Modeling Epidemics in Seed Systems to Guide Management Strategies: The Case of Sweetpotato in Northern Uganda

Seed systems are critical for deployment of improved varieties, but also serve as major conduits for the spread of seed-borne pathogens. We evaluated the structure of an informal sweetpotato seed system for its vulnerability to the spread of epidemics, and its utility for disseminating improved varieties. During the 2014 growing season, vine sellers were surveyed weekly in the Gulu Region of Northern Uganda. Our analysis draws on tools from network theory to evaluate the potential for epidemic spread in this region. Using empirical seed transaction data and estimated spatial spread, we constructed a network of seed and pathogen movement. We modeled the introduction of a pathogen, and evaluated the influence of both epidemic starting point and quarantine treatments on epidemic progress. Quarantine of 30 out of 99 villages reduced epidemic progress by up to 66%, when compared to the control (no quarantine), over 20 time steps. The starting position in the network was critical for epidemic progress and final epidemic outcomes, and influenced the percent control conferred by quarantine treatments. Considering equal likelihood of any node being an introduction point for a new epidemic, villages of particular utility for disease monitoring were identified. Sensitivity analysis identified important parameters and priorities for future data collection. The efficacy of node degree, closeness, and eigenvector centrality was similar for selecting quarantine locations, while betweenness had more limited utility. This analysis framework can be applied to provide recommendations for a wide variety of seed systems.