SIMULTANEOUS DEMONSTRATION OF INFECTIOUS PANCREATIC NECROSIS VIRUS (IPNV) AND FLAVOBACTERIUM-PSYCHROPHILUM IN PARAFFIN-EMBEDDED SPECIMENS OF RAINBOW-TROUT ONCORHYNCHUS-MYKISS FRY BY USE OF PAIRED IMMUNOHISTOCHEMISTRY

The Gram-negative bacterium Flavobacterium psychrophilum, which is the causative agent of rainbow trout fry syndrome (RTFS), and infectious pancreatic necrosis virus (IPNV), the causative agent of infectious pa ncreatic necrosis (IPN), are both highly pathogenic for rainbow trout fry. Several 'persistent' cases of RTFS have been observed concomitant with IPNV. Cultivation alone might not be sufficient for evaluation o f the disease situation as both pathogens can be cultivated from fish that do not show any clinical signs of disease. In such cases it may b e difficult to decide which pathogen should be considered the primary cause of the mortality observed. Further, it may be difficult to culti vate the bacterium in later stages of the disease or from dead fish th at have been transported without cooling. In the case of (suspected) d ouble infections it is therefore suggested that immunohistochemistry b e included as a supplementary diagnostic tool, allowing correlation of the presence of either pathogen with pathological lesions. In the pre sent study, fry representing different stages of RTFS from 3 clinical outbreaks were shown to suffer from ongoing double infections as demon strated by immunohistochemistry and supported by cultivation of the 2 pathogens. The general finding was that single cells of the exocrine p ancreas were positive for the virus, whereas bacteria were mainly demo nstrated in the interstitial tissue surrounding the pancreatic islets. In some endothelial cells of the head kidney, both pathogens were det ected in the same cell. These findings as well as various protocols in relation to the methodology are discussed.