JAC Advance Access originally published online on June 13, 2008
Journal of Antimicrobial Chemotherapy 2008 62(3):490-494; doi:10.1093/jac/dkn235
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Original research |
Emergence and spread of azithromycin-resistant Neisseria gonorrhoeae in Scotland
1 Scottish Bacterial Sexually Transmitted Infections Reference Laboratory (SBSTIRL), Department of Medical Microbiology, Royal Infirmary of Edinburgh, 51 Little France Crescent, Edinburgh EH16 5SA, UK 2 Sandyford Initiative, 2-6 Sandyford Place, Glasgow G3 7NB, UK
* Corresponding author. Tel: +44-131-242-6077; Fax: +44-131-242-6008; E-mail: helen.palmer{at}luht.scot.nhs.uk
Received 19 March 2008; returned 16 April 2008; revised 16 May 2008; accepted 20 May 2008
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Abstract |
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Objectives: The aim of this study was to analyse the trend in azithromycin susceptibility (AzDS) of Neisseria gonorrhoeae in Scotland between April 2004 and December 2007, and to characterize isolates exhibiting decreased AzDS or high-level azithromycin resistance (AzHLR).
Methods: Antibiotic susceptibility testing and N. gonorrhoeae multiantigen sequence typing (NG-MAST) were performed on all gonococcal isolates received by the Scottish Bacterial Sexually Transmitted Infections Reference Laboratory (SBSTIRL) during the study period.
Results: AzHLR isolates were observed for the first time in 2004 and increased from 0.3% to 3.9% in 2007. AzDS declined from 2.1% to 1.3% in the same period. Taken together, AzDS and AzHLR isolates accounted for 5.2% of the gonococcal infections in Scotland in 2007. NG-MAST revealed that only a small number of sequence types (STs) contained AzHLR and AzDS isolates; these STs also included azithromycin-susceptible isolates. Most STs containing AzHLR isolates were genetically related on the basis of their por and tbpB alleles; however, demographic data suggested that they formed discrete sexual networks.
Conclusions: AzHLR strains of N. gonorrhoeae are increasing in Scotland. A 1 g dose of azithromycin should not be considered as an alternative antibiotic therapy for gonococcal infections. The use of azithromycin to treat chlamydia in patients co-infected with N. gonorrhoeae results in a level of azithromycin in vivo that is sublethal for N. gonorrhoeae, which may lead to resistance.
Keywords: chlamydia , co-infection , epidemiology , molecular typing , NG-MAST
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Introduction |
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Antibiotic resistance surveillance for Neisseria gonorrhoeae is performed in order to detect the emergence and dissemination of new resistance phenotypes and to inform national empirical treatment guidelines accordingly. Ideally, resistance monitoring should be combined with a highly discriminatory typing scheme and the collection of epidemiological data: this permits the detection of sexual networks and offers the potential for targeted intervention to control the onward transmission of certain strains.1
The Scottish Bacterial Sexually Transmitted Infections Reference Laboratory (SBSTIRL) performs antibiotic resistance monitoring on isolates from all cases of gonorrhoea diagnosed by culture in Scotland, and in April 2004 adopted N. gonorrhoeae multiantigen sequence typing2 (NG-MAST) as the routine typing method. This is a highly discriminatory sequence-based typing method, which differentiates between strains on the basis of the sequence variation of two genes (por and tbpB).
Isolates with high-level azithromycin resistance (AzHLR; MIC 
256 mg/L) were first detected in 2004.3 The aim of this study was to investigate the characteristics of gonococcal strains demonstrating decreased susceptibility to azithromycin (AzDS) or AzHLR and their associated patient demographics.
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Materials and methods |
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All isolates received by the SBSTIRL during the period April 2004 to December 2007 were included in the study. Isolates were grown on modified New York City medium (bioMérieux, Basingstoke, UK) at 37ºC, 5% CO2. Isolates were confirmed as N. gonorrhoeae, and their antibiotic susceptibility to azithromycin, cefixime, ceftriaxone, ciprofloxacin, penicillin, spectinomycin and tetracycline was determined as described previously.4 Etests (AB Biodisk, Sweden) were also performed according to the manufacturer's instruction for those isolates with an azithromycin MIC
1 mg/L. AzDS was defined as MIC 1 mg/L supported by an Etest 0.75 mg/L but <256 mg/L. Although a resistance category has not been defined for azithromycin, for the purposes of this study, those isolates with an azithromycin Etest 256 mg/L were termed high-level resistant (AzHLR) and were separated from the AzDS category. NG-MAST was performed on all isolates as described previously.4 The demographic data collected included geographical location of the clinic the patient attended, patient gender and age. Epidemiological information on all cases of gonorrhoea in genitourinary medicine clinics in Scotland is submitted to the sexually transmitted infection surveillance Scotland (STISS) web-based coding system; this was used to determine the patient's sexual orientation by matching with case numbers. To better understand the transmission links, one of us (A. W.) reviewed the partner notification interviews of 31 selected cases infected with sequence type (ST) 470. Every case of gonorrhoea was counted as an index case.
