title: The distribution of ocular Chlamydia prevalence across Tanzanian communities where trachoma is declining.
abstract: BACKGROUND
  Mathematical models predict an exponential distribution of infection prevalence across communities where a disease is disappearing. Trachoma control programs offer an opportunity to test this hypothesis, as the World Health Organization has targeted trachoma for elimination as a public health concern by the year 2020. Local programs may benefit if a single survey could reveal whether infection was headed towards elimination. Using data from a previously-published 2009 survey, we test the hypothesis that Chlamydia trachomatis prevalence across 75 Tanzanian communities where trachoma had been documented to be disappearing is exponentially distributed.
  
  
  RESULTS
  We fit multiple continuous distributions to the Tanzanian data and found the exponential gave the best approximation. Model selection by Akaike Information Criteria (AICc) suggested the exponential distribution had the most parsimonious fit to the data. Those distributions which do not include the exponential as a special or limiting case had much lower likelihoods of fitting the observed data. 95% confidence intervals for shape parameter estimates of those distributions which do include the exponential as a special or limiting case were consistent with the exponential. Lastly, goodness-of-fit testing was unable to reject the hypothesis that the prevalence data came from an exponential distribution.
  
  
  CONCLUSIONS
  Models correctly predict that infection prevalence across communities where a disease is disappearing is best described by an exponential distribution. In Tanzanian communities where local control efforts had reduced the clinical signs of trachoma by 80% over 10 years, an exponential distribution gave the best fit to prevalence data. An exponential distribution has a relatively heavy tail, thus occasional high-prevalence communities are to be expected even when infection is disappearing. A single cross-sectional survey may be able to reveal whether elimination efforts are on-track.
authors: F.I. Proctor Foundation, San Francisco, California, United States of America.
authors: Salman A
authors: SA
authors: Rahman
authors: Dana Center for Preventive Ophthalmology, Johns Hopkins University, Baltimore, Maryland, United States of America.
authors: Sheila K
authors: SK
authors: West
authors: Dana Center for Preventive Ophthalmology, Johns Hopkins University, Baltimore, Maryland, United States of America.
authors: Harran
authors: H
authors: Mkocha
authors: Dana Center for Preventive Ophthalmology, Johns Hopkins University, Baltimore, Maryland, United States of America.
authors: Beatriz
authors: B
authors: Munoz
authors: F.I. Proctor Foundation, San Francisco, California, United States of America; Department of Ophthalmology, University of California, San Francisco, San Francisco, California, United States of America; Department of Epidemiology & Biostatistics, University of California, San Francisco, San Francisco, California, United States of America.
authors: Travis C
authors: TC
authors: Porco
authors: F.I. Proctor Foundation, San Francisco, California, United States of America; Department of Ophthalmology, University of California, San Francisco, San Francisco, California, United States of America.
authors: Jeremy D
authors: JD
authors: Keenan
authors: F.I. Proctor Foundation, San Francisco, California, United States of America; Department of Ophthalmology, University of California, San Francisco, San Francisco, California, United States of America; Department of Epidemiology & Biostatistics, University of California, San Francisco, San Francisco, California, United States of America.
authors: Thomas M
authors: TM
authors: Lietman
mesh_terms: D002692:Chlamydia trachomatis; D006801:Humans; D008962:Models, Theoretical; D015995:Prevalence; D011795:Surveys and Questionnaires; D013636:Tanzania / Q000453:epidemiology; D014141:Trachoma / Q000453:epidemiology*