Fig. 5.

Conceptual representations of two different approaches to modeling poultry disease outbreaks. (A) Bottom-up approach. Parameters from epidemiological investigations and other sources are input into a detailed simulation of routes of transmission. Within the simulation, a local area is represented by the dotted box. In this example, contact of an infectious premises (open circle) with a focal premises that becomes infected (open star) is via long distance transmission represented by the dotted line. Other types of contact and potential routes of transmission from the focal infected premises to susceptible premises (filled circles) may be by different routes represented by the black and gray solid lines. The detailed simulation output is often a spatial-temporal prediction of outbreak dynamics. (B) Top-down approach. Spatial-temporal outbreak infection data are the input for model fitting of a local spread model. Some features of the detailed simulation model, such as long distance transmission represented by the dotted line may be maintained. Other features such as specific, local transmission routes can be subsumed into a general, local transmission kernel represented by the shaded circle with highest transmission risk near the infectious premises and decreasing transmission risk with distance. This approach may result in a simpler model that can more easily be fit to outbreak data. The output of a local spread model is often estimated outbreak parameters, such as between-premises transmission rates or the time between initial infection and notification.