Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease (BCWD), inducing severe economic loss in aquaculture worldwide. In Japan, outbreaks of BCWD have often occurred in coho salmon Oncorhynchus kisutch hatcheries and ayu Plecoglossus altivelis farms since the mid-1980s. The disease has been causing serious damages to wild populations of ayu in rivers, and F. psychrophilum has been detected from not only ayu but also other wild fish species. F. psychrophilum in the ovarian fluid can contaminate the surfaces of salmonid eggs and passively enter eggs during water-hardening. The pre-water-hardening disinfection with povidone iodine (50 ppm in isotonic solution, 15 min) of fertilized eggs is a promising method for preventing the transmission of BCWD by eggs. Recently, the disinfection procedure begins to prevail in salmonid hatcheries in Japan.
