AAC Accepts, published online ahead of print on 1 December 2014 Antimicrob. Agents Chemother. doi:10.1128/AAC.04337-14 Copyright © 2014, American Society for Microbiology. All Rights Reserved.
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Neisseria gonorrhoeae azithromycin susceptibility in the United States, the Gonococcal Isolate Surveillance Project: 2005–2013
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Robert D. Kirkcaldy, MD, MPH,a # Olusegun Soge, PhD,b John R. Papp, PhD,a Edward
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W. Hook, III, MD,c Carlos del Rio, MD,d Grace Kubin, PhD,e Hillard S. Weinstock, MD,
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MPHa
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Division of STD Prevention, Centers for Disease Control and Prevention, U.S.
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Department of Health and Human Services, Atlanta, Georgia, USAa; University of
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Washington, Seattle, Washington, USAb; University of Alabama and Jefferson County
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Department of Health, Birmingham, Alabama, USAc; Emory University, Atlanta,
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Georgia, USAd; Texas Department of State Health Services, Austin, Texas, USAe
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Running Head: N. gonorrhoeae azithromycin susceptibility in the US
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# Address correspondence to Robert D. Kirkcaldy,
[email protected]
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Abstract
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Background: Azithromycin, administered with ceftriaxone, is recommended by CDC for
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treatment of gonorrhea. Many experts have expressed concern about the ease with
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which Neisseria gonorrhoeae can acquire macrolide resistance.
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Objective: We sought to describe gonococcal azithromycin susceptibility in the United
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States and determine whether azithromycin susceptibility has changed over time.
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Methods: We analyzed 2005–2013 data from the Gonococcal Isolate Surveillance
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Project, a CDC-supported sentinel surveillance network that monitors gonococcal
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antimicrobial susceptibility.
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Results: 44,144 N. gonorrhoeae isolates were tested for azithromycin susceptibility by
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agar dilution methods. The overall azithromycin MIC50 was 0.25 µg/ml and the MIC90
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was 0.5 µg/ml. There were no overall temporal trends in geometric means. Isolates from
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men who had sex with men had significantly higher geometric mean MICs than isolates
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from men who have sex exclusively with women. The overall prevalence of reduced
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azithromycin susceptibility (MIC ≥2 µg/ml) was 0.4% and varied by year from 0.3%
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(2006 and 2009) to 0.6% (2013).
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Conclusion: We did not find a clear temporal trend in gonococcal azithromycin MICs in
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the United States and the prevalence of reduced azithromycin susceptibility remains
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low. These findings support continued use of azithromycin in the combination therapy
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regimen for gonorrhea.
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Introduction
Gonorrhea is the second most commonly reported notifiable disease in the
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United States, second only to chlamydia. In 2012, 334,826 gonococcal infections were
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reported in the United States, representing a continuing gradual increase over the
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preceding four years.[1] In women, Neisseria gonorrhoeae is a major cause of pelvic
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inflammatory disease, ectopic pregnancy, and infertility.[2,3] Public health control of
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gonorrhea relies on prompt detection and effective treatment. However, treatment has
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been made more challenging because N. gonorrhoeae has successively developed
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resistance to each antimicrobial recommended for treatment. Penicillinase-producing N.
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gonorrhoeae and gonococcal fluoroquinolone resistance emerged first in the United
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States in Hawaii, California and other western states, possibly due to geographic
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proximity to East Asia and travel, before spreading elsewhere in the United States.[4–6]
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Fluoroquinolone resistant gonorrhea also initially became prevalent among men who
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have sex with men (MSM) with gonorrhea before emerging among heterosexuals.[7]
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Similar epidemiological patterns have been more recently observed for reduced
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susceptibility to oral cephalosporins: cefixime minimum inhibitory concentrations (MICs)
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increased sharply in isolates from the western US and from MSM during 2006 and
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2011.[8,9]
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Currently, the Centers for Disease Control and Prevention (CDC) recommends
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only a single first-line regimen for gonorrhea treatment: the combination of ceftriaxone
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(an injectable cephalosporin) 250 mg as a single intramuscular dose plus either
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doxycycline 100 mg orally twice daily or azithromycin as a single 1 g oral dose.[9]
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Because of the high prevalence of tetracycline resistance, azithromycin, an azalide
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macrolide which binds to the bacterial 50S ribosomal subunit and inhibits protein
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synthesis, is preferred over doxycycline as the second agent. Observational outcome
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data of pharyngeal gonorrhea also suggest that, compared to the addition of
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doxycycline, the addition of azithromycin to an oral cephalosporin improves likelihood of
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cure.[10]
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Azithromycin has not been recommended for gonorrhea monotherapy because
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of the reported ease with which N. gonorrhoeae can acquire macrolide resistance.[11]
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Two mechanisms have been commonly implicated in gonococcal reduced azithromycin
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susceptibility: overexpression of an efflux pump due to mtrR-coding region mutations
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[12–15] and decreased antimicrobial affinity due to mutations in genes encoding the
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23S ribosomal subunit.[15,16] Because of the importance of azithromycin in the
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currently recommended treatment regimen and risk of macrolide resistance, careful
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monitoring of azithromycin susceptibility trends is important. We sought to describe
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gonococcal azithromycin susceptibility in the United States and determine whether
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gonococcal azithromycin susceptibility has changed over time between 2005 and 2013.
