Infectious hematopoietic necrosis (IHN) of fish is a viral disease

[Pathogen] The pathogen of infectious hematopoietic necrosis of fish is infectious hematopoietic necrosis virus (abbreviated IHNV).

[Symptoms and pathological changes] Affected fish initially show slow swimming and drift downstream, then swim with a swaying motion, and subsequently float up and roll sideways, often dying shortly after violent swimming. The diseased fish have darkened body color, protruding eyeballs, a distended abdomen, congestion at the base of the fins, and often a long, thick white mucous string hanging from the anus. On necropsy, a large amount of fluid is visible in the abdominal cavity, the liver, spleen and kidney are pale in color, the intestine is hemorrhagic, and there is congestion and hemorrhage in the oral wall, skeletal muscle, adipose tissue, peritoneum, meninges, swim bladder and pericardium, with necrosis of hematopoietic tissue. Fish that survive the disease often develop spinal curvature.

[Epidemiology] Infectious hematopoietic necrosis is prevalent in countries such as the United States and Canada, and was introduced into China in 1985 due to lax port quarantine. It mainly affects the fry and young-of-the-year fingerlings of salmonid fishes such as @#@241@#@, chinook salmon, sockeye salmon, masu salmon and river trout. Fry just hatched through to fingerlings 4 weeks after first feeding are especially susceptible to this disease, with high mortality; 1-year-old fingerlings can contract the disease but mortality is not high; fish 2 years and older do not develop this disease. The disease occurs at water temperatures of 10-14°C. The length of the incubation period varies with water temperature; below 10°C symptoms develop slowly; at 10°C, diseased fish generally begin to die in 4-6 days, mortality is highest from 8-14 days, and deaths can last for several weeks; at 12.5°C the incubation period is 4 days, and at 15°C natural outbreaks disappear. Virus-carrying adult fish are the main source of infection for this disease. The virus can be shed into the water with feces, urine and gonadal products, and enters the fish body through the gills and digestive tract, causing disease.

[Prevention and control methods] For the prevention and control measures of infectious hematopoietic necrosis, refer to those for infectious pancreatic necrosis of fish; immunization by immersion with an inactivated tissue homogenate vaccine from fish with infectious hematopoietic necrosis can also be used, and according to reports, the protection rate can reach 75%.