Diversity of Listeria monocytogenes within a U.S. dairy herd, 2004-2010

Authors: Haley, Bradd; Sonnier, Jakeitha - Jackie; YNTE, H. SCHUKKEN - Cornell University; Karns, Jeffrey; Van Kessel, Jo Ann

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2015
Publication Date: 9/3/2015
Citation: Haley, B.J., Sonnier, J.L., Ynte, H., Karns, J.S., Van Kessel, J.S. 2015. Diversity of Listeria monocytogenes within a U.S. dairy herd, 2004-2010. Foodborne Pathogens and Disease. 12:844-850.

Interpretive Summary: Listeriosis in humans is generally caused by eating foods that are contaminated with the bacterium, Listeria monocytogenes. Generally, immune-compromised individuals are most susceptible and listeriosis is the third leading cause of death from food poisoning. Listeria are found in many environmental niches including dairy farms. Although listeriosis does occur in dairy cows, these animals usually harbor L. monocytogenes without clinical signs of disease. There is considerable known diversity within L. monocytogenes and the potential to cause disease in humans is variable. In order to better understand the relationship between strains found on dairy farms and those involved in human illness, more information in needed on the dairy farm-associated L. monocytogenes. The diversity of 43 L. monocytogenes isolates collected over a six-year study from the feces of dairy cattle on a single dairy farm was assessed using a multi-virulence locus sequence typing (MVLST) assay. This is an assay that allows us to compare the DNA between isolates at each of six locations on the genome that play a role in determining virulence. The the MVLST results from the dairy farm L. monocytogenes isolates were compared with those from strains isolated globally from human clinical cases, foods, and the environment. Results of this study demonstrated that multiple, distantly related, L. monocytogenes strains persisted among members of the herd over the course of the study while other strains were present for only short time periods. Further, some strains isolated during this study were identified as new sublineages in the L. monocytogenes global phylogeny, while others were closely related to epidemic clones previously isolated from human clinical cases. This work demonstrates that dairy cows can be reservoirs of a diverse population of human pathogenic L. monocytogenes which can be a potential risk to consumers of milk, dairy products, and meat. This information will be useful to other scientists and regulatory agencies.

Technical Abstract: Listeria monocytogenes, the causative agent of listeriosis, is frequently isolated from the environment. Dairy cows and dairy farm environments are reservoirs of this pathogen where fecal shedding contributes to its environmental dispersal and contamination of milk, dairy products, and meat. The molecular diversity of 43 L. monocytogenes isolates collected over a six-year study from the feces of dairy cattle on a single dairy farm was assessed using a multi-virulence locus sequence typing (MVLST) assay. The dairy farm L. monocytogenes MVLST patterns were compared to those from other strains isolated globally from clinical cases, foods, and the environment. Results of the study demonstrated that multiple, distantly related L. monocytogenes strains persisted among members of the herd over the course of the study while other strains were only present for short time periods. Further, some strains isolated during this study were identified as new sublineages in the L. monocytogenes phylogeny while others were closely related to epidemic clones associated with human clinical cases. This work demonstrates that dairy cows can be reservoirs of a diverse population of human pathogenic L. monocytogenes which represent potential risk to consumers of milk, dairy products, and meat.