SUMMARYA highly virulent strain of infectious bursal disease virus (IBDV) was isolated from the field and propagated in SPF chickens, causing up to 100% mortality. Although it still belongs to the standard serotype 1 IBD viruses, serological typing with monoclonal antibodies showed an antigenic drift in this pathogenic strain. Conventional 'intermediate' IBD vaccines are probably more antigenically related to the pathogenic strain than the mild ones and were effective in protecting SPF chickens against challenge.

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The potential of a capture enzyme-linked immunosorbent assay (ELISA) specific for chicken interferon-g (ChIFN-g) has been evaluated as a tool to assess cell-mediated immunity (CMI) in the chicken. In a first step, ChIFN-g production and cell proliferation of mitogen-activated chicken splenocytes have been compared. In general, for each of the stimulation conditions where significant proliferation was observed, production of ChIFN-g could be measured by ELISA. In our hands, the combination of ionomycin and phorbol-12-myristate 13-acetate or the use of recombinant chicken interleukin-2 gave the most satisfactory results. Then, the CMI response induced by live or killed Newcastle disease virus (NDV) vaccines has been evaluated sequentially by ex vivo antigen-specific ChIFN-g production and cell proliferation of splenocytes from immune chickens. The ex vivo data showed that both types of NDV vaccines are capable of stimulating CMI responses to NDV in chickens as measured by the ChIFN-g ELISA. However, most of the chickens vaccinated with the live vaccine produced ChIFN-g after antigen recall stimulation, from 2 to 4 weeks after vaccination, when only some chickens vaccinated with the inactivated vaccine showed a specific response 4 weeks after vaccination. No significant proliferative responses to either NDV vaccine were detectable during the 4 weeks of the study. From our results, it appears that antigen-specific ChIFN-g production can be used as a good indicator of actively acquired immunity to NDV and that the sensitivity range of the capture ELISA test is well adequate to measure ex vivo release of ChIFN-g.

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Monoclonal antibodies detect evident antigenic variations in NDV HN and F protein. However, the A/PMV-1 viruses can be identified by HI test using a preparation made of the combination of two different monoclonals. A primary evaluation of the pathogenicity of the isolated viruses can be made by HI test using monoclonal antibodies but needs always confirmation using conventional pathogenicity tests.

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