Increased Incidence of Antimicrobial-Resistant Nontyphoidal Salmonella Infections, United States, 2004-2016

Abstract

Salmonella is a major cause of foodborne illness in the United States, and antimicrobial-resistant strains pose a serious threat to public health. We used Bayesian hierarchical models of culture-confirmed infections during 2004-2016 from 2 Centers for Disease Control and Prevention surveillance systems to estimate changes in the national incidence of resistant nontyphoidal Salmonella infections. Extrapolating to the United States population and accounting for unreported infections, we estimated a 40% increase in the annual incidence of infections with clinically important resistance (resistance to ampicillin or ceftriaxone or nonsusceptibility to ciprofloxacin) during 2015-2016 (≈222,000 infections) compared with 2004-2008 (≈159,000 infections). Changes in the incidence of resistance varied by serotype. Serotypes I 4,[5],12:i:- and Enteritidis were responsible for two thirds of the increased incidence of clinically important resistance during 2015-2016. Ciprofloxacin-nonsusceptible infections accounted for more than half of the increase. These estimates can help in setting targets and priorities for prevention.

Keywords: Salmonella; United States; ampicillin; antibiotic resistance; antimicrobial resistance; bacteria; ceftriaxone; ciprofloxacin; drug resistance; enteric infections; foodborne diseases; incidence; nontyphoidal; resistance trends.

Figure 1

Estimated annual incidence of culture-confirmed nontyphoidal Salmonella infections with any clinically important resistance, by serotype and region, United States, 2004–2016. Estimated changes in resistance incidence (mean and 95% credible intervals of the posterior differences per 100,000 persons per year) were derived using Bayesian hierarchical models. Crude resistance incidence rates were derived by multiplying infection incidence and resistance proportion for state-year. Any clinically important resistance was defined as resistant to ceftriaxone, resistant to ampicillin, or ciprofloxacin nonsusceptible. The “other” category comprised serotypes other than Enteritidis, Typhimurium, Newport, I 4,[5],12:i:-, and Heidelberg. US Census regions were used to define 4 geographic regions. NTS, all nontyphoidal Salmonella serotypes.

Figure 2

Estimated changes in the incidence of resistant culture-confirmed nontyphoidal Salmonella infections, by serotype, resistance category, and geographic region, United States, 2015–2016 versus 2004–2008. Estimated changes in resistance incidence (mean and 95% credible intervals of the posterior differences per 100,000 persons/year) were derived using Bayesian hierarchical models Amp-only, Cef/Amp, and Cipro are mutually exclusive categories of clinically important resistance: Amp-only, resistant to ampicillin but susceptible to ceftriaxone and ciprofloxacin; Cef/Amp, resistant to ceftriaxone and ampicillin; Cipro, nonsusceptible to ciprofloxacin but susceptible to ceftriaxone. Isolates in each category might have resistance to other agents. Multidrug resistance was defined as resistance to >3 classes of antimicrobial agents. The “other” category comprised serotypes other than Enteritidis, Typhimurium, Newport, I 4,[5],12:i:-, and Heidelberg. US Census regions were used to define 4 geographic regions (A, all regions; M, Midwest; N, Northeast; S, South; W, West). MDR, multidrug resistant. NTS, all nontyphoidal Salmonella serotypes.

Figure 3

Estimated changes in the incidence of resistant culture-confirmed nontyphoidal Salmonella infections, by serotype, resistance category, and geographic region, United States, 2015–2016 versus 2010–2014. Estimated changes in resistance incidence (mean and 95% credible intervals of the posterior differences per 100,000 persons/year) were derived using Bayesian hierarchical models. Amp-only, Cef/Amp, and Cipro are mutually exclusive categories of clinically important resistance: Amp-only, resistant to ampicillin but susceptible to ceftriaxone and ciprofloxacin; Cef/Amp, resistant to ceftriaxone and ampicillin; Cipro, nonsusceptible to ciprofloxacin but susceptible to ceftriaxone. Isolates in each category might have resistance to other agents. Multidrug resistance (MDR) was defined as resistance to >3 classes of antimicrobial agents. The “other” category comprised serotypes other than Enteritidis, Typhimurium, Newport, I 4,[5],12:i:-, and Heidelberg. US Census regions were used to define 4 geographic regions (A, all regions; M, Midwest; N, Northeast; S, South; W, West). MDR, multidrug resistant; NTS, all nontyphoidal Salmonella serotypes.