Assessing Onchocerciasis Subcriticality from Pre-Intervention Cross-Sectional Surveys.

Honorat Gustave Marie Zoure,Thomas M Lietman,John Daniel Kelly,Catherine E Oldenburg,Maria Rebollo Polo,Jeremy D Keenan,Travis C Porco

doi:10.4269/ajtmh.19-0758

Abstract

.Elimination of an infectious disease requires subcritical transmission, or a reproductive number less than one, and can be assessed with cross-sectional surveys conducted by neglected tropical disease programs. Here, we assess the distribution of onchocerciasis prevalence taken from surveys across sub-Saharan Africa before the initiation of ivermectin in mass drug administrations. Pre-intervention nodular palpation cross-sectional surveys were available from 15 countries in the Expanded Special Project for Elimination of Neglected Tropical Diseases (ESPEN) database. We determined whether the distribution of the prevalence over communities in an area was consistent with a geometric distribution, which previous studies have suggested indicates a subcritical disease. If not, we fitted a negative binominal distribution (hypothetically supercritical) or a mixture of two distributions: geometric (hypothetically subcritical) and Poisson (hypothetically supercritical). The overall distribution of community-level onchocerciasis prevalence estimates from the ESPEN dataset from 2005 to 2014 was not consistent with a geometric distribution. By contrast, data from several countries and parts of countries were consistent with the geometric distribution, for example, some areas within Nigeria and Angola. Even if the geometric distribution suggested pre-intervention subcriticality in more localized geographical areas, our model using pooled survey data of all geographic areas suggests that the entire pre-intervention prevalence does not fit a geometric distribution. Further work will be required to confirm the significance of a geometric distribution for onchocerciasis.

Highlights

Neglected tropical disease programs typically use the mean incidence or prevalence over a geographical region to assess whether an intervention is required and whether control targets have been achieved
Onchocerciasis nodular palpation pre-intervention surveys from the overall ESPEN database were unlikely to have come from a geometric distribution, which mathematical models have suggested might be expected when an infectious disease is subcritical.[4,5,6]
Given that mass distribution of ivermectin had yet to begin in these places, we expected preintervention surveys to be inconsistent with the geometric distribution

Summary

Introduction

Neglected tropical disease programs typically use the mean incidence or prevalence over a geographical region to assess whether an intervention is required and whether control targets have been achieved. Thresholds allow programs to classify populations into those that have met the criterion. Those populations above the threshold may represent high, sustained transmission areas of endemic disease or the inevitable laggards in what will be a successful elimination program. A different criterion could be achievement of subcritical transmission, where the replacement number for infectious cases (R) is less than unity. Infection would be expected to eventually be eliminated even without additional programmatic efforts. Identification of the prevalence threshold for subcriticality is a goal for programs, as this implies they are moving toward elimination. Assessment could be performed with longitudinal surveys, subcriticality can be assessed in cross-sectional surveys.[1,2,3]

Methods

Results

Conclusion