FMDV is a member of the family Picornaviridae, genus Aphthovirus. Seven immunologically distinct serotypes have been identified: A, O, C, SAT1, SAT2, SAT3, and Asia1. Infection with one serotype does not confer immunity against another. Virions appear smooth and round in outline, are non-enveloped, 27 nm in diameter, and have icosahedral symmetry. The genome is a single linear molecule of ssRNA and is about 7.2-8.4 kb in size. Virus replication is restricted to the cytoplasm of the cell. Genomic RNA is infectious.
Species of the family Bovidae, as well as sheep, goats, swine, all wild ruminants and Suidae are natural hosts of FMDV. More than 70 species of wild mammals belonging to more than 20 families are susceptible to infection. Species of the family Camelidae have low susceptibility. Due to the rapid replication cycle of the virus, the short incubation period, the high levels of virus shed by infected animals and the high susceptibility to infection of different species, foot and Mouth Disease (FMD) is one of the most contagious animal diseases. This in combination with the antigenic variability of the virus which permits sequential infection with different serotypes or subtypes and hampers vaccination, make FMD control very difficult.
The incubation period of FMDV is 2-14 days and virus shedding may precede clinical signs. The virus can cause persistent infection of the pharynx in cattle, sheep, goats, and other ruminants, although the mechanism of persistence is unknown and its epidemiological significance has been difficult to assess. Typical cases of FMD are characterised by a vesicular condition of the feet, buccal mucosa and, in females, the mammary glands. Clinical signs can vary from mild to severe and fatalities may occur, especially in young animals which may suffer myocarditis. The disease is usually most severe in dairy cattle and in pigs, especially in high production and intensively managed stock in which morbidity is high and convalescence is often protracted. The virus occurs in many parts of the world, often causing extensive epidemics in domestic cattle and swine, associated with significant economic losses. In contrast, infection in sheep and goats may pass unnoticed, but may act as a reservoir for infection of indicator species.
FMD cannot be differentiated clinically from other vesicular diseases, including Swine Vesicular Disease (SVD), Vesicular Stomatitis (VS), and Vesicular Exanthema (VE). Laboratory diagnosis of any suspected FMD case is therefore a matter of urgency. Subsequent virus characterisation can help to trace the transboundary origins of outbreaks and the emergence of new strains. It is also essential for vaccine selection.
FMDV is endemic in parts of Asia, Africa, the Middle East and South America (sporadic outbreaks in free areas). The World Animal Health Organisation (OIE) provides a list of countries that are officially FMD free, either with or without vaccination and in their entirety or within a free zone within their national territories. Australia, Europe and the United States are amongst the FMD-free countries. FMD-free regions restrict the introduction of animals and animal products from countries where FMDV occurs and deal with incursions of infection by prompt recognition, tracing the spread of the virus, imposition of animal movement restrictions and slaughter and/or vaccination. In endemic regions vaccination is critical for control, but as it provides short-lived and serotype-specific protection, mass vaccination is very resource intensive.
For more information:
- Veterinary Virology, 3rd Edition (Academic Press, 1999)
- Diseases of Swine, 8th Edition (Iowa State University Press, 1999)
- New edition of Coetzer et al has just come out and has a chapter by Gavin Thomson that gives a very good overview.