Prevalence of Salmonella enterica, Listeria monocytogenes and E. coli Virulence Factors in Bulk Tank Milk and In-line Filters from US Dairies

Authors: Van Kessel, Jo Ann; Karns, Jeffrey; LOMBARD, JASON - US Department Of Agriculture (USDA)

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2011
Publication Date: 5/2/2011
Citation: Van Kessel, J.S., Karns, J.S., Lombard, J.E. 2011. Prevalence of Salmonella enterica, Listeria monocytogenes and E. coli Virulence Factors in Bulk Tank Milk and In-line Filters from US Dairies. Foodborne Pathogens and Disease. 759-768.

Interpretive Summary: Human illnesses caused by the foodborne pathogenic bacteria Salmonella, Listeria, and E. coli have sometimes been associated with the consumption of raw (unpasteurized) milk or products made from it. In this study we cooperated with USDA:APHIS:VS to conduct a survey on the prevalence of the bacteria in raw, bulk tank milk and in inline milk filters from dairy farms across the United States. This study differed from previous surveys in that inline milk filters were examined. Inline filters are designed to filter the debris from the milk as it enters the bulk tank. Other studies have shown that inline milk filters are a more sensitive indicator or bacterial contamination of bulk tank milk than is the milk itself. Salmonella was detected in samples from 36.7% of the dairy farms surveyed while Listeria monocytogenes was found in samples from 8.7% of the dairy farms surveyed. Genes associated with pathogenic forms of E. coli were detected by real time PCR in samples from 58.1% of the dairy farms but the signals were generally very low indicating that potentially pathogenic forms did not constitute a large portion of the coliform population in the samples. These data confirm those from earlier studies that showed significant contamination of bulk tank milk by zoonotic bacterial pathogens and reinforce the fact that consumption of raw milk and raw milk products presents a health risk.

Technical Abstract: The zoonotic bacterial pathogens Salmonella enterica, Listeria monocytogenes, and Escherichia coli are known to infect dairy cows while not always causing overt clinical signs of disease. These pathogens are sometimes found in raw milk and outbreaks of human disease due to these organisms have occasionally been associated with the consumption of raw milk or products made from raw milk. Samples of bulk tank milk (BTM) and in-line milk filters collected from dairy farms across the US during the NAHMS 2007 Dairy Survey were analyzed by real-time PCR for the presence of Salmonella enterica and pathogenic forms of E. coli and by culture techniques for the presence of L. monocytogenes. Salmonella enterica was detected in samples from 36.7% of the dairy operations, primarily in the milk filters. Salmonella was successfully cultured from 36 of 77 PCR-positive BTM samples and 104 of 177 PCR-positive filter samples and serotyped. Cerro, Kentucky, Muenster, Anatum, and Newport were the most common serotypes. Listeria monocytogenes was isolated from samples from 8.7% of the dairy operations with 1/2a complex the most common serotype followed by 1/2b and lineage 1 4b. Shigatoxin genes were detected in enrichments from 15.3% of the BTM samples and from 58.1% of the milk filters by real time PCR. In most cases the cycle threshold values for the PCR indicated that toxigenic strains were not a major part of the population in the enrichments. These data confirm those from earlier studies that showed significant contamination of BTM by zoonotic bacterial pathogens and reinforce the fact that consumption of raw milk and raw milk products presents a health risk.