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Foot and Mouth Disease

Foot and Mouth Disease

Routes of infection

Vaccination

Resources

Foot and mouth disease is caused by a particular type of virus. The disease affects cloven hooved animals; that is, animals with hooves that are split into

two main segments. Examples of domestic cloven hooved animals include cattle, sheep, pigs, and goats. Wild cloven hooved animals that are susceptible to foot and mouth disease include elephants, hedgehogs, and rats.

Foot and mouth disease occurs all over the world. In parts of Asia, Africa, the Middle East, and South America, foot and mouth disease is common to the point of being a continual occurrence among various livestock herds. In other areas of the world, stringent control and inspection measures have made outbreaks infrequent. For example, there has not been an outbreak of foot and mouth disease in domestic animals in the United States since 1929. Canada last experienced an outbreak in 1952, and Mexico in 1954.

Other developed countries have been less fortunate. Outbreaks have occurred in Britain periodically since 1839. An outbreak in 18641866 devastated cattle herds throughout Britain, prompting legislation governing the transport and export of cattle. Outbreaks in the 1910s and from 19221924 saw slaughter introduced as an attempt to limit the spread of the disease. This control measure has been controversial ever since its implementation, because herds that may not be infected are often ordered destroyed.

In 19671968, over 400,000 domestic animals were slaughtered in an attempt to limit the spread of another outbreak. The latest large outbreak occurred in England beginning on February 20, 2001. The outbreak was declared over on January 14, 2002. In between these dates, 2030 cases were confirmed, and over 3,900,000 cattle, sheep, pigs, and goats had been slaughtered in England and Western Europe in order to contain the outbreak.

The virus that is responsible for the disease is a member of the viral family called Picornaviridae. Specifically, the virus is a member of the genus called Aphthovirus. A genus is a more detailed grouping of organisms based on common characteristics. The virus contains ribonucleic acid (RNA) as its genetic material. When the virus infects host cells, the RNA is used to make deoxyribonucleic acid (DNA), using the hosts genetic replicating processes. The viral DNA then forms the template for the production of viral RNA, which is packaged into new viral particles and released from the host cells.

This infectious process is destructive for the host cells that have been housed the virus. Typically, an infection is apparent as blistering in the mouth and feet. Hence, the name of the disease. The blisters cause the feet to become very tender and sore, so that animals have difficulty walking. Often an infected animal will suddenly become lame. Other symptoms of infection include slobbering and smacking of the lips, fever with shivering, and reduced milk production.

Routes of infection

Foot and mouth disease is very infectious. The infection can spread quickly through a herd of cattle and sheep. Large numbers of infectious virus particles are present in the fluid from the blisters. The virus is also present in saliva, milk, feces, and, because lungs cells can also become infected, even in the air that the animals exhales. In an advanced infection, the virus can be widespread through the body.

The virus spreads from animal to animal in a number of ways. Direct contact between an infected animal and non-infected animal can spread the virus. Indirect spread is possible, for example when an animal eats food that has been contaminated by the virus (from saliva, for example). The virus can also become airborne, particularly when it has been exhaled, thus an infection in one herd can quickly become widespread over the countryside. An outbreak of foot and mouth disease causes alarm in farmers many miles away from the site of the infection.

Direct spread of the virus is aided by the fact that the appearance of symptoms does not occur until anywhere from 2 to 21 days after infection has occurred. But, the infected animals can be spreading the virus during this time. Thus, infected animals can be present in a herd, allowing an infection to become established before the farmer becomes aware of a problem.

The virus can also be spread by dogs, cats, poultry, and wild birds and other animals. Usually, there are no symptoms of infection among these animals, as they are often carriers of the virus, without themselves being harmed by the virus. This secondary route of contamination makes an infection difficult to control, particularly on a farm where dogs, cats, and poultry abound.

Another indirect route of virus spread is via contaminated shipping trucks, loading ramps, and market stalls. Because the virus is capable of surviving for at least a month in cold and dark environments, contaminated transport and storage facilities can be reservoirs for virus spread for a long time after becoming contaminated. Stringent cleaning and disinfection of such facilities should be done routinely, and especially during an outbreak of foot and mouth disease.

Not all disinfectants are effective against foot and mouth disease. For example, phenol- and hypochlor-ite-based disinfectants are insufficient to kill the virus. Disinfectants such as sodium hydroxide, sodium carbonate, and citric acid are effective, likely because they destroy the protective structure that surrounds the genetic material.

The death rate among infected animals varies depending on the animal species and age. For example, in pigs and sheep the death rate among adults can be only 5%, while almost 100% of infected young animals will die. Survivors can continue to carry the virus in their bodies for one to two months (sheep) up to 24 months (cattle). Surviving pigs do not continue to carry the virus.

Even though many adult animals survive, they suffer. As well, the animals commercial value is diminished because of weight loss and reduced milk production. The economic losses can be considerable. Estimates of the losses that could result in the United States from a widespread outbreak are in the billions.

Foot and mouth disease is confirmed by the recovery of the virus from infected cells, or by detection of antibodies to the virus in host fluids.

Vaccination

There is a limited vaccine for foot and mouth disease. The vaccine consists of killed viruses. The viruses are unable to cause the disease, but stimulate the immune system to form the antibodies that will help protect a vaccinated animal from developing the disease. The full promise of a foot and mouth vaccine has not yet been fulfilled, however. This is because there are different types of the foot and mouth disease virus. Furthermore, these types have multiple subtle differences.

As of 2006, a single vaccine that is capable of stimulating immunity to all these different versions of the virus does not exist. As with human influenza vaccination programs, vaccination of animals against foot and mouth disease evaluates outbreaks and determines the types/subtypes of the virus that are most active. It is these that vaccination is directed against.

Vaccination against foot and mouth disease must be accomplished each year to confer protection to the virus. The cost of an annual vaccination of the domestic cattle, sheep, and swine of any country would run to the many millions of dollars. And the vaccine protects an animal from developing symptoms, but not from acquiring the virus. Thus, a vaccinated animal could acquire the virus and pass the virus on to other animals that were not vaccinated.

For the reasons of cost and the possible contribution of vaccination to the spread of the disease, the widespread use of foot and mouth vaccine has not been sanctioned. It is conceivable that future versions of the vaccine will be modified using the tools of biotechnology to provide long lasting immunity.

Resources

BOOKS

Mahy, Brian W.J. Foot-and-Mouth Disease Virus (Current Topics in Microbiology and Immunology). New York: Springer, 2005.

Sobrino, Francisco, and Esteban Domingo, eds. Foot and Mouth Disease. Boca Raton: CRC, 2004.

Woods, Abigail. A Manufactured Plague: The History of Foot-and-Mouth Disease in Britain. London: Earthscan, 2004.

Brian Hoyle

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