Estimated Incidence of Antimicrobial Drug-Resistant Nontyphoidal Salmonella Infections, United States, 2004-2012

Abstract

Salmonella infections are a major cause of illness in the United States. The antimicrobial agents used to treat severe infections include ceftriaxone, ciprofloxacin, and ampicillin. Antimicrobial drug resistance has been associated with adverse clinical outcomes. To estimate the incidence of resistant culture-confirmed nontyphoidal Salmonella infections, we used Bayesian hierarchical models of 2004-2012 data from the Centers for Disease Control and Prevention National Antimicrobial Resistance Monitoring System and Laboratory-based Enteric Disease Surveillance. We based 3 mutually exclusive resistance categories on susceptibility testing: ceftriaxone and ampicillin resistant, ciprofloxacin nonsusceptible but ceftriaxone susceptible, and ampicillin resistant but ceftriaxone and ciprofloxacin susceptible. We estimated the overall incidence of resistant infections as 1.07/100,000 person-years for ampicillin-only resistance, 0.51/100,000 person-years for ceftriaxone and ampicillin resistance, and 0.35/100,000 person-years for ciprofloxacin nonsusceptibility, or ≈6,200 resistant culture-confirmed infections annually. These national estimates help define the magnitude of the resistance problem so that control measures can be appropriately targeted.

Keywords: Salmonella; United States; ampicillin; antibiotic resistance; antimicrobial resistance; bacteria; ceftriaxone; ciprofloxacin; drug resistance; food safety; incidence.

Figure 1

Number of nontyphoidal Salmonella isolates with clinically important resistance, by resistance category, United States, 2004–2012. Three mutually exclusive categories were defined. Isolates in each category may have resistance to other agents: 99% of the 599 Cef/Amp isolates, 43% of the 467 Cipro isolates, and 89% of the 1,254 Amp-only isolates were resistant to >1 antimicrobial class other than cephems, quinolones, or penicillins. Amp-only, resistant to ampicillin but susceptible to ceftriaxone and ciprofloxacin; Cef/Amp, resistant to ceftriaxone (MIC >4 μg/mL) and ampicillin (MIC >32 μg/mL); Cipro, nonsusceptible to ciprofloxacin (MIC >0.12 μg /mL) but susceptible to ceftriaxone; NTS, nontyphoidal Salmonella.

Figure 2

Estimated incidence of infection with all NTS and major serotypes with clinically important resistance (no. infections per 100,000 person-years), by state and resistance category, United States, 2004–2012. Estimates were derived by using Bayesian hierarchical models. All NTS includes the 4 major and other serotypes. Isolates in each category may have resistance to other agents. Data on Cipro among Newport (8 isolates), Cipro among Heidelberg (7), and Cef/Amp among Enteritidis (2) were too sparse to use in the Bayesian hierarchical models. Overall resistance was defined as Cipro, Cef/Amp, or Amp-only. Amp-only, resistant to ampicillin (MIC >32 μg/mL) but susceptible to ceftriaxone and ciprofloxacin; Cef/Amp, resistant to ceftriaxone (MIC >4 μg/mL) and ampicillin; Cipro, nonsusceptible to ciprofloxacin (MIC >0.12 μg /mL) but susceptible to ceftriaxone; NTS, nontyphoidal Salmonella.

Figure 3

Estimated incidence of NTS infections with clinically important resistance (no. infections/100,000 person-years), by period, serotype, and resistance category, United States, 2004–2012. Estimates were derived by using Bayesian hierarchical models. All NTS includes the 4 major and other serotypes. Three mutually exclusive resistance categories were defined. Isolates in each category may have resistance to other agents. Data on Cipro among Newport (8 isolates), Cipro among Heidelberg (7), and Cef/Amp among Enteritidis (2) were too sparse to use in the Bayesian hierarchical models. Overall resistance was defined as Cipro, Cef/Amp, or Amp-only. Data were grouped into 3 periods (P): 2004–2006 (P1), 2007–2009 (P2), and 2010–2012 (P3). Error bars indicate 95% credible intervals. Amp-only, resistant to ampicillin (MIC >32 μg/mL) but susceptible to ceftriaxone and ciprofloxacin; Cef/Amp, resistant to ceftriaxone (MIC >4 μg/mL) and ampicillin; Cipro, nonsusceptible to ciprofloxacin (MIC >0.12 μg /mL) but susceptible to ceftriaxone; NTS, nontyphoidal Salmonella; P, period.