American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“An empirical test of the role of small-scale transmission in large-scale disease dynamics”

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Joseph R. Mihaljevic, Carlos M. Polivka, Constance J. Mehmel, Chentong Li, Vanja Dukic, and Greg Dwyer (Apr 2020)

Study shows that small-scale transmission dynamics are insufficient to explain large-scale patterns of disease incidence

Read the Article (Just Accepted)

Larval Douglas-fir tussock moths being reared in captivity for transmission experiments. Various developmental stages (i.e., instars) can be seen, along with some molted exuviae.<br />(Credit: Joseph R. Mihaljevic)
Larval Douglas-fir tussock moths being reared in captivity for transmission experiments. Various developmental stages (i.e., instars) can be seen, along with some molted exuviae.
(Credit: Joseph R. Mihaljevic)

Abstract

A key assumption of epidemiological models is that population-scale disease spread is driven by close contact between hosts and pathogens. At larger scales, however, mechanisms such as spatial structure in host and pathogen populations and environmental heterogeneity could alter disease spread. The assumption that small-scale transmission mechanisms are sufficient to explain large-scale infection rates, however, is rarely tested. Here we provide a rigorous test using an insect-baculovirus system. We fit a mathematical model to data from forest-wide epizootics, while constraining the model parameters with data from branch-scale experiments, a difference in spatial scale of four orders of magnitude. This experimentally-constrained model fits the epizootic data well, supporting the role of small-scale transmission, but variability is high. We then compare this model’s performance to an unconstrained model that ignores the experimental data, which serves as a proxy for models with additional mechanisms. The unconstrained model has a superior fit, revealing a higher transmission rate across forests compared to branch-scale estimates. Our study suggests that small-scale transmission is insufficient to explain baculovirus epizootics. Further research is needed to identify the mechanisms that contribute to disease spread across large spatial scales, and synthesizing models and multi-scale data is key to understanding these dynamics.