Evaluation of Different Strategies for the Use of ELISA Tests as First Screening Tools for Serologic Surveillance of Low Pathogenic Avian Influenza in the Belgian Poultry Sector

Since the emergence of the highly pathogenic avian influenza H5N1, avian influenza surveillance has been expanded in Europe. The serologic monitoring of domestic poultry is usually accomplished using the reference hemagglutination inhibition (HI) test for the detection of H5 and H7 subtypes. However, as the number of tested sera has been increasing, there is a need for another serologic method that could be used as a preliminary screening test. A comparison of four enzyme-linked immunosorbent assay (ELISA) tests (two indirect and two competitive) was conducted, and they showed good specificity and higher sensitivity than the HI test. The selected ELISA tests were then tested using approximately 800 field sera representative of different poultry species, and a simulation was done to determine the best strategy for screening. The first strategy was testing both gallinaceous and nongallinaceous sera with a competitive ELISA and using the HI test for H5 and H7 as a confirmatory test. The second strategy was testing only gallinaceous bird sera with the indirect ELISA with confirmatory H5 and H7 HI and all nongallinaceous sera by the H5 and H7 HI test. In the Belgian poultry context, the best strategy seems to be the use of a blocking ELISA as the primary screening tool to test all the poultry sera, followed by confirmation by H5 and H7 HI test subtyping.