Complexity and critical thresholds in the dynamics of visceral leishmaniasis

Abstract

We study a general multi-host model of visceral leishmaniasis including both humans and animals, and where host and vector characteristics are captured via host competence along with vector biting preference. Additionally, the model accounts for spatial heterogeneity in human population and heterogeneity in biting behaviour of sandflies. We then use parameters for visceral leishmaniasis in the Indian subcontinent as an example and demonstrate that the model exhibits backward bifurcation, i.e. it has a human infection and a sandfly population threshold, characterized by a bi-stable region. These thresholds shift as a function of host competence, host population size, vector feeding preference, spatial heterogeneity, biting heterogeneity and control efforts. In particular, if control is applied through human treatment a new and lower human infection threshold is created, making elimination difficult to achieve, before eventually the human infection threshold no longer exists, making it impossible to control the disease by only reducing the infection levels below a certain threshold. A better strategy would be to reduce the human infection below a certain threshold potentially by early diagnosis, control animal population levels and keep the vector population under check. Spatial heterogeneity in human populations lowers the overall thresholds as a result of weak migration between patches.

Livestock and rodents within an endemic focus of Visceral Leishmaniasis are not reservoir hosts for Leishmania donovani

“…Nonhuman sources of infection are not targeted through current VL elimination program interventions in the ISC. Mathematical modelling and observational studies posited that livestock as hosts for human L. donovani infection in the ISC could potentiate outbreaks or ongoing infection [14,17,[36][37][38]. Finding a significant number of infected animals epidemiologically connected to human L. donovani exposure as well as demonstrating the infectiousness of domestic livestock to sand flies would establish the extent to which these animals contribute to VL ecology in endemic villages [39].…”

“…Mathematical modelling and observational studies posited that livestock as hosts for human L . donovani infection in the ISC could potentiate outbreaks or ongoing infection [ 14 , 17 , 36 – 38 ]. Finding a significant number of infected animals epidemiologically connected to human L .…”