Foot-and-mouth disease

Foot-and-mouth disease (FMD), sometimes called hoof-and-mouth disease, is a highly contagious viral disease of cattle and pigs. It can also infect deer, goats, sheep, and other animals with cloven hooves, as well as elephants, rats, and hedgehogs. Horses are not susceptible to FMD. The cause of FMD was first shown to be viral in 1897 by Friedrich Loeffler. He passed the blood of an infected animal through a fine porcelain-glass filter and found that the fluid that was collected could still cause the disease in healthy animals. FMD occurs throughout much of the world, including parts of Europe, Africa, Asia, and South America. While currently (July 2001) some countries, including Canada, the United States, and Australia, have been relatively free of FMD for some time, it's wide host range and rapid spread represent cause for international concern.

In cattle, foot-and-mouth disease is characterized by high fever that declines rapidly after two or three days; blisters inside the mouth that lead to excessive secretion of stringy or foamy saliva and to drooling; and blisters on the feet that may rupture and cause lameness. Adult animals may suffer weight loss from which they do not recover for several months, and in cows, milk production can decline significantly. Though most animals eventually recover from FMD, the disease can lead to myocarditis (inflammation of the heart muscle) and death, especially in newborn animals. Some infected animals remain asymptomatic, that is, they do not suffer from disease or show signs of the disease; but they are carriers of FMD and can transmit it to others.

Infection with foot-and-mouth disease tends to occur locally, that is, the virus is passed on to susceptible animals through direct contact with infected animals or with contaminated pens or vehicles used to transport livestock. The clothes and skin of animal handlers such as farmers, standing water, and uncooked food scraps and feed supplements containing infected animal products can harbor the virus as well. Cows can also catch FMD from the semen of infected bulls. Control measures include quarantine and destruction of infected livestock, and export bans for meat and other animal products to countries not infected with the disease.

Foot-and-mouth disease is caused by an Aphthovirus [ap-thoh"-vei-r&s] of the viral family Picornaviridae. [pi-kawr-n&-vir"-id-ei] The members of this family are small (25-30 nm), nonenveloped icosahedral [ei-koh-s&-hee-dr&l] viruses that contain single-stranded RNA (ribonucleid acid, the viral genetic material). When such a virus comes in contact with a host cell, it binds to a receptor site and triggers a folding-in of the cell membrane. Once the virus is inside the host cell, it's protein coat dissolves. New viral RNA and components of the protein coat are then synthesized in large quantities and assembled to form new viruses. After assembly, the host cell lyses (bursts) and releases the new viruses.

Humans can be infected with foot-and-mouth disease through contact with infected animals, but this is extremely rare. Because the virus that causes FMD is sensitive to stomach acid, it cannot spread to humans via consumption of infected meat. In the UK, the last confirmed human case occurred in 1967, and only a few other cases have been recorded in countries of continental Europe, Africa, and South America. Symptoms of FMD in humans include malaise, fever, vomiting, red ulcerative lesions (surface-eroding damaged spots) of the oral tissues, and sometimes vesicular lesions (small blisters) of the skin. There is another viral disease with similar symptoms, commonly referred to it as “hand, foot, and mouth disease”, that occurs more frequently in humans, especially in young children; this disease is caused by a different virus of the family Picornaviridae, namely, an Enterovirus called Coxsackie A. Because FMD rarely infects humans but spreads rapidly among animals, it is a much greater threat to the agriculture industry than to human health. Farmers around the world can lose milliards of dollars a year during a foot-and-mouth epidemic, when large numbers of animals are destroyed and revenues from milk and meat production go down.