Insights into Pathogenesis of Trachoma

Abstract

Trachoma is the most common infectious cause of blindness worldwide. This review investigates the pathogenesis of trachoma, focusing on its causative agent, transmission pathways, disease progression, and immune responses. Trachoma is caused by serovars A-C of the bacterium Chlamydia trachomatis (Ct). Transmission occurs through direct or indirect exchanges of ocular and nasal secretions, especially in regions with poor hygiene and overcrowded living conditions. The disease is initiated in early childhood by repeated infection of the ocular surface by Ct. This triggers recurrent chronic inflammatory episodes, leading to the development of conjunctival scarring and potentially to trichiasis, corneal opacity, and visual impairment. Exploring the pathogenesis of trachoma not only unveils the intricate pathways and mechanisms underlying this devastating eye disease but also underscores the multifaceted dimensions that must be considered in its management.

Keywords: chlamydia trachomatis; pathogenesis; trachoma; trachomatous scarring; trachomatous trichiasis; transmission.

Figure 1

Routes of trachoma transmission−direct (fingers between humans) and indirect (fomites and eye-seeking flies) means of Ct spread.

Figure 2

Interplay of factors contributing to trachoma.

Figure 3

Relationship between the appearance of clinical signs of active trachoma and the load of the causative agent (Ct). (A) Quadrilateral table of the four states of infection and disease with states indexed from 1 to 4. (B) Diagram illustrating the kinetics of trachoma, highlighting that active inflammatory disease persists for a period even after the load of Ct becomes undetectable. (1) Time of infection—the load of Ct increases. (2) Incubation period—infection, but no clinical signs are observed. (3) Evident disease—clinical signs have become apparent. (4) Recovery period—infection cleared, and clinical signs persist but gradually resolve. (Disclaimer: the duration of each period is not representative).

Figure 4

The natural history of trachoma. Persistent re-infections contribute to the chronic cicatricial process (1). The recovery of the inflamed conjunctiva (2) involves some residual conjunctival scarring (3). [The clinical images are adapted from Ref. [30], CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/ accessed on 26 May 2024)].

Figure 5

Diagram illustrating the correlation of clinical signs of trachoma with age. In highly endemic regions, blinding complications of trachoma emerge at an earlier age. (TF/TI: Trachomatous Follicular/Intense inflammation, TS: Trachomatous Scarring, TT: Trachomatous Trichiasis, CO: Corneal Opacity).