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Foot-and-mouth disease (FMD) viruses are acid-labile picornaviruses and are grouped into seven immunological types designated O, A, C, SAT 1, SAT 2, SAT 3 and Asia 1 (Brooksby, 1958). Virus strains within each type are further classified into subtypes on the basis of serological tests (Brooksby, 1952).
Whereas several picornaviruses are known to haemagglutinate, including Coxsackie viruses (Goldfield, Srihongse & Fox, 1957), ECHO viruses (Goldfield et al. 1957; Lahelle, 1958), bovine picornaviruses (Moscovici & Maisel, 1958), encephalomyocarditis viruses (Craighead & Shelokov, 1961) the gd vii strain of mouse encephalomyelitis virus (Lahelle & Horsfall, 1949) and human rhinoviruses (Stott & Killington, 1972), this property has so far not been demonstrated conclusively with FMD viruses. Michelsen and colleagues (Michelsen, 1949; Michelsen & Schjerning-Thiesen, 1949; Michelsen & Bachrach, 1950) have reported that rat erythrocytes are agglutinated by the vesicular fluid of guinea pigs infected with type O, A, and C viruses, but this finding has not been confirmed (Singh, Bansal & Malik, 1968).