The epidemiology of gonorrhea in North America and Western Europe was marked in recent years by the emergence and spread of resistance to fluoroquinolones.1,2 Overall, 13.3% of gonococcal isolates tested in United States by the Gonococcal Isolate Surveillance Project were resistant to ciprofloxacin in 2006.3 In Canada, quinolone resistance of Neisseria gonorrhoeae isolates increased to 16% in 2005.4 In Western Europe, a sentinel surveillance study disclosed an even higher resistance rate to ciprofloxacin (30.9%) in 2004.2
In Israel, ciprofloxacin-resistant N. gonorrhoeae was detected for the first time in 19995; a study in 2000 revealed a resistance rate of 61%.6 In response, a recommendation was issued to the medical community to abstain from using fluoroquinolones for the empirical treatment of gonococcal infections.7 The current study was undertaken to examine the subsequent change over time in N. gonorrhoeae antibiotic-resistance patterns.
N. gonorrhoeae strains were isolated from clinical specimens (mostly urethral discharges from male patients) at the Central Laboratory of Maccabi Health Services, Rehovot, Israel. The Central Laboratory serves a population of 1,200,000 from all over Israel. All isolates recovered from 2002 to 2007 were transferred to the Infectious Diseases Research Laboratory at the E. Wolfson Hospital, Holon, Israel and were included in the study. During 2004 to 2007 isolates were also obtained from the Tel Aviv STI clinic (Levinsky Clinic) representing between 30% and 44% of each year(s) total isolates studied. Limited demographic data of the patients from the STI clinic were extracted: patient sex, age, and sexual orientation. The identity of the isolates was confirmed with the API-NH system for the identification of Neisseria and Haemophilus (bioMerieux, Lyon, France).
The MICs of penicillin, tetracycline, ceftriaxone, ciprofloxacin, spectinomycin, and azithromycin were determined by the Etest method (AB Biodisk, Solna, Sweden).8 N. gonorrhoeae ATCC 49,226 was used as the reference strain.
The antimicrobial susceptibility was judged by use of the breakpoint criteria defined by the National Committee for Clinical Laboratory Standards9 (see Table 1 for values). No definition of resistance for MICs of azithromycin was issued by the National Committee for Clinical Laboratory Standards; in this study in vitro resistance to azithromycin was defined as a MIC of ≥1.0 mg/L.10 Isolates with concomitant resistance to penicillin, tetracycline, and azithromycin were designated as “chromosomally mediated resistance in N. gonorrhoeae,” a marker of multidrug resistance.11
The mean log MICs for each antibiotic were compared between the different years by analysis of variance with Bonferroni correction for multiple comparisons. Student t test was used to compare between the mean log MICs of antibiotics tested on the following groups: (1) Isolates from patients seen at the STI clinic versus isolates from patients seen at non-STI clinics. (2) Within the STI clinic, isolates obtained from men having sex with men (MSM) versus isolates from heterosexual patients. (3) Isolates resistant to ciprofloxacin versus strains susceptible to this drug. To compare the frequency distributions of different categorical variables the χ2 test or the Fisher exact test were used.
Four hundred six strains of N. gonorrhoeae isolated during the survey period from 2002 through 2007 were studied. The sample represents 12% of the total number of laboratory confirmed cases in Israel. Three hundred isolates were received from Maccabi Health Services Central Laboratory (2002–2007) serving various non-STI clinics (the non-STI patients came mostly from urology and family physician offices), and 106 strains were isolated from patients seen at the STI clinic (2004–2007); 67.2% of the STI clinic isolates were from MSM, and 14% were from women. The mean age of the patients was 31.75 ± 8.63 (SD) years (range, 16–69 years).
The range of MICs and the percentage of susceptible and resistant strains to the various antimicrobial agents tested are shown in Table 1. The highest resistance rate was against ciprofloxacin. All isolates were fully susceptible to ceftriaxone and spectinomycin. Multidrug resistance was found in 11 (2.7%) strains. The majority of these strains (10/11) were isolated from patients seen in non-STI clinics. The resistance rates over the years have remained stable for penicillin (P = 0.6) and tetracycline (P = 0.5). On the other hand, resistance arates for ciprofloxacin were significantly higher in the second half of the period than in the first half (P = 0.05). For azithromycin, a sharp, transitory increase in the resistance rate was observed in 2004 (P < 0.000); otherwise the resistance to azithromycin has remained unchanged during the study period. There was no statistically significant difference over the years in the geometric mean MICs of the antimicrobials tested, except for ciprofloxacin, P = 0.02 (data not shown).
MIC of ceftriaxone ≥0.06 mg/L was detected in 3.4% of isolates (less susceptible strains). No particular trend was observed over the years: 1.2% in 2002, 0% in 2003, 14.2% in 2004, 5.6% in 2005, 1.0% in 2006, and 3.3% in 2007. All less susceptible strains, except one, were isolated from patients seen at the STI clinic (from heterosexual patients and MSM alike). In only one case (a homosexual patient seen at the STI clinic) was MIC ≥0.06 mg/L to ceftriaxone associated with multidrug resistance.
