A common management strategy for livestock diseases involves culling animals on farms within a ring surrounding infected premises. However, ring culling often results in culling farms without any infected animals. Alternatively, we simulated the culling of farms to a specified target farm density within a ring. With this strategy, we studied the maximum farm density reduction needed to stop the outbreak in order to reduce the total number of farms lost. We simulated our target density strategy on foot-and-mouth disease (FMD) of livestock and we asked whether target density reduces the overall epidemic impact of FMD.
To answer our question, we simulated culling strategies using farm demography of the counties Cumbria and Aberdeen in the United Kingdom. We simulated FMD spread across 7,884 farms in Cumbria and 2,925 farms in Aberdeen. Cumbria has a greater average animals per farm than Aberdeen, but they have similar local farm densities. Since resources are often limited to fight outbreaks, we varied the daily cull capacity for farms as well as the cull radius. We ran 1,000 replicates of 185 unique simulations varying in target density, daily cull capacity, and control radius to determine the effects on epidemic impact.
Results/Conclusions
We found that, in certain cases, target density-based culling is a more effective management strategy compared to scorched earth ring culling. We measured the impact of the epidemic by determining the number of animals and farms lost due to infection and culling as well as the length of the epidemic. Target density was a notably effective strategy in Aberdeen and Cumbria based on lost farms and epidemic length, but not based on lost animals. Capacity was impactful based on epidemic length in both counties and based on lost animals in Cumbria. Finally, radius was important based on all epidemic impact measurements except for epidemic length in Aberdeen.
The fixed effects of daily cull capacity and control radius differed in significance between counties. Aberdeen had an average of 280 animals per farm while Cumbria had 494, which is likely driving these differences. Therefore, farm demography should be considered when determining cull capacities and control radii for disease management. We are currently expanding the study to include target density-based ring vaccination to provide a potential modification to another common livestock management strategy.