A hierarchical network approach for modeling Rift Valley fever epidemics with applications in North America

TL;DR

This paper presents a stochastic, network-based mathematical model to simulate Rift Valley fever spread in North American livestock, incorporating spatial, climate, and vector dynamics to assess outbreak risks and patterns.

Contribution

It introduces a novel hierarchical network model combining stochastic parameters and spatial factors for Rift Valley fever in North America, addressing data gaps on vectors.

Findings

Fewer initial infections can lead to longer delays before outbreaks.

The model shows rapid, widespread epidemics once thresholds are crossed.

Spatial and vector dynamics significantly influence disease spread patterns.

Abstract

Rift Valley fever is a vector-borne zoonotic disease which causes high morbidity and mortality in livestock. In the event Rift Valley fever virus is introduced to the United States or other non-endemic areas, understanding the potential patterns of spread and the areas at risk based on disease vectors and hosts will be vital for developing mitigation strategies. Presented here is a general network-based mathematical model of Rift Valley fever. Given a lack of empirical data on disease vector species and their vector competence, this discrete time epidemic model uses stochastic parameters following several PERT distributions to model the dynamic interactions between hosts and likely North American mosquito vectors in dispersed geographic areas. Spatial effects and climate factors are also addressed in the model. The model is applied to a large directed asymmetric network of 3,621 nodes based on actual farms to examine a hypothetical introduction to some counties of Texas, an important ranching area in the United States of America (U.S.A.). The nodes of the networks represent livestock farms, livestock markets, and feedlots, and the links represent cattle movements and mosquito diffusion between different nodes. Cattle and mosquito (Aedes and Culex) populations are treated with different contact networks to assess virus propagation. Rift Valley fever virus spread is assessed under various initial infection conditions (infected mosquito eggs, adults or cattle). A surprising trend is fewer initial infectious organisms result in a longer delay before a larger and more prolonged outbreak. The delay is likely caused by a lack of herd immunity while the infections expands geographically before becoming an epidemic involving many dispersed farms and animals almost simultaneously.