A comparison of two serological methods for detecting the immune response after rabies vaccination in dogs and cats being exported to rabies-free areas

Levels of rabies virus neutralizing antibody in sera from dogs and cats wer e titrated to endpoint by the Rapid Fluorescent Focus inhibition Test (RFFI T) and retested by the RFFIT and the Fluorescent Antibody Virus Neutralizat ion test (FAVN). The two tests were compared for their ability to detect th e 0.5 international units/ml (I.U.) of antibody required by the World Healt h Organization and the Office International des Epizooties as the minimum r esponse for proof of rabies immunization. No difference was observed in sen sitivity or specificity for either method in tests of 168 sera from unvacci nated animals or 70 sera from vaccinated animals with high levels of neutra lizing antibody tan initial RFFIT titre of greater than or equal to 1.0 I.U .). Test to test variation occurred for results obtained by both RFFIT and FAVN for 95 sera from vaccinated animals with low to moderate levels of neu tralizing antibody (RFFIT titre < 1.0 I.U.). No significant differences wer e detected for the 95 sera in the frequency for one methodology more often than the other to have a positive response (greater than or equal to 0.5 I. U.), nor were significant differences detected for the symmetry (P=0.43) or the marginal homogeneity (P=0.39) of results obtained by the two methods. Both methods can adequately identify unvaccinated animals, but false positi ve and false negative results are possible for either method when a single test is used to measure the antibody response of low-responding vaccinated animals. Nucleotide sequence analysis identified several amino acid differences in s tocks of the challenge rabies virus from different laboratories. The small differences in neutralizing antibody titre that may result from mutations i n the challenge virus are not important for evaluating immunity induced by vaccines which are themselves prepared from a variety of different rabies v irus strains, but differences in the challenge virus, rather than differenc es in methodology, may account for at least some of the discrepant results reported in inter-laboratory surveys. Comparative studies of serological me thods for measuring rabies antibodies should use well-characterized unpassa ged virus stocks obtained from a single reference laboratory. (C) 1998 The International Association of Biological Standardization.