Visceral Leishmaniasis on the Indian Subcontinent: Modelling the Dynamic Relationship between Vector Control Schemes and Vector Life Cycles

Abstract

BackgroundVisceral leishmaniasis (VL) is a disease caused by two known vector-borne parasite species (Leishmania donovani, L. infantum), transmitted to man by phlebotomine sand flies (species: Phlebotomus and Lutzomyia), resulting in ≈50,000 human fatalities annually, ≈67% occurring on the Indian subcontinent. Indoor residual spraying is the current method of sand fly control in India, but alternative means of vector control, such as the treatment of livestock with systemic insecticide-based drugs, are being evaluated. We describe an individual-based, stochastic, life-stage-structured model that represents a sand fly vector population within a village in India and simulates the effects of vector control via fipronil-based drugs orally administered to cattle, which target both blood-feeding adults and larvae that feed on host feces.Principle findingsSimulation results indicated efficacy of fipronil-based control schemes in reducing sand fly abundance depended on timing of drug applications relative to seasonality of the sand fly life cycle. Taking into account cost-effectiveness and logistical feasibility, two of the most efficacious treatment schemes reduced population peaks occurring from April through August by ≈90% (applications 3 times per year at 2-month intervals initiated in March) and >95% (applications 6 times per year at 2-month intervals initiated in January) relative to no control, with the cumulative number of sand fly days occurring April-August reduced by ≈83% and ≈97%, respectively, and more specifically during the summer months of peak human exposure (June-August) by ≈85% and ≈97%, respectively.ConclusionsOur model should prove useful in a priori evaluation of the efficacy of fipronil-based drugs in controlling leishmaniasis on the Indian subcontinent and beyond.

Visceral leishmaniasis on the Indian Subcontinent: Efficacy of fipronil-based cattle treatment in controlling sand fly populations is dependent on specific aspects of sand fly ecology

“…By exploring these uncertainties, we aim to subsequently predict the outcome of vector control schemes implemented under a variety of ecological conditions representative of villages in Bihar. To do this, we relax model assumptions made by Poché et al [34] and re-evaluate the three schemes they identified as most efficacious. More specifically, we use the model of Poché et al [34] to quantify how uncertainties associated with 1) the percentage of female sand flies taking blood meals from cattle, and 2) the percentage of female sand flies ovipositing in organic matter containing feces from treated cattle might impact the efficacy of fipronil-based sand fly control schemes.…”

“…To do this, we relax model assumptions made by Poché et al [34] and re-evaluate the three schemes they identified as most efficacious. More specifically, we use the model of Poché et al [34] to quantify how uncertainties associated with 1) the percentage of female sand flies taking blood meals from cattle, and 2) the percentage of female sand flies ovipositing in organic matter containing feces from treated cattle might impact the efficacy of fipronil-based sand fly control schemes. This information could prove valuable for managers deciding whether to initiate treatment in candidate villages and at what frequency to do so.…”

“…Poché et al recently investigated the dynamic relationship between timing of fipronil-based sand fly control schemes and seasonality of the P. argentipes life cycle using an individualbased model parameterized to represent a village in Bihar, India [34]. These authors assumed that 1) P. argentipes females had a 50% probability of feeding on cattle blood [11], 2) gravid P. argentipes females had a 90% probability of ovipositing in organic matter containing feces from cattle [35], and 3) 100% of the village cattle would be treated.…”

“…P. argentipes has been described as a "chance feeder", feeding opportunistically on humans and cattle relative to host availability [10,12]. The authors of Poché et al hence indicated that the percentage of sand flies feeding on cattle implicitly represented the relative availability of cattle [34]. It seems logical that the encounter rate of sand flies with village cattle, relative to humans, might influence the blood feeding and oviposition tendencies of sand flies.…”

A Review on Major Risk Factors and Current Status of Visceral Leishmaniasis in North India

“…A deeper understanding of both sand fly and host biology and behavior is therefore essential to ensuring effectiveness of vector interventions and avoiding unintended counterproductive (hindrance) consequences etc. at an early age at the domestic or sylvatic areas with the aim of encouraging the use of protective measures that reduce vector exposure [65]. Efforts have been constantly made for the elimination of Kala-azar from the Indian Sub-continent; but the goal has not been yet achieved [67].…”