COMPARISON OF IMMUNOHISTOCHEMISTRY AND VIRUS CULTIVATION FOR DETECTION OF VIRAL HEMORRHAGIC SEPTICEMIA VIRUS IN EXPERIMENTALLY INFECTED RAINBOW-TROUT ONCORHYNCHUS-MYKISS

Immunohistochemistry and virus isolation were compared for their ability to detect viral haemorrhagic septicaemia virus (VHS) in experimentally infected rainbow trout Oncorhynchus mykiss. The fish were divided into 3 groups (I to III), and infected, respectively, by bath challenge with 10(2), 10(3.7), and 10(5) TCID50 ml-1 water of a VHS virus strain serologically similar to reference strain F1. The cumulative mortality in Groups I to III was 44, 64, and 96%, respectively, at 14 d post infection (p.i.). Statistical comparison of the results from all groups showed that virus isolation was significantly more sensitive than immunohistochemistry (p < 0.05). The same result was obtained by separate comparison of Groups I and II, but there was no significant difference between the 2 methods for Group III (10(5) TCID50). Immunohistochemically, virus antigens were detected early (2 to 4 d) in endothelial cells lining venules and sinusoids and in the haematopoietic cells in the head kidney, as well as in interstitial macrophages and melanomacrophages; they were detected subsequently in hepatocytes (4 d p.i.) and exocrine pancreatic cells (6 d p.i.). Presence of virus was accompanied by cell degeneration and necrosis from 4 d p.i. in all positive organs. These findings show that virus cultivation is the most sensitive method for detection of virus, although immunohistochemistry may represent an adjunct to diagnosis of acute VHS virus infections. The main advantage of immunohistochemistry is the possibility of simultaneous demonstration of virus and morphological changes, making it a valuable tool for pathogenesis studies.