Incidence estimates of late stages of trachoma among women in a hyperendemic area of central Tanzania.

The purpose of this study is to estimate 5-year incidences of conjunctival scarring and trichiasis, and 10-year incidence of corneal opacities due to trachoma, using prevalence data from a population sample of 6038 women living in a trachoma-hyperendemic area of central Tanzania. Previous surveys have documented the age-specific prevalence of scarring, trichiasis, and corneal opacities in women in hyperendemic areas. Using the age-stratified prevalences of these different clinical signs, corresponding incidence rates were estimated. Transition rates from one sign to the next were also obtained by restricting the risk group to only women with a specific trachoma sign. Thus, the 5-year incidence of trichiasis among women with conjunctival scarring, and the 10-year incidence of corneal opacities among women with trichiasis were estimated. Incidences of all the signs markedly increased with age. For scarring, 5-year incidence rates increased from 3.1% in the 15-19 age category to 14.3% for women between 55 and 59 years. The 5-year incidence of trichiasis ranged from 0.3% in the 15-19 age category to 7.5% in the age group 55-59. Corneal opacities due to trachoma were highest in the age group 45-54; the 10-year incidence increased to 2.8%. The 5-year incidence of trichiasis among only women with scars increased from 3.2% in the 15-19 age group to 15.1% in women in the 55-59 age group. Once trichiasis is present, almost one-third of the women below 35 and more than 40% of the women older than 45 will develop corneal opacities in a 10-year interval. These estimates are important in understanding the dynamics of progression of trachoma from conjunctival scarring to the potentially blinding signs of trichiasis and corneal opacities. They provide important information for planning adequate services in areas where trachoma is endemic and surgery for trichiasis is a key factor to avoid blindness from trachoma. They also provide clues to the pathogenesis that may be useful in the development of new methods of control.