Comparison of immunohistochemistry and virus cultivation for detection of viral haemorrhagic septicaemia virus in experimentally infected rainbow trout Oncorhynchus mykiss

lmmunohistochem~stry and virus isolation were compared for their ability to detect viral haemorrhagic septicaemia vlrus (VHS) in experimentally ~nfected ralnbow trout Oncorhynchus mykiss. The f ~ s h were divided into 3 g]-oups ( I to I l l ) , and infected, respectively, by bath challenge w ~ t h 102, 103 ', and 10"CID50 ml-l water of a VHS virus strain serologically sllnilar to reference straln F1 The cumulative mortality in Groups I to I11 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 im~nunohistochemistry (p < 0.05). The same result was obtained by separate comparison of Groups 1 and 11, but there was no significant difference between the 2 methods for Group 111 (10' TCID;,,,). Immunohistochemically. virus antigens were detected early (2 to 4 d) in endothelial cells linlng venules and sinusoids and in the haematopoietic cells in the head kidney, a s 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 sensitlve method for detection of VII-us, although immunohistochem~stry may represent an adjunct to diagnosis of acute VHS virus infections. The main advantage of ~mmunohistochemistry is the possibility of simultaneous demonstration of VII-us and morphological changes, making it a valuable tool for pathogenesis studies.