An episode of infection was defined as one or more isolates of N. gonorrhoeae of the same ST isolated from a patient within a 4 week period.
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Results |
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SBSTIRL received isolates from 3326 episodes of infection during the study period of 3 years and 9 months. The percentage of AzHLR rose from 0.3% (2/674) in 2004 to 3.9% (33/845) in 2007, while the percentage of AzDS isolates declined from 2.1% (14/674) to 1.3% (11/845) in the same period.
The 3326 isolates were subdivided by NG-MAST into 642 STs. Thirteen of 642 STs contained AzDS or AzHLR isolates (Table 1). Five STs were represented by only one isolate. The remaining eight STs contained isolates from more than one azithromycin susceptibility category, while the MIC values for the other six antibiotics tested were, with few exceptions, consistent within any given ST. The azithromycin MIC distribution is shown in Table 2; in the STs containing AzDS or AzHLR isolates, the majority of the susceptible isolates had MIC values between 0.12 and 0.5 mg/L, whereas the majority of the isolates of all other STs were more broadly distributed between the 0.03 and 0.25 mg/L MIC categories. The MIC range for AzDS isolates was 1–4 mg/L. There were no isolates with an MIC between 4 and 256 mg/L. The proportion of AzHLR increased every year.
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Among STs containing five or more AzDS or AzHLR isolates, ST470 was the most common ST during 2004–06, but ST649 was the most common by 2007. Most STs containing AzDS or AzHLR isolates (9/13, 69%) shared the same tbpB allele. Of these, ST649, ST1377 and ST1704 each differed from ST470 by 1 nucleotide in their por allele, ST1443 and ST2193 each differed by 2 nucleotides, ST359 differed by 10 nucleotides, and ST1929 and ST2152 each differed by 12 nucleotides. Most AzHLR isolates (46/47, 98%) were found among the STs that differed from each other by only one or two nucleotides.
Patient demographics for those STs represented by more than five AzDS or AzHLR isolates are given in Table 3. ST359 isolates were predominantly cultured from men who have sex with men, whereas the remaining four STs were isolated from heterosexual patients. With one exception (ST359), the mean age of those infected with AzDS and AzHLR isolates was not markedly different from the mean age for patients infected with isolates that were susceptible to azithromycin for any given ST.
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STs represented by AzDS or AzHLR isolates were obtained mainly from patients who attended clinics in the west of Scotland (Greater Glasgow and Clyde, Ayrshire & Arran and Lanarkshire health boards) with the exception of ST359, which was isolated from patients attending clinics in Lothian (principally Edinburgh) and Greater Glasgow and Clyde. The geographical distribution of patients harbouring AzDS or AzHLR isolates did not differ significantly from those harbouring susceptible isolates within a given ST. One possible exception is ST1704, where AzHLR isolates were isolated from patients in Greater Glasgow and Clyde, and AzDS isolates were isolated from patients in Lanarkshire and Tayside; however, the numbers of isolates were small. There were no AzDS or AzHLR isolates in five health boards (Borders, Dumfries & Galloway, Fife, Orkney and Western Isles).
Partner network maps were constructed for 31 patients with ST470 in Greater Glasgow and Clyde. Of 37 contacts, 10 were linked as 5 dyad pairs, all with ST470. A further 8 contacts were culture-negative and the remaining 19 were untraceable.
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Discussion |
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This report shows a rapidly increasing level of gonococcal isolates with AzDS and AzHLR in Scotland to a level of 5.2% in 2007. The Gonococcal Resistance to the Antimicrobials Surveillance Programme (GRASP) for 2006 reported that 2% of the isolates had an azithromycin MIC
1 mg/L in England and Wales, with a significant difference between participating London and non-London clinics (1.1% versus 2.7%);5 our Scottish data mirrors the level for non-London clinics in 2006 and demonstrates an ongoing increase in 2007. Although an azithromycin resistance category has not yet been established for N. gonorrhoeae, decreased susceptibility to azithromycin (with a reported MIC range 1–8 mg/L) has been observed since the early 1990s in Canada, the Caribbean, Cuba, Spain, Sweden and the USA.6,7 This is the first report of N. gonorrhoeae isolates exhibiting azithromycin MIC 256 mg/L. Gonococcal isolates exhibiting AzDS or AzHLR have occurred over several years within a small number of STs that, on the basis of their tbpB and por alleles, are genetically closely related. Most STs containing AzDS or AzHLR isolates also contain susceptible isolates and there is no apparent demographic separation between patients with AzDS, AzHLR and susceptible isolates within any given ST (except perhaps ST1704, but the number of isolates is very small). Conversely, although these STs appear genetically related, their distribution over time suggests that they represent discrete sexual networks. Thus, it would appear that the development of resistance to azithromycin may have occurred several times within these different STs.
