Salmonella enterica burden in harvest-ready cattle populations from the southern high plains of the United States.

Salmonella enterica is an important cause of food-borne disease, with an estimated 1.4 million illnesses and 500 deaths attributed to salmonellosis in the United States annually (20). Of all salmonellosis events, the vast majority, in excess of 90%, are believed to result from contaminated foods or beverages (20, 21, 25). Poultry, pork, and fresh produce are traditionally viewed as the principal vehicles of human salmonellosis, presumably attributable to fecal contamination (16, 21). Salmonella enterica organisms are, however, common inhabitants of the gastrointestinal tracts of all animals, including cattle, and as a consequence, beef and dairy products can also serve as vehicles for human exposure to this organism (4, 5, 10, 22). Not all S. enterica serovars appear equally likely to cause disease. Of the more than 2,000 serovars identified, only 5 serovars (Salmonella enterica serovars Typhimurium, Enteritidis, Newport, Javiana, and Heidelberg) account for approximately 56% of human salmonellosis cases reported to the U.S. Centers for Disease Control and Prevention (7), and an additional 25 serovars account for the majority of the remaining human illnesses. Furthermore, some serovars, such as Salmonella serovar Newport and Salmonella serovar Typhimurium, appear more likely to harbor a diverse ensemble of genetic determinants that convey resistance to a variety of antimicrobial drugs (6, 14). There is a growing body of evidence that disease caused by multiply resistant variants is more severe than disease caused by broadly susceptible S. enterica (15, 24). Presumably, this is not a factor of the genes encoding resistance per se; rather, S. enterica strains that harbor these genes are more likely to possess additional virulence determinants. The factors that contribute to the coselection of resistance and virulence determinants are not certain. Since antimicrobial drug use provides a selection pressure that favors those isolates resistant to that drug and since more-virulent strains are by definition more likely associated with disease, it seems logical to speculate that antimicrobial use at least in part selects for resistant variants and multiply resistant variants when resistance determinants are linked. If so, such selection could occur either in the animal reservoir or in human salmonellosis patients, although some argue that multiply resistant S. enterica strains arise primarily as a direct consequence of antimicrobial use in food animals (12, 15). Regardless of the origin of multiply resistant S. enterica, it is certain that at least a portion of human salmonellosis cases are attributable to the consumption of beef products. Well-publicized outbreaks of human salmonellosis caused by multiply resistant S. enterica strains associated with ground beef (Salmonella serovar Typhimurium DT104 and Salmonella serovar Newport) have been reported (4, 25). In one instance, beef produced from dairy-type cattle was implicated in an outbreak of multiply resistant Salmonella serovar Newport (5). Because specific animal types are often associated with documented outbreaks of salmonellosis, it is conceivable that multiply resistant S. enterica strains might be clustered within definable populations of harvested cattle. If so, further research of these populations that is designed to better understand the factors that contribute to their elevated risk as well as the development of targeted strategies to mitigate their risk may be warranted. The objectives of this study, therefore, were to quantify S. enterica burdens in harvest-ready cattle and to identify specific at-risk populations of cattle most likely to harbor multiply resistant S. enterica.