Rapid quantitative assessment of the distribution of Listeria in silage implicated in a suspected outbreak of listeriosis in calves

A silage clamp was sampled for the presence of Listeria species. Even though covered by a heavy duty black plastic sheet the top few centimetres of silage were heavily contaminated with Listeria species. In the areas nearest the edges of the sheet the pathogenic species Listeria monocytogenes was found in numbers in excess of 12,000 organisms/g silage. Using the methods of isolation described, pathogenic species of Listeria could be differentiated from non-pathogenic species in three to four days.