Staphylococcal coagglutination

Staphylococcal coagglutination can specifically detect and identify IHNV grown in cell cultures or in infected fish tissue (Bootland and Leong, 1992). This test is simple and, since it takes only 15 min, it is one of the most rapid methods of diagnosing IHNV. It has the added benefit of being suitable for field use, because it only requires a light microscope, glass slides and one reagent. The test uses formalin-fixed Staphylococcus aureus cells sensitized with unadsorbed polyclonal rabbit anti-IHNV antiserum. When the antibody-coated cells are mixed with samples containing IHNV, the antibody specifically binds to the virus and causes the bacterial cells to coagglutinate.

For viruses grown in cell culture, the assay is specific for IHNV isolates belonging to all five electropherotypes, but the test is not very sensitive -106 pfu ml-1 is generally required. However, this should not affect its use in diagnosing IHNV, since cell cultures showing complete CPE typically have virus titres greater than 106 pfu ml-1. The test is not suitable for use in cell cultures pretreated with PEG, due to non-specific coagglutination.

The coagglutination test identified IHNV directly from fry homogenates, adult organs and ovarian fluids, as long as the virus titre was at least 106 pfu g-1. No false positives were observed. Detection of IHNV in adult carriers may not be as effective due to the low sensitivity of the coagglutination test. Samples with a low viral titre would have to be passaged in cell culture to amplify the virus before a coagglutination test could be used.

The binding of MAbs to staphylococcal cells or latex beads, although not yet successful, may increase the sensitivity of the assay and allow differentiation between different types of IHNV.

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