Clinical characteristics and ompA serovar correlation in adult inclusion conjunctivitis: a cohort study in China

Introduction

Chlamydia trachomatis is an obligate intracellular bacterium and a well-recognised cause of ocular surface infection.^{1 2} Although classical trachoma continues to account for a substantial burden of preventable blindness in endemic settings, ocular disease caused by non-trachomatous C. trachomatis strains, serovars D-K, has been increasingly encountered in routine clinical practice, particularly among adult patients.^{3 4} This trend parallels the persistently high global prevalence of urogenital chlamydial infection. According to estimates released by the WHO, approximately 129 million new genital C. trachomatis infections occur worldwide each year among individuals aged 15–49 years, underscoring the ongoing public health relevance of this pathogen.⁵

Adult inclusion conjunctivitis (AIC) represents a distinct clinical manifestation of C. trachomatis infection, most commonly caused by urogenital serovars.^6–8 Clinically, AIC typically presents as a subacute or chronic follicular conjunctivitis, characterised by conjunctival hyperaemia, papillae hypertrophy and mucopurulent discharge.^{9 10} Owing to its insidious onset and overlapping features with viral or allergic conjunctivitis, AIC is frequently under-recognised or misdiagnosed, which may lead to delayed diagnosis and prolonged disease duration.¹¹ Nevertheless, data describing the clinical presentation and microbiological characteristics of AIC in Chinese populations remain scarce.

Accurate diagnosis relies on laboratory confirmation, including conjunctival cytology, nucleic acid amplification testing and, in selected cases, cell culture.^{11 12} Molecular typing based on the ompA gene has identified multiple serovars associated with ocular and genital infection, with serovars D–K most commonly linked to AIC.^{13 14} Systemic antibiotic therapy is highly effective in eradicating C. trachomatis infection, yet marked interindividual variability in symptom severity at presentation and time to clinical resolution is commonly observed.¹⁵ However, data examining the relationship between ompA serovars and clinical presentation or disease course in adult conjunctivitis remain limited. In particular, studies describing the clinical characteristics, ompA serovar distribution and their potential associations in patients with AIC are scarce in China, and systematic clinical and molecular investigations in this population remain lacking.

In this retrospective study, we analysed a cohort of patients with laboratory-confirmed AIC diagnosed over a 4-year period. Integrating detailed clinical phenotyping, a structured conjunctival severity scoring system, multimodal laboratory diagnostics and ompA genotyping, we aimed to define the clinical features and serovar distribution of AIC in a Chinese population and to investigate potential associations between C. trachomatis ompA genotypes, clinical presentation and prognosis.

Results

The clinical characteristics of AIC patients are summarised in table 1. The mean age at diagnosis was 34.5±14.0 years. Males accounted for 68.2% (15/22) of the cohort and females for 31.8% (7/22). In addition, 10 cases (45.5%) presented with unilateral red eye and 12 cases (54.5%) presented bilateral. Six patients (27.3%) admitted to having unprotected sex, of whom four had symptoms suggestive of genitourinary tract infection. Two patients (9.1%) reported exposure to public bathing environments, and one case (4.5%) suggested possible indirect transmission via shared personal items within a household setting. Five patients (22.7%) had systemic diseases and three (13.6%) had skin diseases, including facial acne (n=2, 9.1%), rosacea (n=1, 4.5%), AIDS (n=1, 4.5%), syphilis (n=1, 4.5%) and diabetes (n=1, 4.5%). Moreover, 12 patients (54.5%) were diagnosed with AIC at our hospital between 1 and 3 months after the onset of the disease, and three patients (13.6%) did not receive a correct diagnosis until over 6 months. The most common initial misdiagnoses were bacterial conjunctivitis (10/22, 45.5%), adenoviral keratoconjunctivitis (7/22, 31.8%) and allergic conjunctivitis (5/22, 22.7%) (see table 1).

Patients with AIC exhibited a spectrum of characteristic ocular findings. Conjunctival hyperaemia and oedema, accompanied by both lymphoid follicle formation and papillae hyperplasia, were observed in all patients (22/22, 100%). Lymphoid follicle formation predominantly involved the lower palpebral conjunctiva and the inferior conjunctival fornix. Additional ocular signs included conjunctival mucopurulent discharge in 14 patients (63.6%) and corneal epithelial damage in 3 patients (13.6%). Limbal lesions were identified in 4 patients (18.2%) (figure 1).

