Why do farmers kill entire healthy herds over a few mouth blisters?

Foot-and-mouth disease is fought so drastically because it spreads so easily, and often before anyone can see it. The virus causes a high fever, then blisters in the mouth and near the hooves of cattle, sheep, goats, pigs and other even-toed hoofed animals, and most animals eventually recover. But it is highly infectious, travelling on contaminated equipment, vehicles, clothing and feed, and an animal can pass it on before it shows signs of disease, so the virus may already have spread by the time an outbreak is detected. That is why containing it demands vaccination, strict monitoring, trade restrictions, quarantines, and the culling of both infected and healthy animals.

Trade adds a second reason. Vaccinated animals can become carriers, still harboring a little virus while looking healthy, so a country that vaccinates instead of slaughtering must wait longer to prove it is free of the disease. Many FMD-free countries therefore resist emergency vaccination, out of concern for the trade and economic cost of a long period without that status. Culling remains very controversial, given its drastic nature and its psychological effects on farmers: in Britain's 2001 outbreak, millions of animals were killed.

History shows both sides. In 1914, an outbreak that began in Michigan reached the Chicago stockyards and infected about 3,500 herds across the US, more than 170,000 animals. When the disease was found in Mexico in 1946, some Texans proposed an animal-proof wall along the border. Instead, the two countries cooperated: radio broadcasts and loudspeaker trucks explained to Mexican ranchers why US veterinarians were working on their livestock, ranchers whose cattle were culled were compensated, and over 60 million injections were given to livestock by the end of 1950. The disease was eradicated without a wall, though tensions still led to clashes with the military-protected vets.