Plaque purification of foot-and-mouth disease (FMD) type O viruses isolated from cattle in Saudi Arabia showed the presence of mixed serotype infections. Sixteen out of 31 samples collected between 1985 and 1991 also contained Asia 1 virus, a serotype which had previously only been isolated from a single outbreak in that country in 1980. Nucleotide sequences of the Asia 1 component of all these samples revealed little variation and showed that they were closely related to both a Russian lapinized vaccine virus strain (Asia 1/Tadzhikistan/64), and to a field isolate from Turkey (Asia 1/TUR/15/73). Although mixed FMD infections have been observed previously this is a first report of a serotype, considered to be exotic to a country, co-existing undetected for an extended period of time.

Foot-and-mouth disease (FMD) was the first animal disease to be attributed to a virus, and the second virus to be discovered [1]. It is a positive-sense, singlestranded RNA picornavirus and the sole member of the genus Aphthovirus. Each infectious virus particle contains a single strand of RNA approximately 8-5 kb long. This is translated into a single polypeptide which is then cleaved into the structural and non-structural virus proteins.

The foot-and-mouth disease (FMD) virus field specimen SAU/8/88 was previously shown to consist of a mixture of O and Asia 1 serotypes [15]. In this study, plaques representing the O and Asia 1 components isolated from the original epithelial virus suspension were used to construct mixtures of known ratios, and these were serially passaged in tissue culture. After each passage, the ratio of O to Asia 1 virus was calculated. The two virus populations were shown to be cycling through time. This cycling phenomenon has not been described before for FMD virus in tissue culture, but is consistent with current population theory.

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