Serological and immunological relationships among type A foot-and-mouth disease strains in South America.

Field strains of foot-and-mouth disease (FMD) virus which differ immunogenically from vaccine strains appear from time to time and may present serious problems for FMD campaigns. In order that appropriate measures can be taken it is important to determine the degree of differences between the vaccine strain and the variant strain soon after its appearance. With the A strains studied it was observed that a serological relationship by complement fixation test of greater than or equal to 0.40 would indicate sufficient protection of the vaccine strain for the field strain. With a lower relationship, however, the vaccine strains usually do not fully protect against the field strain. The mouse protection test can be used to determine how serious the lack of protection may be. With this test routine it is possible to issue alerts for intensified epidemiological surveillance within 2-3 weeks after the field strain is received at the Center.