Azithromycin-resistant syphilis-causing strains in Sydney, Australia: prevalence and risk factors.

Azithromycin has shown high efficacy in randomized trials when used for treating infectious syphilis in Africa. However, its use in clinical practice has been limited by the development of antimicrobial drug resistance. Resistance has not previously been reported from Australasia. The aim of this study was to determine the prevalence of and risk factors for azithromycin-resistant syphilis-causing strains in Sydney, Australia. We evaluated 409 samples that were PCR positive for Treponema pallidum DNA collected between 2004 and 2011 for the presence of the A2058G mutation, which confers resistance to macrolide antibiotics such as azithromycin. Overall, 84% of samples harbored the mutation. The prevalence of the mutation increased during the study period (P trend, 0.003). We also collected clinical and demographic data on 220 patients from whom these samples had been collected to determine factors associated with the A2058G mutation; 97% were from men who have sex with men. Reporting sex in countries other than Australia was associated with less macrolide resistance (adjusted odds ratio, 0.25; 95% confidence interval, 0.09 to 0.66; P = 0.005), with other study factors showing no association (age, HIV status, recent macrolide use, stage of syphilis, or history of prior syphilis). Azithromycin cannot be recommended as an alternative treatment for syphilis in Sydney.