Abstract
BackgroundTrachoma, the worldwide leading infectious cause of blindness, is due to repeated conjunctival infection with Chlamydia trachomatis. The effects of control interventions on population levels of infection and active disease can be promptly measured, but the effects on severe ocular sequelae require long-term monitoring. We present an age-structured mathematical model of trachoma transmission and disease to predict the impact of interventions on the prevalence of blinding trachoma.Methodology/Principal FindingsThe model is based on the concept of multiple reinfections leading to progressive conjunctival scarring, trichiasis, corneal opacity and blindness. It also includes aspects of trachoma natural history, such as an increasing rate of recovery from infection and a decreasing chlamydial load with subsequent infections that depend upon a (presumed) acquired immunity that clears infection with age more rapidly. Parameters were estimated using maximum likelihood by fitting the model to pre-control infection prevalence data from hypo-, meso- and hyperendemic communities from The Gambia and Tanzania. The model reproduces key features of trachoma epidemiology: 1) the age-profile of infection prevalence, which increases to a peak at very young ages and declines at older ages; 2) a shift in this prevalence peak, toward younger ages in higher force of infection environments; 3) a raised overall profile of infection prevalence with higher force of infection; and 4) a rising profile, with age, of the prevalence of the ensuing severe sequelae (trachomatous scarring, trichiasis), as well as estimates of the number of infections that need to occur before these sequelae appear.Conclusions/SignificanceWe present a framework that is sufficiently comprehensive to examine the outcomes of the A (antibiotic) component of the SAFE strategy on disease. The suitability of the model for representing population-level patterns of infection and disease sequelae is discussed in view of the individual processes leading to these patterns.
Highlights
Trachoma is the leading infectious cause of blindness in the world; 8 million people are blind or severely visually impaired due to trachoma and 63 million have active disease [1]
We present a mathematical model of trachoma transmission and disease to predict the impact of interventions on blinding trachoma
The model is based on the concept of multiple re-infections leading to progressive scarring of the eye and the potentially blinding disease sequelae
Summary
Trachoma is the leading infectious cause of blindness in the world; 8 million people are blind or severely visually impaired due to trachoma and 63 million have active disease [1]. The effects of control programs on community infection and active disease can be rapidly measured, but their effects on the severe sequelae (trichiasis, corneal opacity and blindness) will not be properly ascertained until decades after their implementation, so mathematical modeling provides an invaluable method for the prediction of program performance. Previous mathematical models of trachoma infection at the population level have primarily looked at the effects of treatment with antibiotics, and the rebound in the prevalence of active disease that follows treatment cessation [3,4,5,6]. The worldwide leading infectious cause of blindness, is due to repeated conjunctival infection with Chlamydia trachomatis. The effects of control interventions on population levels of infection and active disease can be promptly measured, but the effects on severe ocular sequelae require long-term monitoring. We present an age-structured mathematical model of trachoma transmission and disease to predict the impact of interventions on the prevalence of blinding trachoma
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