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Foot-and-mouth disease (FMD) is a highly infectious viral infection of cattle, pigs, sheep, goats, buffalo, and artiodactyl wildlife species. It is characterized by fever; vesicles in the mouth and on the muzzle, teats, and feet; and death in young animals. All species of deer and antelope, elephant, and giraffe are susceptible to FMD, but camels are resistant to natural infection, and the smaller camelids such as alpacas and llamas, although susceptible, are probably of no epidemiologic significance. Camels, mice, and guinea pigs can be infected experimentally.
FMD is endemic in the Middle East; in Africa, excluding South Africa, Botswana, Namibia, and Zimbabwe and possibly Tunisia and Morocco; and in South America, excluding Chile, southern Argentina, Uruguay, Guyana, Surinam, and French Guiana. Europe is free of FMD, although there have been occasional outbreaks that have quickly been eliminated. Australasia, Japan, Indonesia, and Korea are free of FMD, but FMD is endemic in all other Asian countries, including the Philippines and Malaysia.
Etiology: FMD is caused by an aphthovirus of the family Picornaviridae. There are seven immunologically distinct serotypes: A, 0, C, Asia 1, and SAT (Southern African Territories) 1, 2, and 3. Within each serotype, there is a large number of strains that exhibit a spectrum of antigenic characteristics; therefore, a number of vaccine strains for each serotype, particularly 0 and A, are required to cover the antigenic diversity. Strains are characterized by their genomic relationships and their antigenic similarities with established vaccine strains. (Previous classification into subtypes became untenable as the number of subtypes rapidly increased.)
The virus is quickly inactivated outside the pH range of 6.0-9.0 and by desiccation and temperatures >56ºC, although residual virus may survive a considerable time when associated with animal protein (for instance, a proportion of FMD virus in infected milk will survive pasteurization at 72ºC for 15 sec). The FMD virus is resistant to lipid solvents such as ether and chloroform. Because of the sensitivity of the virus to acid and alkaline pH, sodium hydroxide, sodium carbonate, and citric or acetic acid are effective disinfectants.
Transmission, Epidemiology, and Pathogenesis: Transmission of FMD is generally by contact between susceptible and infected animals. Infected animals have a large amount of aerosol virus in their exhaled air, which can infect other animals via the respiratory or oral routes. All excretions and secretions from the infected animal contain virus, and virus may be present in milk and semen for up to 4 days before clinical signs appear. Aerosol FMD virus can spread a considerable distance as a plume, depending on weather conditions, particularly when the relative humidity is >60% and when the topography of the surface over which it is dispersing does not cause turbulence. FMD has been transmitted to calves via infected milk, and milk tankers carrying infected milk have been implicated in the spread of disease between farms. Fodder can become contaminated after contact with infected animals, and iatrogenic spread of FMD has been reported, but mechanical spread by nonsusceptible species of wildlife such as birds and dogs is unlikely. A typical scenario for the introduction of FMD into a previously clear area is for pigs to be fed imported food derived from an infected animal (as meat, offal, or milk) and then virus spreads by aerosol from the infected pigs to cattle, which are the species most likely to be infected by the respiratory route because of their large respiratory volume. FMD virus can survive in dry fecal material for 14 days in summer, in slurry up to 6 mo in winter, in urine for 39 days, and on the soil between 3 (summer) and 28 days (winter).
Ruminants that have recovered from infection and vaccinated ruminants that have contact with live FMD virus can remain infected and carry the virus in the pharyngeal region--for up to 2 1/2 years in cattle, 9 mo in sheep, and probably lifelong in the African buffalo. Experimentally, it has not been possible to show transmission from a carrier bovid to an in-contact susceptible, but there is evidence that under field conditions these carrier animals initiate new outbreaks of disease. FMD virus can be recovered from carrier animals by culturing a sample of pharyngeal mucus and superficial cells (collected using a probang cup) on susceptible tissue culture, such as primary bovine thyroid cells. However, the technique is probably only 50% reliable in identifying a carrier using a single sample because the quantity of virus found in the pharynx varies on different occasions.
The primary site of infection and replication is usually the mucosa of the respiratory tract, although the virus can enter through skin abrasions or the GI tract. Replication then occurs in the local lymph node, and infection subsequently spreads via the blood stream to predilection sites in the epithelium of the mouth, muzzle, feet, and teats, and also to areas of damaged skin (eg, the knees and hocks of lame pigs kept on concrete). Vesicles develop at these sites and rupture, usually within 48 hr. The viremia persists for ~5 days.
