Publication Abstract

Problems arising from the use of immunoassays for the detection of viral pathogens directly in fish tissues

K. Way

The enzyme immunoassay (EIA) is a widely used technique to aid viral identification in diagnostic laboratories. Specific EIAs are commonly used to confirm the isolation of fish pathogenic viruses in cell cultures and have also been employed to detect virus directly in fish tissue homogenates which also makes the EIA a useful tool in research areas such as viral pathogenicity studies. Enzyme immunoassays have been developed for rapid diagnosis of acute infections of epizootic hematopoietic necrosis virus (EHNV) (Hyatt et al., 1991), infectious pancreatic necrosis virus (IPNV) (Dixon & Hill 1983, Hattori et al., 1984, Sanz & Coll 1992b), spring viraemia of carp virus (SVCV) (Way 1991, Rodak et al., 1993) and viral haemorrhagic septicaemia virus (VHSV) (Way & Dixon 1988, Olesen & Jorgensen 1991, Mourton et al., 1992, Sanz & Coll 1992a & 1992b, Way I 996).

When EIAs are used for detection of antigen in fish tissues the important requirements are that the assay retains specificity and sensitivity. These two parameters are often assessed in terms of the signal or noise produced by a sample when tested in an EIA. The signal can be defined as the colour reaction (absorbance value) produced by a specific antigen positive sample in an EIA while the noise is the 'background' colour produced by non-specific interfering components in a sample. Thus, more accurate assays will generally have high signal to noise (S/N) ratios. However, it cannot be assumed that an assay which provides an accurate identification of viral antigen in cell culture extracts will retain this accuracy when applied to tissue homogenates. A number of factors can combine to affect the accuracy of the assay and some authors have acknowledged that it is often necessary to modify EIAs for detection of antigen in tissue homogenates to maintain high S/N ratios. Immunoassays for detection of viral pathogens have usually been modified in order to enhance the specific signal produced (Way 1991, Mourton et al., 1992, Sanz & Coll 1992a). Additionally, this author has investigated the reduction of noise levels produced by tissue homogenates in an EIA (Way 1991) and has also highlighted other factors which must be considered in order to develop assays that are sufficiently robust and accurate for field use (Way 1996).

This paper describes the examination of a number of factors which can affect the accuracy of EIAs and investigations into modifications to the assays which have been designed to reduce their influence.

Reference:

K. Way, 1996. Problems arising from the use of immunoassays for the detection of viral pathogens directly in fish tissues. pp247-256 In: Methodology in Fish Diseases Research. (Barnes, A.C. et al. (eds)). Fisheries Research Services, Aberdeen