Infectious hematopoietic necrosis (IHN)


Infectious hematopoietic necrosis (IHN) is a viral disease of rainbow trout and other salmonids. The virus belongs to the genus Novirhabdovirus 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 virus infection is listed together with VHS as a Category C disease under the new Animal Health Act implemented in April 2021. That is, a listed disease that is relevant to some Member States and for which measures are needed to prevent it from spreading to parts of the EU that are officially disease-free or that have eradication programs for that disease.

Until 18 May 2021, IHNV had never been detected in Denmark, and the whole country was declared disease-free. With the IHN outbreak at a fish farm in Stouby in South-east Jutland in May 2021 and later at other farms, Denmark’s status regarding this disease, will depend upon the on-going epidemiological analyses that will determine whether the disease is concentrated in one area or has spread to several fish farms. There is an ongoing dialogue between the Competent authorities and the fish farmer association and the National Reference Laboratory in Denmark on how to control the disease.

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.



Molecular detection of IHNV

Information regarding molecular detection of IHNV in Denmark:

RT-qPCR performances and variation in targeting region of the N-gene

Senior Researcher Argelia Cuenca, arcun@aqua.dtu.dk


23 OCTOBER 2021