FMD Current Topic

Spread of Type O PanAsia Strain from the Far East

Dr James Irvine, FRSE DSc FRCPEd FRCPath FInstBiol

Teviot Scientific Consultancy, Edinburgh, UK

The strain of FMDV responsible for the 2001 epidemic that occurred in the UK is serotype O (Pan Asia Strain), which belongs to the Near East-South Asia topotype. This particularly virulent strain was first identified in northern India in 1990, it spread westwards into Saudi Arabia and neighbouring countries, and by 1996 it had reached Turkey, Greece and Bulgaria (Figure 1). The virus also spread east into Bangladesh and China. By 2000 the PanAsia strain had reached Mongolia, the Russian Federation, South Africa, Japan and Korea. Table 1 shows those countries that have experienced outbreaks of Serotype O since 1990.

Figure 1: Conjectured spread (as estimated in 2000) of the Pan-Asia lineage of the Near East-South Asia topotype of FMDV-O. (Reproduced from reference 1)

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As stated by the FAO (Food and Agriculture Organisation) in 2000 (1), the viruses causing these outbreaks of FMD all belong to a single genetic lineage and the nucleotide sequences of their VP1 genes differ by no more than 5 percent despite having been isolated over an 11-year period. In East Asia the strain coexisted with other strains of serotype O, particularly the pig-adapted strain which is still causing outbreaks in Viet Nam, in China, Hong Kong Special Administrative Region and in the Philippines. It is not clear why this pandemic strain of serotype O has been so successful; clinically it has caused very high mortality rates in lambs in Iraq, but has also been associated with subclinical infection in cattle in Taiwan Province of China and Japan. The rapid spread of a pandemic strain such as this clearly demonstrates the ability of newly emerging FMD viruses to infiltrate a wide geographic area and to cause epidemics in countries which have been free from the disease for many years.

Table 1: Spread of serotype O PanAsia strain from India 1990.

The spread of this single strain of FMD is unique. It appears to have a competitive advantage over other O strains which it has out-competed (2). Exactly what this advantage is remains unclear. Studies have shown that pigs infected with the PanAsia strain excrete considerably less virus as aerosol in comparison to pigs infected with the A strain that caused the 1967/8 outbreak. Another characteristic of the PanAsia strain is the absence or mildness of clinical symptoms in adult sheep. This is one reason for delayed detection during the UK outbreak.

References

1. FAO (2000). EMPRES Transboundery Animal Disease Bulletin. No 14/2. (View article)