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

Abstract

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. KEY WORDS: Cold water disease · Flavobacterium psychrophilum · Pathogenesis · Ayu · Plecoglossus altivelis Resale or republication not permitted without written consent of the publisherDis Aquat Org 67: [93][94][95][96][97][98][99][100][101][102][103][104] 2005 salmonids in which the disease occurs most frequently when the water temperature is between 4 and 10°C (Shotts & Starliper 1999). The most notable clinical sign of CWD in ayu is anaemia as revealed by pale gills (Iida & Mizokami 1996). Ulcerative lesions are often found on the caudal peduncle of affected wild ayu, including stocked fish (Iida & Mizokami 1996). However, when CWD occurs in cultured ayu, ulcerative lesions on the surface of the body trunk are often not observed, although haemorrhagic lesions or ulcers develop on the lower jaw of many of the affected fish. The ulcerative lesions found on the caudal peduncle of affected wild ayu might develop following infection by the bacteria into skin wounds caused by fishing (snagging) or by territorial aggression by other ayu. Erosion of fins (fin rot) or tail loss, which are typical clinical signs for CWD in salmonids (Nematollahi et al. 2003), are not observed in ayu. Furthermore, the F. psychrophilum strain(s) that causes CWD in ayu is distinct serologically and genetically from those in other fish species including salmonids...

Detection of Flavobacterium psychrophilum in Chum Salmon Oncorhynchus keta and Virulence of Isolated Strains to Salmonid Fishes

“…Further quantitative study for 50% lethal dose will be required on the virulence of the pathogen and susceptibility of chum salmon using fish with different body weight. Moreover, other challenge methods including bath infection combination with formalin treatment (stress) conducted in rainbow trout (Madsen and Dalsgaard, 1999;Aoki et al, 2005) and ayu (Miwa and Nakayasu, 2005) will also be needed because i.p. injec-tion bypasses the non-specific defence mechanisms located in the skin and may not be an appropriate method of evaluation of the virulence (Madsen and Dalsgaard, 1999).…”

Protection against Flavobacterium psychrophilum Infection (Cold Water Disease) in Ayu Fish (Plecoglossus altivelis) by Oral Administration of Humus Extract

“…In Japan, local and farmed ayu fish (Plecoglossus altivelis) [2,24], salmonid fish [23] and other freshwater fish are affected by cold water disease. This manifests in ayu fish as severe skin erosion and ulcers on the trunk and peduncle, and hemorrhages on the gill cover and lower jaws [12]. The disease is frequently fatal.…”

Experimental Scuticociliatosis in Japanese Flounder (Paralichthys olivaceus) Infected with Miamiensis avidus: Pathological Study on the Possible Neural Routes of Invasion and Dissemination of the Scuticociliate inside the Fish Body

“…The same experimental design was used by Miwa and Nakayasu [14]. Few mortality cases were observed in the control group till day 10 post-infection.…”

“…The fish in each tank were sampled in the manner to ensure the minimum number for the investigation and to avoid an intentional selection of weakened individuals. Because of the predicted increased risk of mortality of the fish associated with progression of the disease, the initial numbers of fish in latter tanks were increased so that the total number of the fish examined in each tank would be equal (Miwa and Nakayasu [14]). …”