There were no statistically significant differences between resistance rates of isolates in the different groups (Table 2). However, significant differences were observed in the geometric mean MICs for some of the antimicrobials: for ceftriaxone, geometric mean MICs were higher for isolates from the STI clinic than from the non-STI clinics, and for isolates from heterosexual patients seen in the STI clinic compared to MSM visiting the same clinic. For tetracycline, the geometric mean MICs were higher for isolates from heterosexual patients seen in the STI clinic compared with MSM. For spectinomycin and azithromycin geometric mean MICs were higher for isolates from the STI clinic compared with the non-STI clinics (data not shown).
Isolates resistant to ciprofloxacin were significantly more resistant to penicillin (44.8%) and tetracycline (30.1%) than isolates susceptible to ciprofloxacin (8.5% and 11.2%, respectively; P < 0.001). There was no significant difference in the resistance rates to azithromycin (12.0% and 16.3%, respectively, P = 0.2). Similarly, the geometric mean MICs of penicillin, tetracycline, and ceftriaxone, but not of spectinomycin and azithromycin, were significantly higher in the group of ciprofloxacin-resistant isolates than in the group of ciprofloxacin-susceptible isolates (data not shown). The rate of multidrug resistance among ciprofloxacin-resistant isolates (5.2%) was higher than among ciprofloxacin-susceptible organisms (P = 0.02).
The ciprofloxacin resistance rates for gonococcal strains isolated in Israel in 2005–2007 (27.4%–39.6%) are similar to those reported in Europe in recent years.2 Compared with the findings in United States,12,13 the resistance rates of the Israeli isolates were more than 2-fold higher, and the geometric mean MICs of ciprofloxacin were approximately 10-fold higher. Ciprofloxacin was the only drug against which resistance rates and the geometric mean MICS continued to rise, despite the decrease in reporting of gonorrhea cases to the Ministry of Health.14
In the United States quinolone-resistant N. gonorrhoeae infections were associated with homosexual intercourse, antibiotic use, age above 35 years, HIV infection, and travel to Asia. Public health data from some provinces in Canada, but not from others, have also suggested an association between quinolone-resistant infection and MSM.11 In the current study no association was found between ciprofloxacin resistance and age or sexual preference (we had no data on travel abroad or antibiotic use). The association between ciprofloxacin resistance and resistance to penicillin and tetracycline was also documented for American isolates.12
Resistance rate to azithromycin (MIC ≥1 mg/L) was relatively high although not showing a rising tendency. Approximately 10% of the isolates in the second half of the study period were azithromycin-resistant, following a steep transitory rise to 38% in 2004. This outbreak was mostly noticeable among heterosexual patients of the STI clinic, although no additional details were available. Using the same criteria for resistance, the American Gonococcal Isolate Surveillance Project documented a resistance rate to azithromycin of 0.4% in 2003.12 In Western Europe, the prevalence of azithromycin resistance for 2004 varied widely between 0% (France, Greece, and Portugal) and 31% (Austria).2 The prevalence of azithromycin resistance remains sufficiently high to preclude the empirical use of this antimicrobial for treating gonorrhea in Israel.
All of the isolates in the current study were susceptible to ceftriaxone and spectinomycin, indicating that these drugs remain suitable for empirical therapy of gonococcal genital infections in Israel. However, treatment failures and raised MICs have been reported recently in the literature with oral third-generation cephalosporins (cefixime and ceftibuten). The affected strains also have decreased susceptibility to ceftriaxone, although not yet at MIC levels associated with loss of clinical efficacy.15 The rate of isolates with ceftriaxone MICs in the less susceptible range (0.064–0.25 mg/L)16 in the current study (3.4%) was higher than that reported in Australia (0.8% in 2007).17 The full susceptibility to spectinomycin, documented also by other surveillance systems,2,11,15 probably reflecting the limited use of this agent nowadays. However, more extensive exposure to spectinomycin may be associated with rapid emergence of gonococcal resistance to that drug.18
When differences in susceptibility were observed between different groups, they were significant only for the geometric mean MICs and had the same pattern: mean MICs were higher for isolates from the STI clinic compared with non-STI clinics, and for isolates from heterosexual patients compared with MSM. The differences in resistance rates followed usually the same trend (except for ciprofloxacin) but were not statistically significant possibly because of the small size of patients in each group. One can tentatively explain the higher mean MICs in the STI clinic by a possible higher exposure of patients seen in that clinic to antimicrobials (no data on antibiotic use were available to support the assumption). However, we have no explanation for the higher mean MICs for isolates from heterosexual patients compared to MSM.
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