Fig. 1: The natural history of trachoma.

Pathogenesis of visual impairment in trachoma and the SAFE strategy interventions intended to prevent or slow those processes. The two linked chronic processes that are involved in pathogenesis are shown: a recurrent, generally subclinical infectious–inflammatory disease that mostly affects children; and a non-communicable, cicatricial, increasingly painful, eventually blinding disease that supervenes in some individuals later in life. (1) Resolution of Chlamydia trachomatis infection and restoration of the uninflamed conjunctiva is accompanied by (2) some residual conjunctival scarring; (3) repeated reinfection drives the chronic cicatricial process. (4) Conjunctival scarring reduces the volume and alters the composition of the tear fluid, which normally acts as a mechanical barrier (via continual flow) and contains molecules that are part of the innate immune system. Where trichiatic eyelashes abrade the cornea, breaching of the epithelium (the most superficial layer of the cornea) enables penetration of bacteria into the corneal stroma. (5) Antibiotics treat prevalent C. trachomatis infections (and some other prevalent bacterial infections), whereas facial cleanliness and environmental improvement are intended to reduce the incidence of new infections, that is, reduce transmission intensity. (6) No specific interventions have been trialled or deployed to try to reduce the likelihood of incident trichiasis in individuals who have conjunctival scarring. Of note, SAFE strategy components are intended to reduce the accumulation of conjunctival scarring in those who do not yet have it and in those who do. (7) Surgery reduces progressive corneal opacification and ocular surface irritation, which is crucial for symptom relief. Surgery has no effect to correct existing ocular surface dryness. (8) In many conditions that blind through corneal opacification, keratoplasty (corneal transplantation) is used to reduce or reverse visual impairment. In an individual with corneal opacity due to trachoma, changes in the tear film and corneal vascularization are predicted to reduce the likelihood of long-term graft survival. In addition, populations affected by trachoma tend to have very poor access to keratoplasty services.