Foot-and-mouth virus spreads before symptoms show, which makes it a farmer's nightmare
Foot-and-mouth disease rarely kills adult livestock, yet countries will cull healthy herds and ban meat exports to stop it. The reason is its sheer ease of travel: on boots, lorries, feed, even the wind. Infected pigs breathe out 30 times more airborne virus than cattle or sheep, and animals can pass it on before looking ill.
The virus mainly strikes even-toed hoofed animals such as cattle, pigs, sheep, goats, deer and bison, and it has turned up in hedgehogs and elephants. After an incubation of one to 12 days comes a high fever, then blisters in the mouth and on the feet that cause drooling and lameness. Most animals recover, but milk yields can drop sharply, weight loss can linger for months, and the young may die of heart inflammation. People are infected only extremely rarely; the childhood illness hand, foot and mouth disease is a different infection altogether.
Its routes are many: direct contact, drifting aerosols, contaminated vehicles and fodder, farm workers' clothing and skin, food scraps, and bull semen. Animals that cannot catch it can still carry it. In Canada in 1952, an outbreak flared again after dogs dragged bones away from carcasses. Pigs are especially vulnerable through what they eat, and the virus can survive for long periods in soybean meal. One 2016 experiment found infected pigs spreading disease for a full day before showing any signs.
Some recovered cattle, buffalo, sheep and goats become silent carriers, holding a trace of virus in the throat for up to one or two years. Pigs never do. Transmission from carriers is thought very unlikely in the field, though a laboratory experiment showed it can happen. Because vaccinated animals can also become carriers, proving a country is free of the disease takes longer after vaccination than after culling, so many disease-free nations resist emergency vaccination for fear of losing trade status.
First documented in 1870, the virus has seven serotypes and mutates at a rate typical of RNA viruses, which undermines vaccines. Genetic analysis traces a common ancestor to roughly 481 years ago, in the early 16th century.
Source: Foot-and-mouth disease