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Methods
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The Gonococcal Isolate Surveillance Project (GISP)
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The Gonococcal Isolate Surveillance Project (GISP) is a US-based national
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sentinel surveillance system established in 1986 to monitor trends in antimicrobial
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susceptibility in N. gonorrhoeae and to establish a rational basis for the selection of
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gonococcal therapies in the US.[17] Sexually transmitted disease (STD) clinics in 25–30 4
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cities throughout the US participate in GISP as sentinel sites. At each participating STD
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clinic, urethral N. gonorrhoeae isolates are collected from the first 25 men presenting
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with symptomatic gonococcal urethritis each month. Isolates are submitted to regional
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reference laboratories for antimicrobial susceptibility testing by agar dilution according
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to a common protocol (available at: www.cdc.gov/std/gisp), and results are sent to CDC.
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GISP also collects de-identified data on patient characteristics and clinical information
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from clinic medical records.
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Laboratory methods
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Gonococcal isolates collected at each sentinel clinic are sub-cultured at the
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clinic’s local public health laboratory on supplemented chocolate medium and frozen in
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trypticase soy broth containing 20% glycerol. Isolates are shipped monthly to one of the
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regional reference laboratories where they are tested for β-lactamase production and
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susceptibility to azithromycin, penicillin, tetracycline, ciprofloxacin, spectinomycin,
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cefixime, and ceftriaxone using the agar dilution method. Isolates were inoculated on
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Difco GC Medium Base supplemented with 1% IsoVitalex Enrichment (Becton-
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Dickinson Diagnostic Systems, Sparks, Maryland). During 2005–2007, the lowest
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azithromycin concentration tested was 0.008 µg/ml; this was increased to 0.03 µg/ml in
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2008. The routine testing range for azithromycin extended to 16.0 µg/ml during 2005–
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2013. Laboratories were asked to conduct agar dilution testing to identify an endpoint
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for isolates with MICs ≥ 16.0 µg/ml on the initial testing run; testing to an endpoint was
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not conducted on three isolates collected during 2005–2013. In the absence of Clinical
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and Laboratory Standards Institute (CLSI) breakpoints for gonococcal azithromycin
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susceptibility or resistance,[18] we defined reduced azithromycin susceptibility as MICs 5
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≥2.0 µg/ml for this analysis. Quality assurance processes are described in detail in the
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GISP protocol.[19]
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To ensure accuracy and reproducibility of antimicrobial susceptibility results from
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the regional reference laboratories, a set of seven control N. gonorrhoeae strains with
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known MICs of various antimicrobials are included with each susceptibility run. In
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addition, reference laboratories test a CDC-provided panel of 15 unidentified strains
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twice yearly to compare results and ensure consistency among laboratories.[19]
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Results obtained from testing of control strains and CDC-provided panels are used for
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internal quality assurance.
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Statistical Analysis
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Data were limited to sites that continuously participated during the analytic time
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period: Albuquerque, New Mexico; Atlanta, Georgia; Baltimore, Maryland; Birmingham,
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Alabama; Chicago, Illinois; Cleveland, Ohio; Dallas, Texas; Denver, Colorado;
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Greensboro, North Carolina; Honolulu, Hawaii; Las Vegas, Nevada; Los Angeles,
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Orange County, San Diego and San Francisco, California; Miami, Florida; Minneapolis,
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Minnesota; New Orleans, Louisiana; Oklahoma City, Oklahoma; Philadelphia,
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Pennsylvania; Phoenix, Arizona; Portland, Oregon; and Seattle, Washington. Clinical
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sites were grouped into US census regions. The Northeast and South were combined
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because of the small number of sites in the Northeast.
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Azithromycin MIC values ≤0.03 µg/ml were considered to have MICs of 0.03
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µg/ml for these analyses. Similarly, the four values reported as ≥16.0 µg/ml without
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reported endpoints were considered to have MICs=16.0 µg/ml for the primary analysis. 6
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Sensitivity analyses was performed by considering the azithromycin MIC value for these
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three isolates to be 256.0 µg/ml (the highest azithromycin MIC detected thus far in the
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United States was ≥256.0 µg/ml [1]). MICs50, geometric means with 95% confidence
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intervals (CIs), and the percentage of isolates with reduced susceptibility with 95% CIs
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were calculated. Proportions were compared using chi-square and medians were
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compared using the Wilcoxon-Mann-Whitney test. Two-sided P values of
