What is infectious hematopoietic necrosis in salmonids?

Pathogen The pathogen is infectious hematopoietic necrosis virus.

Transmission routes Vertical transmission: eggs and milt of virus-carrying broodfish, with transmission through eggs being more likely; horizontal transmission: contamination of water sources, etc. The virus mainly enters the fish body through the gills and digestive tract.

Symptoms The disease often causes sudden mass mortality during the fry and juvenile stages, with onset generally occurring 7 to 14 days after infection. Diseased fish swim sluggishly, exhibit spinning movements, sometimes sink to the bottom, have darkened body color, a distended abdomen, congested fin bases, opaque mucous-like feces hanging from the anus, and "V"-shaped hemorrhage in the flank muscles. In chronic cases, the eyeballs of diseased fish often protrude.

Epidemiology and harm It mainly affects salmonid fish such as @#@241@#@, chum salmon, river trout and other fry and fingerlings of the year; the disease occurs at water temperatures of 4°C to 13°C, with the highest incidence at 8°C to 10°C, and it stops occurring above 15°C. At a water temperature of 10°C, the mortality rate of fry under 2 months of age reaches 100%, the mortality rate of fingerlings aged 2 to 6 months exceeds 50%, and the mortality rate of fingerlings older than 7 months is about 10%. In recent years, cases have also been found in rainbow trout weighing 7 g to 100 g.

Prevention Aquaculture facilities and tools should be strictly disinfected; fish eggs should be soaked in povidone-iodine (containing 1% available iodine) at 50 mg/L for 15 minutes, but when the pH is alkaline the concentration can be increased to 60 mg/L, because povidone-iodine is less effective under alkaline conditions; diseased fish should be thoroughly destroyed; fry ponds should be located at the uppermost reaches of the hatchery.

Treatment Raising the water temperature of ponds containing diseased fish to 17°C to 20°C can control severe worsening of the condition. In addition, the feces trailing from the anus of fish with infectious hematopoietic necrosis is thicker and longer than the feces trailing from fish with infectious pancreatic necrosis.