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How to Stop Foot and Mouth Disease

A new mathematical analysis suggests that nothing short of preemptive killing will limit the current epidemic's ultimate reach
aging



J. W. Stewart
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HEALTHY LIVESTOCK living near farms affected by foot and mouth disease may face slaughter to contain the current epidemic.

Since early February foot and mouth disease (FMD) has spread like wildfire through the English countryside, fanning out from a single pig herd in Northumberland to more than a million animals on some 45,000 farms. The disease¿which infects not only pigs but cattle, sheep and other cloven-footed mammals¿has also cropped up in Ireland, France and the Netherlands. As of last Thursday, April 12, the British Ministry of Agriculture, Fisheries and Food (MAFF) reported that 1,580,000 animals had been slaughtered or marked for slaughter so far.

More bad news followed that update on Friday, April 13th, when scientists suggested in Science that the best way to contain the epidemic may be to kill the healthy livestock on farms neighboring those affected¿a tactic known as ring culling. "Extensive culling is, sadly, the only option for controlling the current British epidemic," wrote researchers from the Imperial College School of Medicine, "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 scientists¿Neil Ferguson, Christl Donnelly and Roy Anderson¿based their recommendation on a detailed mathematical model of the disease's spread during February and March.

In fact, the current outbreak of FMD is thought to be reaching its peak, thanks in part to increasing restrictions and control efforts. The number of veterinarians involved has grown from just over 200 in February to more than 1,600 now. And the daily average of animals slaughtered rose from 19,000 in mid-March to 34,000 in the beginning of April. FMD almost never affects people (there is only one recorded case in England from 1966) and doesn't typically kill adult animals. But the epidemic has been economically devastating for many farmers, particularly those in Cumbria, Dumfries, Galloway and Devon.

England's last major outbreak of FMD, in 1967, claimed many fewer animals (a total of 440,000), but a number of factors may have helped this year's epidemic flare up so quickly. "The initial spread was greatly influenced by the frequency of movement of animals around the country and their mixing in livestock markets," Ferguson and his colleagues note. (When the government did restrict the movements of people and animals, the incidence of infections over nine kilometers dropped from 38 percent to 12 percent, the authors say.) And "due to logistical difficulties in processing very large numbers of animals," they add, "there were initially significant delays between the reporting of a suspect case in the U.K. and culling of the farm."

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CLOVEN-FOOTED MAMMALS--cattle, goats, pigs, sheep, deer and the like--succumb to foot and mouth disease. Adult animals rarely die from the infection but do suffer permanent weight reduction and a loss of milk yield. Young animals can die without ever showing symptoms.

In addition, the virus that causes FMD, an aphthovirus from the family picornaviridae, has remarkable reach, especially in cold temperatures. And the strain now in play, pan-Asiatic O type, is highly contagious. FMD manifests itself as a fever, followed by namesake blisters in the mouth or on the feet. Because sick animals begin excreting the virus a few days before symptoms appear, they can readily pass it on to others through direct contact.

The virus can also survive outside a host for a month or more in damp soil or in meat from the carcass of infected animals. What is particularly dangerous is that FMD travels by air within water droplets. Scientists estimate that wind can transmit virus particles some 60 kilometers over land and 250 kilometers over water.

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."

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