Clinical outcomes in cases of foodborne salmonellosis are significantly worse when the pathogen of infection exhibits any antimicrobial resistance (AMR), according to recent research from the U.S. Centers for Disease Control and Prevention (CDC). However, infections caused by Salmonella with resistance only to clinically important antibiotics were not associated with poorer outcomes, suggesting that factors other than treatment failure—such as strain virulence, strain source, and host factors—may be important.

According to CDC, nontyphoidal Salmonella causes an estimated 1.35 million infections in the U.S. annually. AMR is recognized as one of the greatest emerging threats to global public health by national and international agencies.

For the present study, CDC scientists examined the association between AMR in Salmonella isolates and the clinical outcomes of hospitalization, a length of stay greater than or equal to three days, and death. The researchers linked epidemiologic data from CDC’s Foodborne Diseases Active Surveillance Network (FoodNet) with AMR data from the National Antimicrobial Resistance Monitoring System (NARMS) for nontyphoidal Salmonella infections from 2004–2018. Outcomes were compared before and after adjusting for patient age, state, race/ethnicity, and international travel, as well as outbreak association and isolate serotype and source.

Isolates were defined as having “any resistance” when exhibiting resistance to one or more antimicrobial, and as having “clinical resistance” when exhibiting resistance to important antibiotics ampicillin, azithromycin, ceftriaxone, ciprofloxacin, or trimethoprim-sulfamethoxazole.

Of the 5,549 isolates included in the study, 20 percent (1,105) had any resistance, and 16 percent of the subset of isolates tested for all five important antibiotics (469 of 2,969 isolates) had clinical resistance. Patients whose isolates had any resistance were more likely to be hospitalized (31 percent) than those whose isolates had no resistance (28 percent) or have a length of stay greater than or equal to three days (20 percent for AMR isolates versus 16 percent for isolates with no resistance). Deaths were rare, but more common among those with any than no resistance (1 percent and 0.4 percent, respectively).

Interestingly, outcomes for patients whose isolates had clinical resistance did not differ significantly from those with no resistance. However, after adjustment, any resistance and multidrug resistance (AMR for one or more antibiotic from three or more classes) remained significantly associated with hospitalization.

The CDC researchers call for further studies to assess the direct and indirect economic costs attributed to resistant Salmonella and to clarify the role of host and strain factors in determining clinical outcomes.