Our results suggest that the total amount of virus inhaled by the herd should be used to evaluate the risk instead of individual amount. This would be a way to rank the herds according to airborne risk. An important question is the choice of a cut-off value. With a too small value, too many herds will be identified at risk. At this point, it has to be noticed that there is a need for standardization of the mathematical model used to calculate concentration of virus.
Spatial analysis is complicated by the absence of information on non-infected herds.
Our results demonstrate that outbreaks are clustered. But the next step, the estimation of the probability of infection according to the distance, was not possible. In spatial analysis two situations can lead to the formation of cluster. On one hand the population is homogeneously distributed in space and there is an infectious process which creates a spatial dependence between cases. On the other hand the population is heterogeneous, there is no spatial link between cases but cases appear clustered in one place because there are more individuals at this place.