Molecular typing of Treponema pallidum strains in western Canada: predominance of 14d subtypes

Abstract

Background: Resurgence of syphilis in Canada and worldwide requires laboratories to update their methods for molecular epidemiology investigation and surveillance. This study utilizes polymerase chain reaction diagnostic tests for syphilis, identifies macrolide resistance, and uses a molecular typing system to characterize Treponema pallidum clinical strains causing syphilis in Alberta and Northwest Territories, Canada.

Methods: In total 449 specimens including genital swabs, whole blood, sera, and cerebrospinal fluid were obtained from 374 patients with suspect syphilis in Alberta and Northwest Territories. Molecular subtyping was based on genetic characterization of treponemal repeat genes, arp and tpr. Detection of macrolide resistance was accomplished by identification of the 23S rRNA gene mutation associated with the resistance pattern.

Results: Forty-nine specimens obtained from 43 patients were found to be positive for T. pallidum DNA using bmp, tpp47 and polA polymerase chain reaction assays. Four molecular subtypes were identified, with one type, 14d, accounting for 70% of all cases and 83% of typeable strains. Seven patients (16%) were found to be infected by macrolide-resistant strains, of which 6 were men who have sex with men and 1 whose infection was acquired in China.

Conclusions: A single molecular type of T. pallidum, characterized as 14d, caused the majority of the syphilis cases identified in this study. A more discriminatory typing method would be required to determine if these strains are clonal. Treatment of infectious syphilis with macrolide antibiotics should be restricted to patient populations where resistance is rare and clinical and serological follow up of patients is possible.