The transmission dynamics of Strongyloides stercoralis and the impact of mass drug administration

Abstract

The epidemiology of Strongyloides stercoralis is briefly reviewed with an emphasis on cross section and longitudinal studies of infection prevalence stratified by age, performance of different diagnostic tools, mass drug administration (MDA) impact and estimates of key population parameters within the complex life cycle of the parasite that determine transmission intensity and response to control measures. The paucity of studies is highlighted, and gaps in current knowledge identified about the population biology of this very prevalent infection in tropical and sub-tropical regions around the world. A stochastic individual based stochastic model is described in part to highlight gaps in knowledge. The impact of repeated MDA is simulated to illustrate some aspects of transmission dynamics of this helminth infection. Specifically, the impact and bounce back times in either the intervals between treatment rounds, or post the cessation of treatment, depend critically on the magnitude of two distinct components of the basic reproductive number R 0 . The absence of data on these key components is highlighted, as is the value of studies of longitudinal cohorts of people in regions of endemic infection post rounds of MDA to record how infection levels bounce back post treatment at individual and population levels of study. This article is part of the Theo Murphy meeting issue ‘ Strongyloides : omics to worm-free populations’.

Cost-effectiveness of mass drug administration with ivermectin against strongyloidiasis: a modelling study

“…In another recent mathematical modelling study of strongyloidiasis, 45 others explicitly modelled the complexities of the parasite life cycle in the environment (mating of free-living female and male worms) as well as the auto-infection process and regulation of female worm egg production by the host immune response. In comparison, our model here is simpler and has fewer parameters and effectively assumes that (1) the mating process of free-living adult worms amplifies the size and longevity (somewhat) of the environmental reservoir of infection by a constant and can therefore be left out and absorbed into the central parameter for overall intensity of transmission (ζ); (2) the host immune response and the resulting regulation of female worm fecundity reaches it maximum in negligible little time (weeks to months) compared to the duration of untreated infection (years to lifetime) and can therefore be ignored; and (3) infection is lifelong due to auto-infection, which in terms of the other model 45 is an “internal reproduction number” R I > 1, which is consistent with the slow bounce-back of infection levels after treatment observed in the field. 17 An important benefit of our simplifications is that they allow us to avoid several unidentifiable and highly uncertain parameters, while still allowing us to capture the effects of the associated uncertainty in the three transmission-related parameters of the model.…”