Testing Mechanistic Models to Explain How Variation in Host Susceptibility Arises and Why It Shifts Across Thermal Gradients.

Shifting environmental conditions resulting from climate change have the potential to alter established ecological interactions, including the dynamics between pathogens and hosts. As a result, developing a thorough understanding of both the mechanisms that drive host-pathogen dynamics and the influence of environmental conditions on these mechanisms is of paramount importance to predicting and preparing for the effects of future environmental change.

This work tests novel hypotheses about the general mechanisms by which temperature influences pathogen susceptibility in Pacific herring, an ecologically and economically important marine forage fish. Hypotheses are being tested by fitting mathematical models to laboratory and field-collected data that evaluate the responses of herring and cold- and warm-water adapted pathogens across thermal gradients.

The foundational information resulting from this work increases predictive capacities to assess the effects of a changing climate on the long-term stability of host-pathogen interactions and to build links between fundamental evolutionary processes and disease ecology. Further, results inform fisheries managers about impacts of increased water temperatures on herring epizootics; thereby allowing for a more nuanced use of disease information in herring stock assessment models and contributing to sustainable aquatic resource management.

Beyond the study’s relevance to fisheries, the broader impacts focus on creating and evaluating multi-lingual outreach materials that educate high school and university students on how climate and disease interact to affect agricultural and wildlife species. The investigators are also developing a simple, interactive web application for managers to explore the model.

The distribution of susceptibility in a host population determines short- and long-term epidemic characteristics. However, little is known about the mechanisms that generate variation in susceptibility, and current theory assumes that the distribution of susceptibility is fixed across environmental conditions. Our capacity to understand and predict epidemic outcomes under climate change is therefore limited.

The central hypothesis of this proposal seeks to test this assumption by positing that the variation in pathogen susceptibility arises as a stochastic process defined by the relative performance of host and pathogen traits under a given temperature condition. Alternatively, variation in susceptibility may be explained by genetic variation in the thermal reaction norms of host susceptibility.

These hypotheses are tested using warm- and cold-water adapted pathogens in Pacific herring and will involve the use of mathematical models to link the within-host infection dynamics to among-host transmission dynamics. Developed theory is being further scrutinized against experimental data obtained from laboratory and field epizootic experiments. By rigorously testing theory against collected data, this project provides insights into general mechanisms that generate variation in disease susceptibility and quantifies the roles that thermal gradients play in shaping susceptibility distributions.

As a result, this project has the potential to provide new approaches to assess the consequences of a changing climate on the long-term stability of host-pathogen interactions. Furthermore, by describing the interplay between the distribution of susceptibility and temperature, this project identifies basic mechanistic processes that could be the foci of disease mitigation under climate change.

This Ecology and Evolution of Infectious Diseases project is jointly funded by the Biological Oceanography Program and the Established Program to Stimulate Competitive Research (EPSCoR).

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.