Trachoma is the leading cause of infectious blindness worldwide, with the majority of affected populations occur in low income countries.
Trachoma is estimated to occur in 53 countries and affects 24 million people, of which 1.2 million people are irreversibly blind (Source: WHO). Further information is available from the WHO, International Trachoma Initiative (ITI) and the Carter Center. This page gives an overview of the transmission dynamics for modellers who are new to modelling trachoma. At the bottom of the page are some questions about trachoma which modelling can help address as well as some relevant modelling papers.
Causative organism: ocular bacterial pathogen Chlamydia trachomatis
Ocular infection leads to the development of conjunctivitis. Initial infection, normal in childhood, can be self- limiting but repeat infection may manifest to ‘active disease’ of the conjunctiva, characterised by inflammation and follicles on the conjunctiva (TF and TI) (see WHO simplified grading system).
Repeat episodes of infection and active disease leads to scarring of the conjunctiva (TS). Severe scarring from multiple episodes results in contraction of the upper eyelid, causing eyelashes to rub against the cornea – a phenomenon known as Trichiasis (TT).
The resulting scratching of eyelashes against the cornea of the eye results in trauma and scarring, leading to corneal opacity (CO) and ultimately blindness. The stages TS, TT, CO normally occur in adults but in highly endemic populations they can occur earlier.
Stopping repeat ocular infection, through reducing transmission of the bacteria, should stop the progression to blindness.
The global burden of disease has been mapped by the Global Atlas of Trachoma.
The bacteria are transmitted between people by flies (Musca sorbens), fingers and fomites.
Typically the disease occurs in populations with a relatively low level of sanitation. The disease was previously prevalent in Europe and United States before sanitation improved. The house flies breed in human and animal faeces and a lack of latrines has shown to be associated with an increased risk of trachoma. Populations with poor access to clean water are unable to wash their faces to prevent transmission.
Children are considered to be the principal reservoir of infection in most communities as the prevalence of active disease and infection peaks in young children and declines into adulthood. The chronic stages of trachoma (TS, TT and CO) are more prevalent in women, suggesting they may contribute more towards transmission over their lifetime due to close contact with children.
Infection and active trachoma disease cluster by household and transmission has shown to be very efficient within households.
Active trachoma has shown to be strongly seasonal in some areas. Studies have not directly assessed whether transmission of C. trachomatis is seasonal and inference from the active trachoma data must be done with caution as other seasonal ocular pathogens may also cause active disease.
The Global Elimination of blinding Trachoma by 2020 (GET 2020) alliance advocates the ‘SAFE’ strategy to achieve their goal:
S: Surgery – to reverse in-turned eyelashes from trichiasis and prevent corneal opacity
A: Antibiotics – to clear ocular C. trachomatis infection and hence reduce transmission. Pfizer donates Zithromax™ (azithromycin) and annual mass community-wide distribution is advocated in areas where the prevalence of TF in 1-9 year olds is 10% or greater.
F: Face-washing – this is advocated to remove the infectious ocular secretions on individuals’ faces to prevent transmission
E: Environmental Improvement – to increase access to water and sanitation. Part of the ‘E’ component is aimed at installing latrines to prevent breeding of the mechanical fly vector to reduce transmission. Provision of clean water is also included in this component to enable face- and hand- washing together with education programmes to help affected communities understand how the disease is caused and how to prevent the spread of infection.
The ‘AFE’ components aim at reducing transmission of C. trachomatis. Programmes rely on changes in prevalence of active trachoma disease as a marker of infection to evaluate interventions, as testing for infection is currently too expensive. Studies have found active disease to correlate poorly with infection, particular after multiple rounds of mass antibiotic distributions and further work is needed to understand this relationship more.
The costs and benefits of different diagnostic approaches and the cost-effectiveness of targeted control depend on the transmission dynamics in a particular location, as well as the age and household structure of that population. Modelling papers which consider these issues will appear in the list below shortly.
There are a number of questions about trachoma biology, dynamics and interventions which modelling can help address, such as:
- How often should we be treating? And for how long?
- How can we measure success when the main monitoring measure does not measure infection?
The publications on this page include modelling analyses which address some of these questions.
We are committed to making our model code available for use by other modellers. Below is a link to code used in a recent publication:
Pinsent, Liu et al Probabilistic forecasts of trachoma transmission at the district level: A statistical model comparison Epidemics 2017. The code for both the transmission and statistical models are available as Appendix A. Supplementary data.