Spread of avian influenza in The Netherlands: identifying areas at high risk

A risk map of the between-farm spread of avian influenza (AI) in The Netherlands was constructed during the Dutch AI outbreak as a tool to support decision-making for control strategies. The risk map was based on an estimated distance-dependent probability of transmission of AI from an infectious farm to a susceptible farm. The estimated probability function was used to derive a threshold value for the local density of farms. In areas with local farm density above this threshold ('high risk'), transmission could lead to major outbreaks by local spread alone. In below-threshold areas ('low risk'), local spread alone will not cause a propagating epidemic. Distance-dependent probability was estimated from the data available at the start of the 2003 AI epidemic in The Netherlands. The resulting map predicted that two areas were at high risk. This approach might be applicable for other livestock infections and to other countries.