Infectious hematopoietic necrosis (IHN) is a viral disease of rainbow trout and other salmonids. The virus belongs to the genusNovirhabdovirus in the family Rhabdovidiae. IHN virus is related to viral haemorrhagic septicaemia (VHS) virus and is genetically more related to Lyssa virus genus (rabies virus) than to vesiculovirus genus (VSV). IHNV is serologically very homogeneous and can be identified using mono- and polyclonal antibodies. 
IHN has been known in the U.S. and Japan since 1960s and in 1987 the disease was recorded in France and Italy and has since spread to many parts of Europe. IHN has never been detected in Denmark and the whole of the country have Category I (disease free) status under EU legislation. 
Symptoms and pathology 
The incubation period is under farming conditions usually 1-3 weeks and depends on fish age, virus concentration and especially water temperature, incubation periods of up to three months has been observed in winter periods. The clinical symptoms evolve from acute / sub acute in 2-3 weeks. Initially sudden nervousness and restlessness among the fish without detectable specific symptoms is usually seen. Later darkening, protruding eyes, pale gills and lethargy is observed. Hamorrhages can often be seen around the eyes and in skin and muscle. The abdomen can be distended by fluid. Towards the end of an outbreak haemorrhages are usually less pronounced, while abnormal swimming behaviour with rotation around the longitudinal axis takes over. 
Pseudocasts (gray-white filaments consisting of faeces and intestinal epithelium) hanging from the anus can often be seen. Many of the surviving fish after an IHN outbreak, up to 20% may have severe abnormalities of the spine. High mortality have been seen in water temperatures up to 15 ºC. IHN occur mainly in fish less than 100 g. Mortality among juveniles may be 80-90%, while in larger fish rarely exceed 20-30%. 
When dissecting the fish haemorrhages are typical findings. These are usually in muscles, skin, gills, liver, spleen, kidneys, heart, brain, swim bladder and in the perivisceral adipose tissue.

28 FEBRUARY 2020