The first Canadian study investigating the prevalence of rectal chlamydia has reported increased detection of Chlamydia trachomatis after the implementation of universal rectal screening for women at two sexually transmitted infection (STI) clinics.

C. trachomatis is the most common notifiable disease in Canada. Infection rates have been steadily rising since 1998, and two-thirds of all reported cases occur in women. Extragenital sites, such as the rectum, are thought to serve as hidden reservoirs for ongoing infection transmission, but currently there are no specific guidelines referring to the screening of extragenital sites when no symptoms of infection are reported.

This study analysed samples from 3,055 women screened for rectal chlamydia between July and December 2012. Samples were collected from 1,570 women attending an STI clinic in Calgary, and 1,485 women were screened for rectal C. trachomatis at an STI clinic in Edmonton. Results were analysed separately owing to significant differences in patient populations, and also in screening criteria.

The Calgary clinic had an overall infection rate of 13.0%, and the prevalence of rectal chlamydia was 11.7%. In total 89.0% of positive samples were positive for rectal infection, and 42.2% of samples were positive for genitourinary and rectal chlamydia. Notably, 46.6% of cases were positive solely at the rectal site, which increased the detection rate by 87.3%.

Results from the Edmonton clinic showed that the total infection rate was 17.2%, and the prevalence of rectal infection was 13.5%. Overall, 77.1% of C. trachomatis cases were positive in the rectum, with 59.3% positive at genitourinary and rectal sites, 17.8% of samples were positive solely in the rectum, which increased the detection rate by 21.7%.

These results show that the implementation of universal rectal screening increased detection at both clinics. “Our results suggest that the anorectal area might be a hidden reservoir for chlamydia infection and could contribute to the ongoing transmission of chlamydia,” Ron Read, corresponding author, told Nature Reviews Urology. “Including the rectum as a routine sampling site for women attending STI clinics could significantly increase case finding, which should then lead to higher rates of treatment.”