ARS | Publication request: Prevalence, species distribution and antimicrobial resistance of enterococci isolated from U.S. dairy cattle

Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 3, 2010
Publication Date: January 20, 2011
Citation: Jackson, C.R., Lombard, J.E., Dargatz, D.A., Cray, P.J. 2011. Prevalence, species distribution and antimicrobial resistance of enterococci isolated from U.S. dairy cattle. Letters in Applied Microbiology. 52(1):41-48.

Interpretive Summary: The contribution of food animals as reservoirs of antimicrobial resistant enterococci remains largely undefined. This is increasingly important considering the possibility of transfer of bacteria from food animals to humans via the food chain. In this study, prevalence and antimicrobial resistance of enterococci in fecal samples from U.S. dairy operations participating in the 2007 National Animal Health Monitoring System (NAHMS) survey of dairy cattle health and management practices was examined. The NAHMS Dairy 2007 study represented 79.5% of U.S. dairy operations and 82.5% of U.S. dairy cows and was conducted in 17 states. Approximately 30-35 healthy cows were sampled on each of 122 dairy operations from the end of February through August 2007. Of the 122 operations sampled, 118 (96.7%) had at least one dairy cow positive for enterococci and 88.7% (637/718) of dairy cattle fecal samples were positive for enterococci. Enterococcus hirae, E. faecalis, and E. faecium were the most prevalent enterococcal species isolated. The highest levels of resistance were to lincomycin, flavomycin, and tetracycline; none of the isolates were resistant to chloramphenicol, gentamicin, linezolid, tigecycline, or vancomycin. Results from this study indicate that fecal samples from U.S. dairy cattle contain high numbers of antimicrobial resistant enterococci and may act as a reservoir of antimicrobial resistant enterococci that can be transferred to the human host via the food chain. This research will be useful to policy makers and researchers studying the role that food animals have in the dissemination and persistence of antimicrobial resistance in humans which may impact human health.

Technical Abstract: Aims: To estimate prevalence and antimicrobial susceptibility of enterococci in feces collected in 2007 from U.S. dairy cattle. Methods and Results: A total of 718 fecal samples from 122 dairy cattle operations from 17 U.S. states were collected and cultured for the presence of enterococci. One hundred eighteen of the 122 operations (96.7%) had at least one dairy cow positive for enterococci and 88.7% (637/718) of the fecal samples were positive. At least ten different enterococcal species were found on the dairy operations and 90.7% (107/118) of the operations were positive for Enterococcus hirae followed by E. faecalis (40.7%; 48/118) and E. faecium (39%; 46/118). The highest percentage of resistant isolates were to lincomycin (92.3%; 587/636), flavomycin (71.9%; 457/636), and tetracycline (24.5%; 156/636). Multi-drug resistance (MDR) (resistance >2 antimicrobials) was observed to as many as seven antimicrobials regardless of class. Conclusion: In contrast to previous studies, fecal shedding of enterococci in dairy cattle occurred in almost 90% of cows sampled and represented a variety of enterococcal species. Significance and Impact of Study: Although this study demonstrated a high prevalence of antimicrobial resistant enterococci from dairy cattle feces in the U.S., the contribution of dairy cattle as a source of antimicrobial resistant enterococci that can be transmitted to humans remains unclear.