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Open Access Open Badges Case Study

A case of bovine raw milk contamination with Listeria monocytogenes

Karen Hunt13, Niall Drummond2, Mary Murphy2, Francis Butler3, Jim Buckley2 and Kieran Jordan1*

Author Affiliations

1 Teagasc Food Research Centre, Moorepark, Fermoy, Co. Cork, Ireland

2 Cork County Council Veterinary Food Safety Laboratories, Iniscarra, Co. Cork, Ireland

3 Veterinary Sciences Centre, School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Belfield, Dublin

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Irish Veterinary Journal 2012, 65:13  doi:10.1186/2046-0481-65-13

Published: 6 July 2012


During routine sampling of bulk raw milk on a dairy farm, the pathogenic bacteria Listeria monocytogenes was found to be a contaminant, at numbers < 100 cfu/ml. A strain with an indistinguishable pulsed-field gel electrophoresis pattern was isolated from the bulk milk two months later. Environmental swabs taken at the dairy environment were negative for the presence of L. monocytogenes, indicating a possible case of excretion of the L. monocytogenes directly into the milk. Milk samples were collected from the individual cows and analysed, resulting in the identification of L. monocytogenes excretion (at 280 cfu/ml) from one of the 4 mammary quarters of one dairy cow out of 180. When the infected cow was isolated from the herd, no L. monocytogenes was detected from the remaining herd. The pulsed-field gel electrophoresis pattern of the strain from the individual cow was indistinguishable from that originally isolated from the bulk milk. The infected cow did not show any clinical signs of disease, nor did the appearance of the milk have any physical abnormalities. Antibiotic treatment of the infected mammary quarter was found to be ineffective. This study shows that there can be risks associated with direct contamination of raw milk with L. monocytogenes.

Listeria monocytogenes; Raw milk; Subclinical mastitis; Milk quality