the environment into the milk. In a study carried out by Vilar et al., 2010 in the same region of the present study, Listeria species were found in 32.2% of silage and faeces samples and in 16.3% of bulk tank milk samples, confirming the importance of environmental sources of Listeria in dairy cattle farms.