Foot and Mouth Disease
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 out-break of foot and mouth disease.
Not all disinfectants are effective against foot and mouth disease. For example, phenol- and hypochlorite- 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 animal's 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.