Another argument not to use animal density is the clustering of animals in herd. When we use animal density we imply that animals are uniformly distributed in space. But the way animals are clustered in farms suggest that animals from the same farm have a more similar risk and behaviour than animals from different farms. Then analyzing the effect of animal density is reduced to the effect of herd size, distance and their interaction. We have collected information from previous outbreaks that can give some information about density effect.
Data come from many sources such as researcher database, report and articles.
To assess airborne spread we used the mathematical model developed by Moutou et al. Spatial analysis was performed using Splus statistical package. Every time it was possible we tried to find risk factors at the herd level. Taiwan is very informative as it happened nowadays within an industrialized pig production. As accurate data are missing, this is just a descriptive study.