The estimated risk maps (Figure 4 and Figure 5 ) match the areas of the country where highest case incidence rates were seen -with the notable exception of Wales. The discrepancy between the high predicted risk in Wales and the small number of cases observed may reflect inaccuracies in the input data set -Keeling et al. [5] reduced farm-level sheep population numbers by 30% in Wales and obtained a better geographic match to the data (Matt Keeling, personal communication). However, the discrepancy may also reflect model inadequacy. We have not here allowed for other farm-level risk factors, such as the farm fragmentation index considered by Ferguson et al. [3] . We have not explored more complex non-linear models of the dependence of susceptibility and infectiousness on the number of animals on a farm or relaxed our implicit assumption that contact rates between farms scale linearly with the local density of farms. All these assumptions are being relaxed in ongoing work.