Clinical Findings: The incubation period for FMD is 2-14 days, depending on the infecting dose, susceptibility of the host, and strain of virus. The clinical signs are more severe in dairy cows and intensively reared pigs than in sheep and goats, and FMD has frequently been ignored or misdiagnosed in small ruminants. After the incubation period, a fever of up to 106ºF (41ºC) develops, the animal is anorectic and salivates and stamps its feet as vesicles develop on the tongue, dental pad, gums, lips, and on the coronary band and interdigital cleft of the feet. Vesicles may also appear on the teats and udder, particularly of lactating cows and sows, and on areas of skin subject to pressure and trauma, such as the legs of pigs. Young calves, lambs, kids, and piglets may die before showing any vesicles because of virus-induced damage to the developing cells of the myocardium. Milk yield drops dramatically in milking animals, and all animals show a loss in condition and growth rate that may persist after recovery. Sheep and goats may develop only a few vesicles on the coronary band and in the mouth. Vesicles in the mouth, even when severe, usually heal within 7 days, although recovery of the tongue papillae takes longer. Lesions on the mammary gland and feet frequently develop secondary infections, resulting in mastitis, underrunning of the sole, and chronic lameness. In pigs, the complete horn of the toe may be lost. Cattle and deer may also lose one or both horns of the foot.
Diagnosis: In cattle and pigs, the clinical signs of FMD are indistinguishable from those of vesicular stomatitis, and in pigs from those of swine vesicular disease and vesicular exanthema. Samples of vesicular epithelium or vesicular fluid should be sent in phosphate-buffered saline (pH 7.4) to the national laboratory responsible for the diagnosis of FMD, or otherwise to the OIE/FAO World Reference Laboratory for FMD, Pirbright, UK, after having previously made arrangements for submission of samples. Samples are prepared as a 10% suspension and inoculated onto susceptible tissue culture and directly typed by ELISA. Isolated FMD virus is characterized by antigenic comparison with existing FMD vaccine strains, and the nucleotide sequence of a segment of the 1D gene is determined for comparison with other strains of the same serotype from the same region to identify a possible origin of the outbreak. It is essential that samples are kept as close as possible to pH 7.4 to prevent destruction of the FMD virus and antigen, and that they are securely packed in double leak-proof containers that comply with national and, when appropriate, international regulations for the shipment of pathologic and hazardous material.
Treatment and Control: The occurrence of FMD in countries previously free of the disease can have a major effect on local and international trading arrangements. Many countries free of FMD have a policy of slaughter of all affected and in-contact susceptible animals and strict restrictions on movement of animals and vehicles around infected premises. After slaughter, the carcasses are either burned or buried on or close to the premises, and the buildings are thoroughly washed and disinfected with mild acid or alkali and by fumigation. Tracing is done to identify the source of the outbreak and premises to which FMD virus could have already been transmitted by infected animals or animal products, by contaminated vehicles or people, or by aerosol. In areas or countries free of FMD in which this is not possible, control is by movement restriction, quarantine of affected premises, and vaccination around (and possibly within) the affected premises. This has the disadvantage that many carrier animals may remain after the outbreak, and quarantine may not be sufficiently long to prevent their subsequent movement. In countries in which FMD is endemic, protection, particularly of high-yielding dairy cattle, is by a combination of vaccination and prevention of FMD virus entering the dairy premises. This can be difficult if prevalence of FMD in the unvaccinated population is high and climatic conditions are suitable for aerosol transmission of the virus. FMD vaccine is a killed preparation and, at best, affords good protection against challenge for 4-6 mo. However, the antigenic diversity of strains of FMD virus within each of the serotypes is an additional complication, so it is necessary to ensure that vaccines contain strains antigenically similar to the potential outbreak strains. Otherwise, the duration of immunity provided by vaccines containing dissimilar strains may be very short. FMD vaccines for pigs require an oil adjuvant, whereas those for ruminants may contain an oil or aluminum hydroxide/saponin adjuvant. There are currently no alternatives to vaccine antigens derived from whole virus grown in tissue culture and then chemically inactivated.