Comparison of laboratory techniques for the evaluation of the antigenic potency of foot-and-mouth disease virus cultures and vaccines

Abstract

Forty-seven suspensions of type A virus grown in surviving bovine tongue epithelium (Frenkel) cultures were compared in a quantitative complement fixation test (CFT) and an antibody combining test (ACT) to evaluate the antigenic mass of the 140 S component, and with three assay methods for infectivity titration. The cultures were then converted into vaccines and the potency of these was measured with a guinea-pig PD 50 test and again with the ACT. The relationship between the results obtained with different methods used for titration of infectivity, i.e. baby mice and plaque counting in BHK cells in monolayer (BHK-M) or suspended in agar (BHK-S) was poor and, with the exception of the BHK-S technique, showed little correlation with the results of the serological tests. The correlation coefficient between values obtained by the CFT and the ACT on virus cultures was 0.82 and that between CFT and ACT on vaccines 0.88. The guinea-pig PD 50's of 19 vaccines were compared with the ACT and with the CFT and ACT of the corresponding cultures yielding r values of 0.74, 0.62 and 0.62 respectively. Regression lines are presented for the different relations. An accurate cattle PD 50 was determined for two vaccines of the series, using groups of 20 animals per vaccine dilution. The relationship between guinea-pig PD 50 ( X) and cattle PD 50 ( Y), expressed by the regression formula Y = 1.05 X + 0.84, showed that differences in cattle PD 50's are of the same magnitude as those observed in the guinea-pig test, provided that sufficient numbers of animals are used for both tests. The results of the quantitative CFT and ACT were in good agreement with the guinea-pig test and as such these tests can provide valuable information on the quality of Frenkel cultures.