As for diagnosis, Giemsa-stained conjunctival smears were positive in 3 patients (13.6%), as representative cap-like inclusion body structures shown in figure 2A-B. C. trachomatis DNA was detected by qPCR in all 22 patients (100%) (figure 2C). Respectively, cell culture was positive in 19 patients (86.3%), presenting as apple green inclusion body structures visualised by IF staining (figure 2D-E).

The ompA gene of C. trachomatis was successfully genotyped in all 22 PCR-positive samples using PCR amplification followed by Sanger sequencing and phylogenetic analysis. Initial genotype assignment was performed by BLAST analysis and subsequently confirmed by phylogenetic tree construction based on local sequence alignment (figure 3). Serovar D was the most frequently identified, accounting for 36.4% of samples (8/22). Serovars F, J and K were each detected in three samples (13.6%), whereas serovars E and G were identified in two samples each (9.1%). Serovar I/Ia was detected in one sample (4.5%).

Based on ompA genotyping, patients were classified into a serovar D group and a non-D serovars group (figure 4A–D). At initial consultation, the overall clinical severity score appeared higher in the non-D serovars group compared with the serovar D group (p = 0.0489) (figure 4E). Analysis of individual clinical signs suggested a possible difference in papillary reaction between the two groups (p=0.0437). In contrast, no significant between-group differences were observed in conjunctival hyperaemia, follicular reaction or conjunctival discharge (figure 4F–I). Given the small sample size, these findings should be interpreted with caution.

The mean time to clinical resolution of the AIC patients was 23.1±6.4 days, and follicular hyperplasia and papillae hypertrophy were among the slowest clinical signs to resolve. The mean healing time was 23.9±4.4 days in the serovar D group and 23.2±7.0 days in the non-D serovars group. Survival analysis of the cure time showed no statistically significant difference between the two groups (p=0.6373) (figure 4J).

Discussion

This study provides a systematic clinical and molecular characterisation of AIC in a Chinese patient cohort and, to our knowledge, represents one of the first studies in China to combine detailed clinical phenotyping with ompA genotyping. It therefore contributes important region-specific evidence to a field where integrated clinical and molecular data remain scarce. Given that C. trachomatis infection remains one of the most commonly reported sexually transmitted infections worldwide, such data are of clear clinical and epidemiological relevance. In the USA, more than 1.7 million cases were reported in 2017, with a steady increase observed over recent years.¹⁹ A similar increasing trend has been observed in China, particularly among younger populations.²⁰ Given that this group represents a key at-risk population for urogenital C. trachomatis infection, it may also contribute to the burden of AIC. In addition to its urogenital manifestations, C. trachomatis is estimated to account for 1.8%–5.6% of acute conjunctivitis cases.¹⁹ Taken together, these findings suggest that AIC, while often under-recognised, represents a clinically relevant and potentially underdiagnosed form of conjunctivitis that should not be overlooked in routine practice.

In this cohort, AIC presented with a broad but largely non-specific spectrum of clinical features, including conjunctival hyperaemia, follicular and papillary reactions, and mucous discharge, which overlap with other forms of acute and chronic conjunctivitis.^21–24 Such overlap contributes to diagnostic uncertainty in routine clinical practice, particularly in settings where laboratory testing is not routinely performed or where clinicians may prioritise more common causes of conjunctivitis such as viral or allergic disease. Among the 22 patients with AIC, 10 (45.5%) were misdiagnosed as bacterial conjunctivitis, 7 (31.8%) as adenovirus keratoconjunctivitis and 5 (22.7%) as allergic conjunctivitis. Notably, prominent follicular hyperplasia of the inferior palpebral conjunctiva and papillary hypertrophy were commonly observed. These features, especially when associated with a subacute or chronic course, may reflect a sustained host inflammatory response to intracellular infection and could serve as clinical clues to chlamydial aetiology in adults.

