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Trachoma epidemiology: Prevalence

Trachoma is a disease of poverty, with some 6 million people afflicted by trachoma-induced blindness or severe visual loss in some of the world's poorest communities (at least 15% of the world's blind people) [Thylefors et al., 1995]. Approximately 146 million people are estimated by WHO to be affected by trachoma. Some 540 million people, almost 10% of the world's population living in some of the world's poorest communities live in trachoma endemic areas and are at risk from the disease. However some of the difficulties in estimating the true prevalence of trachoma globally and the resulting disease burden are detailed in the studies of Evans & Ranson, 1996 and Ranson & Evans, 1996. Frick et al., 2003 estimated the burden (disability-adjusted life years) and economic impact (productivity loss) of trachoma.  The number of cases was based on the year 2000 while calculation of the years of life with disability used the age-sex distribution of trachomatous visual loss. The one-year potential productivity loss calculation used the agricultural value added per worker in 1998. They concluded that countries with known or suspected blinding trachoma have 3.8 million cases of blindness and 5.3 million cases of low vision and a potential productivity loss of $2.9 billion (1995 US $). Prevalent cases of trachomatous visual loss yield 39 million lifetime disability adjusted life years. It was concluded that from a resource allocation viewpoint,  the economic burden of disease was roughly comparable  with the expected costs of eradicating blindness due to trachoma.

The areas most affected by trachoma are shown in the figures below, courtesy of the World Health Organisation Programme for the Prevention of Blindness and Deafness.

Key factors in the geographical distribution of trachoma are a lack of adequate clean water supplies for washing and basic hygiene, plus inadequate health care resources. As socio-economic conditions have improved in the world through the last few decades, the prevalence of active trachoma has gradually decreased. In The Gambia, for example, a small country in W. Africa where trachoma and blindness are intensively researched, a national blindness survey showed that, from 1986 to 1996 the prevalence of active trachoma fell by 54% and there was an 80% relative reduction in blinding trachomatous corneal opacities [Dolin et al., 1998]. However, as the blinding sequelae take years to develop and as developing world populations gain longer life expectancies, it may be that in other areas it will be many years before the impact of reduced active trachoma leads to reduced blinding disease [Courtright et al., 1989; Schachter & Dawson, 1990]. Nevertheless the World Health Organisation have set the year 2020 as the date for achieving the global eradication of trachoma.

In areas with hyper-endemic trachoma, (primarily North Africa, the Middle East and northern India) most infants become infected by age 2 or 3. In endemic areas, active trachoma may peak a little later but is still primarily a disease of childhood. However, although active infection is uncommon by adulthood, the scarring and potentially blinding sequelae may continue to progress into old age. In common with other forms of blindness [Abou-Gareeb et al., 2001] it is women in particular who suffer the blinding consequences of trachoma because, in this case, they care for the actively infected children.

Solomon et al., 2003 used quantitative PCR to try and establish the burden of ocular C. trachomatis infection in two trachoma endemic communities in Tanzania and in one community in The Gambia. Conjunctival swabs were obtained at examination from 3146 individuals, tested first by the qualitative Amplicor PCR, and positive samples rescreened by a quantitative PCR directed against the omp1 gene encoding the chlamydial major outer membrane protein. As would be expected, children had the highest ocular loads of C. trachomatis and individuals with intense inflammatory trachoma had higher loads of ocular chlamydial infection than did those with other conjunctival signs. However children are often omitted from treatment because they do not have active trachoma (ie follicles) on examination even though they are often infected [Taylor, 2003]. At the site with the highest prevalence of trachoma, Solomon et al., 2003 found that 48 of 93 (52%) individuals with conjunctival scarring but no sign of active disease were positive for ocular chlamydiae. Nevertheless the authors conclude that an antibiotic distribution programmes targeted at children less than 10 years old and those with intense active trachoma should cover the main sources of epidemiologically significant ocular C. trachomatis infection in developing communities. An unexpected feature of their results was the finding that infectious load increased with decreasing endemicity and severity. This might or might not be artifactual . Quantitative PCR is a cutting edge research technique which would be difficult to apply as a routine test in the field in developing countries [Taylor, 2003].

Economic burden of trachoma

Frick et al., 2003 attempted to assess the burden and economic impact of trachomatous visual loss using national survey data on trachomatous blindness or visual impairment since 1980. It was concluded that countries with known or suspected blinding trachoma have 3.8 million cases of blindness and 5.3 million cases of low vision and a potential productivity loss of 2.9 billion US dollars (1995 US dollars). Prevalent cases of trachomatous visual loss yield 39 million lifetime disability adjusted life years.

[MEW] August 2003]

References

Abou-Gareeb, I., Lewallen, S., Bassett, K. & Courtright, P. (2001). Gender and blindness: a meta-analysis of population-based prevalence surveys. Ophthalmic Epidemiology 8, 39 - 56.

Courtright, P., Sheppard, J., Schachter, J., Said, M. E. & Dawson, C. R. (1989). Trachoma and blindness in the Nile Delta: current patterns and projections for the future in the rural Egyptian population. British Journal of  Ophthalmology 73, 536 - 540.

Dolin, P. J., Faal, H., Johnson, G. J., Ajewole, J., Mohamed, A. A. & Lee P. S.  (1998). Trachoma in The Gambia. British Journal of Ophthalmology 82, 930 - 933.  Full article

Evans, T. G. & Ranson, M. K. (1996). The global burden of trachomatous visual impairment: II. Assessing burden. International Ophthalmology 19, 271 - 280.

Frick, K., Basilion, E., Hanson, C. & Colchero, A. (2003). Estimating the burden and economic impact of trachomatous visual loss. Ophthalmic Epidemiology 10, 121 - 132.

Ranson, M. K. & Evans, T. G. (1996). The global burden of trachomatous visual impairment: I. Assessing prevalence. International Ophthalmology 19, 261-270.

Schachter, J. & Dawson, C. R. (1990). The epidemiology of trachoma predicts more blindness in the future. Scandinavian Journal of Infectious Diseases Supplement 69, 55 - 62.

Solomon, A. W., Holland, M. J., Burton, M. J., West, S. K., Alexander, N. D., Aguirre, A., et al., (2003). Strategies for control of trachoma: observational study with quantitative PCR. Lancet 362, 198 - 204. Full article

Taylor, H. R. & Dax, E. M. (2003). New precision in measuring trachoma infection. Lancet 362, 181 - 182. [Interesting commentary on the results of Solomon et al., 2003].

Thylefors, B., Negrel, A. D., Pararajasegaram, R. & Dadzie, K. Y. (1995). Global data on blindness. Bulletin of the World Health Organisation 73, 115 - 121. 

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