Monitoring of Outbreaks of Bacterial Cold Water Disease among Ayu in a River Where Asymptomatic Carriers of Flavobacterium psychrophilum Were Released

Occurrences of bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum were monitored among released and wild ayu Plecoglossus altivelis in Hirose River in 2006 and 2007 in order to clarify the infection source. When hatchery-reared juvenile ayu were examined immediately prior to release, F psychrophilum was not isolated. However, when fish from the same lots were reared until maturation, the bacterium was isolated from ripe fish, indicating that the released juvenile ayu were carriers. In epizootiological investigations in Hirose River, outbreaks of BCWD occurred from June to August in both years, but the source of BCWD seemed to differ: released ayu in 2006 and wild ayu in 2007. Additionally, the disease occurred in released ayu by the pathogen carried by themselves in 2007. The disease subsided in August when water temperature exceeded 20 C. Thereafter, the prevalence of F psychrophilum in ripe fish in the river increased to above 90% between October and November in both years.