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 abili ty to detect viral haemorrhagic septicaemia virus (VHS) in experimenta lly infected rainbow trout Oncorhynchus mykiss. The fish were divided into 3 groups (I to III), and infected, respectively, by bath challeng e with 10(2), 10(3.7), and 10(5) TCID50 ml-1 water of a VHS virus stra in serologically similar to reference strain F1. The cumulative mortal ity 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 grou ps showed that virus isolation was significantly more sensitive than i mmunohistochemistry (p < 0.05). The same result was obtained by separa te comparison of Groups I and II, but there was no significant differe nce between the 2 methods for Group III (10(5) TCID50). Immunohistoche mically, 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 melanomacr ophages; they were detected subsequently in hepatocytes (4 d p.i.) and exocrine pancreatic cells (6 d p.i.). Presence of virus was accompani ed by cell degeneration and necrosis from 4 d p.i. in all positive org ans. These findings show that virus cultivation is the most sensitive method for detection of virus, although immunohistochemistry may repre sent an adjunct to diagnosis of acute VHS virus infections. The main a dvantage of immunohistochemistry is the possibility of simultaneous de monstration of virus and morphological changes, making it a valuable t ool for pathogenesis studies.