Abstract
The 2001 foot-and-mouth disease epidemic was controlled by culling of infectious premises and pre-emptive culling intended to limit the spread of disease. Of the control strategies adopted, routine culling of farms that were contiguous to infected premises caused the most controversy. Here we perform a retrospective analysis of the culling of contiguous premises as performed in 2001 and a simulation study of the effects of this policy on reducing the number of farms affected by disease. Our simulation results support previous studies and show that a national policy of contiguous premises (CPs) culling leads to fewer farms losing livestock. The optimal national policy for controlling the 2001 epidemic is found to be the targeting of all contiguous premises, whereas for localized outbreaks in high animal density regions, more extensive fixed radius ring culling is optimal. Analysis of the 2001 data suggests that the lowest-risk CPs were generally prioritized for culling, however, even in this case, the policy is predicted to be effective. A sensitivity analysis and the development of a spatially heterogeneous policy show that the optimal culling level depends upon the basic reproductive ratio of the infection and the width of the dispersal kernel. These analyses highlight an important and probably quite general result: optimal control is highly dependent upon the distance over which the pathogen can be transmitted, the transmission rate of infection and local demography where the disease is introduced.
Highlights
The control options for infectious diseases of livestock commonly include culling of both infected animals and animals considered to be at increased risk of infection, the latter referred to as ‘pre-emptive’ culling
We looked for differences in the implementation of all pre-emptive culling dependent upon disease control centres (DCCs) and epidemic phase
If an epidemic was seeded in Cumbria, we found that, while well-targeted contiguous premises (CPs) culling was more efficient than a policy of not culling CPs, optimal radius (2 km) ring culling minimized epidemic impact in this case, as well as minimizing epidemic duration
Summary
The control options for infectious diseases of livestock commonly include culling of both infected animals and animals considered to be at increased risk of infection, the latter referred to as ‘pre-emptive’ culling. One wellknown and much discussed example of culling to control livestock disease occurred during the UK 2001 epidemic of FMD This was an exceptionally well-recorded epidemic, providing valuable data on the spread of an infection between farms over a complex landscape. In addition to the 2026 IPs, 250 farms were culled as suspected FMD cases, and animals on a further 8570 premises were culled pre-emptively These data were recorded in the disease control system (DCS) database and the reasons for the pre-emptive culls can be broken down into two main categories. It was anticipated that of those farms that were infected, many farms would have been ‘pre-clinical’— the animals were too early in the course of infection to display clinical signs These farms were the main target of ‘at-risk’ culling, requiring the identification of farms with elevated risk of having been exposed to infection. Welfare culling was not targeted at farms at elevated risk of infection and constituted a relatively small fraction of the farms culled in areas directly affected by FMD; welfare culls are not considered further here
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