Antibiotic Resistance Threats in the United States, 2013 report

Technical Appendix
Drug-Resistant Shigella
Methods
Estimates of the number of illnesses and deaths from infections with
Shigella resistant to azithromycin or ciprofloxacin are reported. They
were derived by multiplying an estimate of the annual number of
Shigella illnesses or deaths in the United States 1 by the prevalence
of resistance among Shigella tested by the National Antimicrobial
Resistance Monitoring System (NARMS) in 2011, the year azithromycin testing began.
Resistance breakpoints from the NARMS 2011 Human Isolates Report were used. 2 As
clinical azithromycin breakpoints have not been established for Shigella, the values used
here were based on epidemiological cut-off values used in the NARMS report. Isolates with
azithromycin minimal inhibitory concentrations of ≥32 µg/ml were considered resistant.
Many assumptions were made in deriving these estimates. The estimated number of
illnesses from resistant Shigella was divided by the U.S. population and multiplied by
100,000 to calculate the estimated number of illnesses from resistant infections per
100,000 people. The U.S. population in 2006 (approximately 299 million people) was
used for the calculations because the estimated number of Shigella illnesses in the United
States was based on this population. 1 The sentinel county survey data displayed were
previously reported. 3,4,5
References
1 Scallan E, Hoekstra RM, Angulo FJ, et al. Foodborne illness acquired in the United
States—major pathogens. Emerg Infect Dis 2011;17:7–15.
2 CDC. National Antimicrobial Resistance Monitoring System for Enteric Bacteria
(NARMS): Human Isolates Final Report, 2011. Atlanta, Georgia: U.S. Department of
Health and Human Services, CDC, 2013.
3 Tauxe RV, Puhr ND, Wells JG, Hargrett-Bean N, Blake PA. Antimicrobial resistance of
Shigella isolates in the USA: the importance of international travelers. J Infect Dis
1990;162:1107–11.
4 CDC. National Antimicrobial Resistance Monitoring System for Enteric Bacteria
(NARMS): 2003 Human Isolates Final Report. Atlanta, Georgia: U.S. Department of
Health and Human Services, CDC, 2006.
5 Sivapalasingam S, Nelson JM, Joyce K, et al. High prevalence of antimicrobial resistance
among Shigella isolates in the United States tested by the National Antimicrobial
Resistance Monitoring System from 1999 to 2002. Antimicrob Agents Chemother
2006;50:49–54.
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