Modelling foot and mouth disease.

A simple generic model of foot and mouth disease (FMD) is presented. The dynamics of FMD at the index site (the farm where the initial infection occurs) is represented. Spread of disease from the index site is simulated with a three-term radial function, the terms corresponding to natural processes, short- and long-range movements. Parameterization includes the processes of threshold for clinical disease reporting, time delay before action is initiated, removal (slaughter) rate at the index farm; movement restrictions, culling of farms out to a given radius (from the index site), rate of culling, and vaccination (timing; rate of build-up of immunity). Between-farm basic reproductive ratio, R0bf, the number of primary (direct) infected farms arising from the initially infected (index) farm during the lifetime of the infection at the index farm, is calculated numerically. A "predictor-corrector" method is developed to estimate an upper limit to the number of secondary infections which could arise during this time. The model is easily programmed and is quick to execute. Simulation results are discussed in relation to the processes assumed in the model. They suggest that, given current UK farm practice, early disease detection, combined with immediate rapid slaughtering at the index site and restriction of short-range and long-range movements are relatively effective strategies of disease control. However, farm practice changes continually, and any control strategies need re-assessment in the light of current farm practice when and where an FMD outbreak occurs. A transparent and "simple" model can facilitate this process. The uncorrected model, without tuning, is compared with the cattle data from the 2001 UK FMD epidemic (Section 6; Fig. 8).