Sexual transmission is a recognised route of infection in AIC patients, which was confirmed by 6 of 22 patients (27.3%) who admitted to a recent history of unprotected sexual contact.^{7 8 25 26} In addition, potential non-sexual exposures were identified, including the use of shared or public bathing facilities. These findings suggest that, beyond direct transmission, indirect exposure via contaminated fomites may contribute to infection, particularly in environments with close contact or suboptimal hygiene.^{27 28} Notably, a small proportion of patients had a history of skin disease. Although this association has been infrequently reported, it may reflect compromised epithelial barrier function or increased opportunities for autoinoculation, thereby facilitating ocular infection. Taken together, these observations highlight the heterogeneity of transmission routes and the importance of considering both behavioural and environmental factors in the clinical assessment of AIC.

The diagnosis of AIC remains challenging due to its non-specific clinical presentation, which often overlaps with allergic, viral and bacterial conjunctivitis.^{24 29} A variety of diagnostic approaches are available for the detection of C. trachomatis, broadly encompassing cytological, culture-based and nucleic acid-based methods, each with distinct advantages and limitations.^30–36 In this study, conjunctival cytology, nucleic acid amplification testing and cell culture were used in combination to support diagnosis, improving overall diagnostic confidence. While nucleic acid amplification testing provides sensitive molecular evidence and cytology offers supportive morphological findings, cell culture is technically demanding and influenced by preanalytical factors such as specimen handling and transport.^{37 38} The relatively high culture positivity rate in our cohort (86.3%) may reflect optimised specimen handling, including cold-chain transport and timely inoculation, which are critical for maintaining organism viability.

OmpA genotyping revealed a heterogeneous distribution of C. trachomatis serotypes among patients with AIC, with serovar D being the most frequently identified genotype (8/22, 36.4%). Patients infected with non-D serovars tended to present with higher clinical scores at initial examination, however, ompA genotype was not associated with differences in treatment response or the time to clinical resolution. All patients achieved favourable outcomes following standard systemic antibiotic therapy. This pattern suggests that variation in ompA serovars may influence the initial clinical expression of infection without substantially affecting short-term therapeutic response. Such differences may relate to serovar-specific variation in epithelial tropism, local replication dynamics or host immune activation, which could modulate the intensity of conjunctival inflammation at presentation. Nevertheless, these findings should be interpreted with caution, and the underlying mechanisms require further investigation.

Chlamydial conjunctivitis is typically managed with systemic antibiotic therapy, most commonly doxycycline or azithromycin, often combined with adjunctive topical treatment.^39–41 In our cohort, all patients received oral azithromycin alongside topical antibiotics and achieved clinical resolution with conversion to qPCR negativity within a mean of 23.1±6.4 days. Despite observed differences in initial clinical severity, treatment outcomes were comparable across ompA serovars. This may reflect the high intracellular activity and broad efficacy of macrolides against C. trachomatis, which likely override potential strain-specific differences. In addition, host immune clearance and adequate treatment adherence may further contribute to the consistent therapeutic response observed.

This study has several limitations. First, the sample size was relatively small, reflecting the low incidence and frequent underdiagnosis of AIC in routine clinical practice. However, all cases were stringently confirmed by cytology, molecular testing and/or cell culture, ensuring diagnostic accuracy. Second, this was a single-centre observational study, which may limit the generalisability of the findings. Third, although ompA genotyping revealed associations with clinical severity, causal relationships cannot be inferred. Larger, multicentre studies are warranted to further validate the clinical relevance of ompA serotypes in AIC. Another limitation relates to the grouping of multiple ompA serovars (E, F, G and J) into a single non-D category. This approach was adopted because of the small number of cases for each individual serovar in our cohort. However, such grouping may obscure potential serovar-specific differences in clinical presentation or disease course, and future studies with larger sample sizes will be required to evaluate these associations in greater detail.

In summary, this study provides a systematic clinical characterisation of AIC in a Chinese population. Our findings suggest that non-D serovars may be associated with more pronounced clinical manifestations at presentation, although this observation should be interpreted with caution given the study design. Overall, treatment outcomes were favourable irrespective of genotype. These results highlight the potential value of integrating genotypic information into the clinical assessment of AIC, while underscoring the need for further validation in larger prospective studies.

Data availability statement

Data are available upon reasonable request.