The relative haemagglutinin-
neuraminidase (HN)
antigen content of inactivated Newcastle disease virus (NDV)
vaccines from different manufacturers was determined by means of an
Enzyme-Linked
Immunosorbent Assay (ELISA) according to Monograph 870 of the European Pharmacopoeia (Ph. Eur.). Wide ranges of reactivity of the different products were observed. When comparing the antibody responses from chickens vaccinated with
vaccines showing either high or low reactivity in the
antigen ELISA it was found that approximately the same titres of
antibodies were induced in the chickens. One hypothesis is that the inactivation procedures used to inactivate the
Newcastle disease antigen may alter the
antigenic determinant recognised by the
monoclonal antibody used. An alteration of the
antigen would influence the binding by the
monoclonal antibodies used as catching and detection
antibodies in the ELISA which may result in a lower ELISA reactivity. It was also found that HN
antigen of two inactivated Paramyxovirus 1 (PMV-1)
vaccines for pigeons could not be measured in the ELISA. For these
vaccines the
antigen-ELISA based on
monoclonal antibody IDNDV134.1 cannot be used. Our experience shows that a thorough knowledge of the products tested with the ELISA and their influence on the test method is essential to avoid misinterpretations of the test results. The level of ELISA reactivity should not be used for the comparison of
vaccines. Furthermore, prediction of the ability of an unknown
vaccine to induce
antibodies based on the level of ELISA reactivity is not possible. The results (level of reactivity) of the
antigen ELISA for the in vitro potency testing of inactivated
Newcastle disease vaccines should therefore be carefully interpreted. However, by knowing the performance characteristics of the NDV
antigen ELISA and the characteristics of the
vaccines to be tested it becomes a valuable tool for the control of inactivated
Newcastle disease vaccines in our laboratory. The implementation of this ELISA method for the batch release testing markedly reduces the number of chickens and the time required for batch release testing.