Serological methods used for rabies post vaccination surveys: an analysis

The assessment of fox immunity following oral rabies vaccination (ORV) is commonly applied to assess the efficacy of an ORV campaign in the field. Several ELISA kits have been developed and validated for their use for rabies serology in wildlife as an alternative to neutralizing techniques (NT), such as the fluorescent antibody virus neutralization test (FAVN) and the rapid fluorescent foci inhibition test (RFFIT). At a European level, NT and ELISA tests are used interchangeably and on different types of samples collected for vaccination follow-up. This has resulted in a difficulty in comparing the results generated with different diagnostic tools. We have evaluated (a) the effect of two different matrices commonly used for serology in red foxes on the results of the FAVN and (b) the performance of two commercially available ELISAs in comparison with the FAVN, as a gold standard, using a panel of over 700 field fox samples. Moderate agreement was observed when comparing results from different matrices. We found a very low level of agreement and low values of relative sensitivity and specificity of the ELISAs tested in comparison with the FAVN. Our findings confirm, using a vast collection of field samples obtained during post-vaccination surveillance campaigns in Italy, the need for improved reliability of certain serological tests.