Abstract
To more fully characterize the burden of Salmonella enterica in bovine peripheral lymph nodes (PLN), PLN (n = 5,450) were collected from healthy cattle at slaughter in 12 commercial abattoirs that slaughtered feedlot-fattened (FF) cattle exclusively (n = 7), cattle removed (or culled) from breeding herds (n = 3), or both FF and cull cattle (n = 2). Qualitative and quantitative methods were used to estimate prevalence and concentration of Salmonella in PLN. Isolates were subjected to a variety of phenotypic, serological, and molecular assays. Overall, Salmonella prevalence in PLN from FF and cull cattle was 7.1 and 1.8%. However, burden varied by season in that observed prevalence in PLN collected in cooler or warmer seasons was 2.4 and 8.2%, respectively. Prevalence in PLN from cull cattle in the southwest region of the US was 2.1 and 1.1% for cool and warm seasons, respectively; however, prevalence in FF PLN was far greater in that it was 6.5 and 31.1%, respectively. Salmonella was recovered from 289 (5.6%) PLN and 2.9% (n = 160) of all PLN tested had quantifiable concentrations that varied from 1.6 to 4.9 log10 colony forming units/PLN. The most common serotypes isolated from PLN were Montevideo (26.9%), Lille (14.9%), Cerro (13.0%), Anatum (12.8%), and Dublin (6.9%). In all, 376 unique isolates were collected from the 289 Salmonella-positive PLN. Antimicrobial susceptibility testing revealed the majority (80.6%) of these isolates were pansusceptible; however, 10.7% of isolates were found to be resistant to two or more antimicrobial classes. We were able to document an observed increased in prevalence of Salmonella in PLN during the warmer season, particularly in FF cattle from the southwest region of the US. The mechanisms underlying the observed association between season, region, and production source have yet to be elucidated. Nevertheless, these findings increase our understanding of the sources of contamination of beef products and shed light on transmission dynamics that may be useful in targeting these sources.
Highlights
Salmonella enterica subspecies enterica is an important group of foodborne pathogens resulting in an estimated 1.2 million illnesses, more than 23,000 hospitalizations, and 450 deaths in the United States (US) each year (Scallan et al, 2011)
Salmonella prevalence in peripheral lymph nodes (PLN) from cull animals was generally low in every region throughout 11months while that in FF cattle PLN was found to be low in the cooler season (2.7%), yet peaked during the warmer season (11.6%)
PLN collected from FF cattle originating from Region B abattoirs had a prevalence of Salmonella in the cooler and warmer seasons of 6.5% and 31.1%, respectively
Summary
Salmonella enterica subspecies enterica (here after referred to as Salmonella) is an important group of foodborne pathogens resulting in an estimated 1.2 million illnesses, more than 23,000 hospitalizations, and 450 deaths in the United States (US) each year (Scallan et al, 2011). Ground beef is an important vehicle for human exposure to foodborne pathogens (including Salmonella), and was implicated in three outbreaks of salmonellosis between 2010 and 2015 (McLaughlin et al, 2006; Centers for Disease Control and Prevention (CDC), 2012b, 2013). The same control strategies that have been implemented generally mitigate both E. coli O157:H7 and Salmonella (Wheeler et al, 2014) These interventions appear to have effectively reduced the burden of foodborne illness attributed to E. coli O157:H7 (Centers for Disease Control and Prevention (CDC), 2015); in 2001, the USDA Food Safety Inspection Service (FSIS) recovered E. coli O157:H7 from 0.9% of ground beef samples, whereas in 2014 the recovery was just 0.04%. Despite the implementation of interventions, the prevalence of Salmonella in ground beef products has remained relatively constant and ranges between 1.6 and 4.2% depending on the size of the sample and the analysis methods employed (Bosilevac et al, 2009; Food Safety Inspection Service, 2011)
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