# Population-based concordance of Chlamydia trachomatis genotypes between ocular and urogenital samples in Nauru

## TL;DR

This study found that in Nauru, eye infections in children and genital infections in adults are caused by different strains of Chlamydia trachomatis, with no evidence of cross-transmission.

## Contribution

The study is the first to investigate genotype concordance between ocular and urogenital Chlamydia infections in a high-prevalence setting.

## Key findings

- Only genotype C was found in children's eye samples, indicating ocular-specific transmission.
- Six urogenital genotypes were detected in adult urine samples, with genotype D being the most common.
- There was no evidence of crossover transmission between ocular and urogenital strains at the population level.

## Abstract

Chlamydia trachomatis causes sexually transmitted urogenital infections, as well as ocular infections, the latter mostly spread between children and responsible for the blinding disease trachoma. The strains causing these two types of infection are generally distinct, but there is some evidence that urogenital strains can infect the eye and cause conjunctivitis, possibly indicating transmission on fingers of adults or adolescents to children. In what we believe is the first study of its kind in a setting of high prevalence of both urogenital and ocular infection, we characterized C. trachomatis strains from 107 ocular and 95 urine samples collected through cross-sectional population surveys in Nauru, employing household-based sampling. Ocular samples were collected during a national baseline prevalence survey in 2019 in children aged 1–9 years. Urine samples were collected from adults participating in a national survey before (March 2020) and after (December 2020) a national mass drug administration (MDA) of azithromycin for trachoma in April 2020. We used a cluster survey design to recruit participants, with clusters based on Nauru’s districts and a predetermined number of households randomly selected in each cluster. There was no attempt to recruit from the same households across the three survey rounds. Only genotype C, recognized as an ocular strain, was identified in eye specimens from children, whereas six urogenital genotypes (D, E, G, J, Ja, and L1) were detected in adult urine samples, with genotype D most frequent. We, therefore, concluded that, in this highly endemic setting for both urogenital and ocular infection with C. trachomatis, there were two distinct networks of transmission with no evidence of crossover, at least at the population level.

Chlamydia infections are a public health issue with two broad manifestations: ocular infections, mostly found in children, and sexually transmitted infections of the genital tract and anus that can lead to adverse reproductive health outcomes. While generally caused by different C. trachomatis strains, there is some evidence that strains considered to be predominantly sexually transmitted can infect the conjunctiva and cause signs resembling trachoma. Possible strain crossover has raised concern about whether eye infection with genital C. trachomatis strains confers a drive toward visual impairment and blindness, and the potential for such infections to lead to overestimates of trachoma prevalence. In what we believe to be the first study of its kind, we identified distinct ocular strains in pediatric ocular swabs and urogenital strains in adult urine specimens in Nauru and concluded that urogenital C. trachomatis strains are not contributing to ocular disease despite the high prevalence of urogenital chlamydia.