Foot and Mouth Disease (FMD) outbreaks in eight separate dairy farms in Saudi Arabia from February 1992 to February 1998 have been studied. Four outbreaks were caused by serotype 0, three with serotype A and one with serotype Asia 1. The morbidity rates ranged from 4 to 69%. Clinical cases were observed at between 2 and 167 days after the last herd vaccination using a quadrivalent commercial vaccine containing O, AC and Asia 1 strains of FMD virus. The relationship between the vaccine strains and the viruses isolated during the outbreaks was assessed by the two dimensional neutralisation test and results expressed as r values. The 3 viruses caused type A outbreaks showed antigenic variation as to compared to the reference vaccine strain A22/Iraq (r= Manisa and Asia 1/India/79. The epizootiological data of each outbreak and the relationship between the outbreak virus isolate and the relevant vaccine strain(s) are interpreted and the possible factors contributing to the recurrence of FMD outbreaks in Saudi dairy farms are discussed.
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