The network level reproduction number for infectious diseases with both vertical and horizontal transmission

TL;DR

This paper derives an explicit expression for the basic reproduction number of diseases transmitted both vertically and horizontally across multiple species in heterogeneous networks, aiding understanding of disease invasion potential.

Contribution

It introduces a novel explicit formula for the reproduction number in complex multi-species, multi-location models with vertical and horizontal transmission, extending prior simpler models.

Findings

Reproduction number depends on vertical and horizontal transmission parameters.

Heterogeneity across locations influences the reproduction number.

Application to Rift Valley fever demonstrates the model's practical utility.

Abstract

A wide range of infectious diseases are both vertically and horizontally transmitted. Such diseases are spatially transmitted via multiple species in heterogeneous environments, typically described by complex meta-population models. The reproduction number is a critical metric predicting whether the disease can invade the meta-population system. This paper presents the reproduction number for a generic disease vertically and horizontally transmitted among multiple species in heterogeneous networks, where nodes are locations, and links reflect outgoing or incoming movement flows. The metapopulation model for vertically and horizontally transmitted diseases is gradually formulated from two species, two-node network models. We derived an explicit expression of the reproduction number, which is the spectral radius of a matrix reduced in size with respect to the original next generation matrix. The reproduction number is shown to be a function of vertical and horizontal transmission parameters, and the lower bound is the reproduction number for horizontal transmission. As an application, the reproduction number and its bounds for the Rift Valley fever zoonosis, where livestock, mosquitoes, and humans are the involved species are derived. By computing the reproduction number for different scenarios through numerical simulations, we found the reproduction number is affected by livestock movement rates only when parameters are heterogeneous across nodes. To summarize, our study contributes the reproduction number for vertically and horizontally transmitted diseases in heterogeneous networks. This explicit expression is easily adaptable to specific infectious diseases, affording insights into disease evolution.