A field survey was carried out for four years in order to investigate the occurrence of Flavobacterium branchiophila infection at the Okutama Trout Hatchery affiliated to Tokyo Metropolitan Fisheries Experimental Station. The gills and the skin of fish collected from rearing ponds were examined by indirect fluorescent antibody technique (IFAT), plating method and light microscopy. The water from the fish ponds was also examined.
IFAT were more sentitive and specific in detection of F.branchiophila than plating method or light microscopy. F. branchiophila was always associated with bacterial gill disease outbreaks that exhibited typical clinical signs. In other words it was detected from both fish and water during the period of the outbreaks only. Outbreaks of F. branchiophila gill disease occurred intermittently in the same fish population. Therefore the recovered fish appeared not to obtain immunity against the pathogen.