IHN

Aetiology 

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). Infectious hematopoietic necrosis virus (IHNVI 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 17 May 2021, IHNV had never been detected from fish farms in Denmark, and the whole country was declared IHN free. With the IHN outbreak at a fish farm in South-east Jutland in May 2021 and further spreading and isolations from seven other farms and three put and take lakes, Denmark lost its status regarding this disease 10 December 2021. In order to control the disease and its further spreading, six restrictions zones comprising whole water catchment areas were established during the course. The restrictions put on all aquaculture production businesses in these zones were lifted when the IHN free status was given up. However, before lifting the restrictions 28 farms in Denmark successfully applied for status as IHN free compartments. Currently, it is not known if and when a 2 or 4 year national program for re-gaining IHN free status will be initiated. 
For more information on IHN in Denmark, visit the Danish Veterinary and Food Administration's website  

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.