Science paper calls for rapid, pre-emptive slaughter called 'ring culling' to combat foot-and-mouth disease

Research published 13 April by the journal, Science, endorses the rapid, pre-emptive slaughtering of livestock that may be infected with foot-and-mouth disease (FMD) as the best way to slow the epidemic.

"Ring culling," the pre-emptive slaughter of all animals on farms bordering infection sites, dramatically reduces the spread of disease, scientists at London's Imperial College School of Medicine reported.

Aggressive ring culling would help prevent the epidemic from spreading beyond the 45,000 farms within the areas currently classified as affected, said Neil Ferguson, Christl Donnelly, and Roy Anderson, whose research is based on data provided by Britain's Ministry of Agriculture, Fisheries, and Food (MAFF). If it isn't stopped, FMD will ultimately threaten farms throughout Great Britain, they said.

"Extensive culling is sadly the only option for controlling the current British epidemic, and it is essential that the control measures now in place are maintained during the long decay phase of the epidemic (several months) to ensure eradication," the authors wrote in Science.

Ferguson's team used a mathematical model to calculate the potential for disease transmission, given different scenarios. Primary conclusions presented in the Science paper were as follows:

• The faster that animals are slaughtered following the report of a suspected infection, the bigger the reduction of the epidemic.
• Even when infected animals are slaughtered within 24 hours after symptoms appear, some 30 percent of all U.K. farms will be affected by FMD over the months-long course of the epidemic--unless pre-emptive ring culling is also implemented. Within the heavily affected area of Cumbria, Dumfries and Galloway (CDG), some 79 percent of farms will be affected throughout the epidemic, assuming no ring culling.
• An even more aggressive approach--slaughtering sick animals within 24 hours, followed by ring culling over a 1.5-kilometer area within 48 hours--would protect many more farms. If such drastic measures were implemented consistently, only 16 percent of British farms (or 53 percent within the badly affected CDG region) would be culled over the course of the epidemic.
• Vaccinating surviving animals within a 3-kilometer radius of an infection site further reduces the number of farms affected by the epidemic.

First reported in Great Britain in February this year, FMD infects cloven-footed mammals including cattle, sheep, goats, and pigs. Adult animals may survive FMD with permanently reduced weight or milk yield, but the disease can kill the young. The epidemic is concentrated in Great Britain, but outbreaks have been reported in Ireland, France, and The Netherlands.

Logistical difficulties in processing large numbers of animals, and significant delays between reporting cases and culling of farms, are hindering efforts to fight FMD, the authors note. Restricting the movement of people and animals has helped to lower the incidence of infections that occur over a 9-kilometer distance, from 38 percent to 12 percent.

By 3 April, some 631,000 animals had been slaughtered, compared with 440,000 during the last FMD epidemic, in 1967. The latest outbreak, now believed to be approaching its peak, is expected to continue through the summer, scientists said.