An Age-Structured Extension to the Vectorial Capacity Model

Abstract

BackgroundVectorial capacity and the basic reproductive number (R0) have been instrumental in structuring thinking about vector-borne pathogen transmission and how best to prevent the diseases they cause. One of the more important simplifying assumptions of these models is age-independent vector mortality. A growing body of evidence indicates that insect vectors exhibit age-dependent mortality, which can have strong and varied affects on pathogen transmission dynamics and strategies for disease prevention.Methodology/Principal FindingsBased on survival analysis we derived new equations for vectorial capacity and R0 that are valid for any pattern of age-dependent (or age–independent) vector mortality and explore the behavior of the models across various mortality patterns. The framework we present (1) lays the groundwork for an extension and refinement of the vectorial capacity paradigm by introducing an age-structured extension to the model, (2) encourages further research on the actuarial dynamics of vectors in particular and the relationship of vector mortality to pathogen transmission in general, and (3) provides a detailed quantitative basis for understanding the relative impact of reductions in vector longevity compared to other vector-borne disease prevention strategies.Conclusions/SignificanceAccounting for age-dependent vector mortality in estimates of vectorial capacity and R0 was most important when (1) vector densities are relatively low and the pattern of mortality can determine whether pathogen transmission will persist; i.e., determines whether R0 is above or below 1, (2) vector population growth rate is relatively low and there are complex interactions between birth and death that differ fundamentally from birth-death relationships with age-independent mortality, and (3) the vector exhibits complex patterns of age-dependent mortality and R0∼1. A limiting factor in the construction and evaluation of new age-dependent mortality models is the paucity of data characterizing vector mortality patterns, particularly for free ranging vectors in the field.

Abscisic acid induces a transient shift in signaling that enhances NF-κB-mediated parasite killing in the midgut of Anopheles stephensi without reducing lifespan or fecundity

“…Changes in mosquito lifespan have a fourth order effect on vectorial capacity, with senescence notably increasing this effect size [5, 26, 27]. Reduced egg production yields fewer adults in the next generation, which impacts biting rate and, therefore, parasite transmission.…”

Vector competence of Aedes aegypti mosquitoes for filarial nematodes is affected by age and nutrient limitation

“…It is common for models of disease transmission to assume that mortality rates are constant with age (Novoseltsev et al, 2012;Styer et al, 2007), but our results suggest that the extent to which this will be the case depends on the diet of the mosquito.…”

Analysis of the vectorial capacity of vector-borne diseases using moment-generating functions

“…Deterministic models described by Novoseltsev et al [11] and Liu et al [12] typically use mean values but accounting for external factors (temperature) or age-structured populations. Our work is complementary to these approaches, as the expected value given by deterministic model is a particular case of the stochastic model.…”

“…For instance, Jansen et al [10] used the concept to evaluate risks by different Aedes species of introducing chikungunya in Australia. Novoseltsev et al [11] derive the vectorial capacity assuming age-structured population model. Liu et al [12] propose estimations of temperature-dependent vectorial capacity.…”