Abstract
BackgroundVisceral 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 findingsTo 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/SignificanceThese 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.
Highlights
Visceral leishmaniasis (VL), the world’s second most lethal vector-borne parasitic disease, has been targeted for elimination as a public health problem in the Indian subcontinent (ISC) by 2020 [1]
Average household size was relatively consistent across the three paras, but there were considerable differences in VL incidence and the average number of VL cases per household with VL
The para-level incidences are similar to those that were observed during 1-3 year ‘micro-epidemics’ in highly endemic clusters of hamlets in the data from Muzaffarpur, Bihar State, India, analysed by Bulstra et al [106]
Summary
Visceral leishmaniasis (VL), the world’s second most lethal vector-borne parasitic disease, has been targeted for elimination as a public health problem in the Indian subcontinent (ISC) by 2020 [1]. The target is an incidence of less than 1 VL case/10,000 people/year at subdistrict level in Bangladesh and India and district level in Nepal. The reported annual number of cases in the ISC has decreased by approximately 80% since 2011 (from *37,000 to *6,750) [2], and as of 2017 the target had been reached in all subdistricts in Bangladesh, in approximately 85% of endemic subdistricts in India, and in all districts in Nepal for the previous 4 years [3,4,5,6,7]. 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
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