The most successful of these STs (ST470 and latterly ST649) was transmitted heterosexually among a young age group (20–26 years old) primarily within Greater Glasgow and Clyde, and neighbouring health boards. The use of traditional partner notification fails to identify such clusters of related infections as patients often only report anonymous partners or contacts that prove to be untraceable. Indeed, in the absence of highly discriminatory typing data, it would have been highly likely that the relationship between isolates with such different azithromycin susceptibilities would have remained undetected. The proven successful spread of these STs over several years make them prime candidates for enhanced surveillance and, if highly specific behaviours are identified (e.g. meeting partners at particular venues), targeted intervention may be possible to interrupt onward transmission as demonstrated for ST225 in Sydney, Australia.1
Development of azithromycin resistance may have occurred as a response to exposure to sublethal levels of azithromycin resulting from treatment for Chlamydia trachomatis according to the national guidelines.8 First-line use of azithromycin is likely to increase particularly since generic prescribing has been possible since 2006. It is interesting to note that the mean age of patients for the three STs containing the majority of the AzHLR isolates (ST470, ST649 and ST1704) is 20–26 years, given that the focus of chlamydial testing is on the under 25 age group.9
The demonstration of increasing levels of azithromycin resistance in N. gonorrhoeae argues strongly against the use of a 1 g azithromycin dose as a treatment for gonorrhoea that has been suggested by Habib and Fernando.10 Instead, it is now vital to ensure that all patients with dual infection who were initially treated only for chlamydia are recalled and treated with a recommended antibiotic therapy for gonorrhoea. This also highlights the potential value of dual molecular testing for gonorrhoea and chlamydia in order not to under-treat gonorrhoea with inadequate azithromycin dosing.
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Funding |
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This work was funded by the National Services Scotland, a division of Health Protection Scotland, as part of the routine work of the Scottish Bacterial Sexually Transmitted Infections Reference Laboratory (SBSTIRL).
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None to declare.
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Acknowledgements |
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The authors wish to thank Mr K. McIntyre for provision of STISS data.
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References |
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1 Todd K, Durrheim D, Pickles R, et al. Using epidemiological and molecular methods to investigate an outbreak of gonorrhoea associated with heterosexual contact in Newcastle, NSW, Australia. Sex Health (2007) 4:233–6.[CrossRef][Web of Science][Medline]
2 Martin IC, Ison C, Aanensen DM, et al. Rapid sequence-based identification of gonococcal transmission clusters in a large metropolitan area. J Infect Dis (2004) 189:1497–505.[CrossRef][Web of Science][Medline]
3 Young H, Palmer HM. Gonococcal antibiotic surveillance in Scotland (GASS): prevalence, pattern and trends in 2005. HPS Wkly (2006) 40:54–5.
4
Palmer HM, Young H. Dramatic increase in TRNG ciprofloxacin resistant Neisseria gonorrhoeae isolates in Scotland in 2003 was associated with transmission of one strain amongst men who have sex with men. Int J STD AIDS (2006) 17:254–6.
5 GRASP Steering Group. The Gonococcal Resistance to Antimicrobials Surveillance Programme (GRASP) Year 2006 Report. (2007) London: Health Protection Agency.
6 Lundback D, Fredlund H, Berglund T, et al. Molecular epidemiology of Neisseria gonorrhoeae—identification of the first presumed Swedish transmission chain of azithromycin-resistant strain. APMIS (2006) 114:67–71.[CrossRef][Web of Science][Medline]
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Arreaza L, Vazquez F, Alcala B, et al. Emergence of gonococcal strains with resistance to azithromycin in Spain. J Antimicrob Chemother (2003) 51:190–1.
8 2006 UK National Guideline for the Management of Genital Tract Infection with Chlamydia trachomatis. Clinical Effectiveness Group, BASHH. http://www.bashh.org/guidelines/2006/chlamydia_0706.pdf (21 April 2008, date last accessed).
9 Management of Genital Chlamydia trachomatis infection. Sign Publication 2000 No. 42. http://www.sign.ac.uk/guidelines/fulltext/42/index.html (22 May 2008, date last accessed).
10
Habib AR, Fernando R. Efficacy of azithromycin 1 g single dose in the management of uncomplicated gonorrhoea. Int J STD AIDS (2004) 15:240–2.
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