The role of case proximity in transmission of visceral leishmaniasis in a highly endemic village in Bangladesh

PLoS Negl Trop Dis. 2018 Oct 8;12(10):e0006453. doi: 10.1371/journal.pntd.0006453. eCollection 2018 Oct.

Abstract

Background: Visceral leishmaniasis (VL) is characterised by a high degree of spatial clustering at all scales, and this feature remains even with successful control measures. VL is targeted for elimination as a public health problem in the Indian subcontinent by 2020, and incidence has been falling rapidly since 2011. Current control is based on early diagnosis and treatment of clinical cases, and blanket indoor residual spraying of insecticide (IRS) in endemic villages to kill the sandfly vectors. Spatially targeting active case detection and/or IRS to higher risk areas would greatly reduce costs of control, but its effectiveness as a control strategy is unknown. The effectiveness depends on two key unknowns: how quickly transmission risk decreases with distance from a VL case and how much asymptomatically infected individuals contribute to transmission.

Methodology/principal findings: To estimate these key parameters, a spatiotemporal transmission model for VL was developed and fitted to geo-located epidemiological data on 2494 individuals from a highly endemic village in Mymensingh, Bangladesh. A Bayesian inference framework that could account for the unknown infection times of the VL cases, and missing symptom onset and recovery times, was developed to perform the parameter estimation. The parameter estimates obtained suggest that, in a highly endemic setting, VL risk decreases relatively quickly with distance from a case-halving within 90m-and that VL cases contribute significantly more to transmission than asymptomatic individuals.

Conclusions/significance: These results suggest that spatially-targeted interventions may be effective for limiting transmission. However, the extent to which spatial transmission patterns and the asymptomatic contribution vary with VL endemicity and over time is uncertain. In any event, interventions would need to be performed promptly and in a large radius (≥300m) around a new case to reduce transmission risk.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Aged, 80 and over
  • Bangladesh
  • Child
  • Child, Preschool
  • Disease Transmission, Infectious*
  • Endemic Diseases
  • Epidemiologic Studies
  • Female
  • Geography
  • Humans
  • Infant
  • Infant, Newborn
  • Leishmaniasis, Visceral / epidemiology
  • Leishmaniasis, Visceral / transmission*
  • Longitudinal Studies
  • Male
  • Middle Aged
  • Models, Statistical
  • Rural Population
  • Spatio-Temporal Analysis
  • Young Adult

Grant support

This study was supported by the Bill and Melinda Gates Foundation in partnership with the Task Force for Global Health through the NTD Modelling Consortium [OPP1053230] (LACC, SEFS, GFM, TDH). The views, opinions, assumptions or any other information set out in this article are solely those of the authors and should not be attributed to the funders or any person connected with the funders. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.