Because FMD is such a mobile pathogen, Ferguson and his colleagues argue that unless aggressive ring culling is put in place, it will spread farther and threaten a vast number of farms throughout Great Britain and elsewhere. The team used data from MAFF to model the potential for disease transmission between farms under different sorts of conditions. In doing so, they generated several predictions.
Most grim, one forecast showed that even if infected animals are slaughtered within 24 hours after symptoms appear (which became U.K. government policy in late March), FMD will eventually affect 30 percent of all British farms. So, too, without culling, 79 percent of the farms in Cumbria, Dumfries and Galloway will face FMD. MAFF reports that on April 10, about 60 percent of affected farms managed to complete slaughtering within 24 hours¿and the remaining cases took no longer than 30 hours.
The model also revealed that limiting exposure to only 16 percent of all British farms would require nothing short of slaughtering sick animals within 24 hours¿and killing healthy animals within a 1.5-kilometer radius within 48 hours. Were such a measure put instituted, Ferguson and his colleagues say that only 53 percent of the farms in the most severely affected areas would be culled over the full course of the epidemic.
Moreover, the model demonstrated that vaccinating animals within three kilometers of an infection site would further reduce FMD's spread but that vaccination alone would not work as well as culling. "Ring vaccination policies need to be more extensive than comparable culling policies, since vaccination has little effect on the infectiousness of animals already infected with the virus," the scientists observe. Other problems with vaccination are that it doesn't always work, and inoculated animals can no longer be distinguished from previously infected animals.
Containing the current outbreak is, of course, top priority now, but Ferguson and his colleagues also make several suggestions for the future. "Ever-increasing international trade has greatly increased the potential for the spread of FMD with more frequent movement of animals over long distances," they write. "A thorough international review of policy options is required."