Pathogenesis of experimentally induced bacterial cold water disease in ayu Plecoglossus altivelis.

Ayu Plecoglossus altivelis were experimentally infected with Flavobacterium psychrophilum, which is the causative agent of bacterial cold water disease (CWD). The fish infected by immersion usually died within an hour after they became moribund. The blood volume and haematocrit values of moribund fish were low, while those values of many infected fish that were not moribund were in the range of controls. Most of the affected fish in the immersion-infected groups had ulcerative lesions on their lower jaw. No histological evidence of haemolysis was observed. These results suggest that rapid bleeding occurred through ulcerative lesions, probably causing hypoxia which killed the fish. Ulcerative lesions developed on the dorsal skin when this area had been slightly abraded artificially prior to immersion challenge. Histologically, F. psychrophilum was initially found on the skin that had microscopic injuries, but not on normal skin. The bacterium then entered the dermis and migrated through connective tissues. The lesions subsequently expanded into the underlying musculature through the myosepta, developed necrotic myositis and formed externally open ulcers. Only in later stages of infection did mild lesions develop in the internal organs and the gill, probably caused by the bacterium migrating through blood vessels. This suggests that infection with CWD through the gill or digestive tract is unlikely. Virtually no open lesions were found in ayu challenged by intramuscular injections except at the injection sites. The results suggest that skin injuries are major portals of entry for F. psychrophilum in ayu, and the bacterium has affinity for collagenous connective tissues.