Prevalence of Salmonella enterica, Listeria monocytogenes, and pathogenic E. coli in bulk tank milk and milk filters from U.S. dairy operations in 2014

Authors: Sonnier, Jakeitha - Jackie; KARNS, JEFFREY - US Department Of Agriculture (USDA); LOMBARD, JASON - Animal And Plant Health Inspection Service (APHIS); KOPRAL, CHRISTINE - Animal And Plant Health Inspection Service (APHIS); Haley, Bradd; KIM, SEON-WOO - US Department Of Agriculture (USDA); Van Kessel, Jo Ann

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2017
Publication Date: 12/21/2017
Citation: Sonnier, J.L., Karns, J.S., Lombard, J.E., Kopral, C.A., Haley, B.J., Kim, S., Van Kessel, J.S. 2018. Prevalence of Salmonella enterica, Listeria monocytogenes, and pathogenic Escherichia coli in bulk tank milk and milk and milk filters from U.S. dairy operations in the National Animal Health Monitoring System Dairy 2014 study. Journal of Dairy Science. 101:1943-1956. https://doi.org/10.3168/jds.2017-13546.
DOI: https://doi.org/10.3168/jds.2017-13546

Interpretive Summary: The dairy farm environment is a well-documented reservoir for zoonotic pathogens such as Salmonella enterica, Shiga-toxigenic Escherichia coli, and Listeria monocytogenes, and humans may be exposed to these pathogens via consumption of unpasteurized milk and dairy products. As part of the National Animal Health Monitoring System Dairy 2014 survey, bulk tank milk (BTM, n=234) and milk filters (n=254) were collected from a total of 234 dairy operations from 17 major dairy states and analyzed for the presence of these pathogens. Salmonella enterica spp. were isolated from in 18.0% of operations. There are more than 1000 serotypes (sub-groups) of Salmonella and some are more virulent (disease-causing) than others. Sixteen serotypes were isolated, and Cerro, Montevideo, and Newport were the most common. The isolates were tested for their sensitivity to 14 antibiotics that are important in human and animal medicine. Most of the isolates (86%) were sensitive to all 14 antibiotics but the remaining were resistant to 4-9 antibiotics. Resistance such as this can complicate treatment options for infected animals or humans. Listeria spp. were isolated from 19.9% of operations, and L. monocytogenes, the Listeria species that is of human health significance, was isolated from 3.0% of operations and the serogroups that were detected are those that have been identified in human outbreaks. One or more E. coli virulence factors were detected in the milk from 30.5% of operations and in the filters from 75.3% of operations. A combination of genes (stx2, eaeA, and '-tir) that are common in pathogenic E. coli was detected in the BTM from 0.5% of operations and in the filters from 6.6% of operations. The results of this study indicate a low but appreciable prevalence of bacterial pathogens in milk and filters at the farm level and support the concern regarding the risks of consumping raw milk or dairy products made with raw milk.

Technical Abstract: The dairy farm environment is a well-documented reservoir for zoonotic pathogens such as Salmonella enterica, Shiga-toxigenic Escherichia coli, and Listeria monocytogenes, and humans may be exposed to these pathogens via consumption of unpasteurized milk and dairy products. As part of the National Animal Health Monitoring System (NAHMS) Dairy 2014 survey, bulk tank milk (BTM, n=234) and milk filters (n=254) were collected from a total of 234 dairy operations from 17 major dairy states and analyzed for the presence of these pathogens. Salmonella enterica spp. were isolated from in 18.0% (Weighted Percentage,WP) of operations. Similarly, the invA gene was detected in 18.5% (WP) of operations, and of these, Salmonella serovar Dublin was detected in 4% (WP) of operations. Sixteen serotypes were isolated, and Cerro, Montevideo, and Newport were the most common. Antimicrobial susceptibility testing against a panel of 14 antibiotics was performed on 137 representative Salmonella isolates, of which 118 were pansusceptible; the remaining were resistant to 4-9 antimicrobials, and the most common profile was resistance to Ampicillin/clavulanic acid, ampicillin, cefoxitin, ceftiofur, ceftriaxone, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline. Listeria spp. were isolated from 19.9% (WP) of operations, and L. monocytogenes was isolated from 3.0% (WP) of operations. Serogroups 1/2a and 1/2b were the most common, followed by 4b and 4a. One or more E. coli virulence factors were detected in the BTM from 30.5% (WP) of operations and in the filters from 75.3% (WP) of operations. A combination of stx2, eaeA, and '-tir genes was detected in the BTM from 0.5% (WP) of operations and in the filters from 6.6% (WP) of operations. The results of this study indicate a low but appreciable prevalence of bacterial pathogens, which include serovars known to infect humans.