Infectious Haematopoietic Necrosis

Also Known As – IHN – Chinook Salmon Disease – Coleman Disease – Columbia River Sockeye Disease – Cultus Lake Virus Disease – Oregon Sockeye Disease – Sacramento River Chinook Disease – Sockeye Salmon Viral Disease

Caused By – Infectious Haematopoietic Necrosis Virus – IHNV – Sockeye Salmon Virus

Introduction

Infectious Haematopoietic Necrosis is a disease of salmonid fish caused by a bullet-shaped RNA Rhabdovirus.

IHN has a significant economic impact upon freshwater rainbow trout farms and salmon farming systems. It also causes large mortalities among wild fish.

There is no risk to humans from this disease.

Distribution

Western North America, Continental Europe and Asia. Transmission between countries is a significant consideration due to the international trade in eggs, fry and fish.

The virus is transmitted by insect, annelid and crustacean vectors. It is shed by infected and carrier fish in faeces, urine, sexual fluids and external mucus.

Signalment

Pacific salmonid fish including rainbow or steelhead trout, chinook salmon, sockeye salmon, chum, masou, coho and atlantic salmon. Outbreaks are more common and more severe in units with high fish density.

Juvenile fish shed high levels of virus. Fish become increasingly resistant to infection as they age. The virus can also be shed at spawning by adult fish.

Clinical disease only occurs in water between 8 and 15⁰C. Water temperature often influences mortality rates but this can occur in either direction dependent upon the species of fish involved. Presence of pollutants and water salinity can also exacerbate outbreaks.

Clinical Signs

IHN causes a characteristic “pop-eye” appearance in infected fish. The skin and fins may be darkened and demonstrate pinpoint haemorrhages. Abdominal distension sometimes occurs and the gills may be pale. Casts may be visible trailing from the vent. Fish often behave abnormally sporadically.

Signs can also vary with species, for example Chinook salmon often show subdermal haemorrhage immediately behind their heads and sock-eye salmon smoults have clubbed/fused lamellae.

In some outbreaks a sudden increase in mortality may be the first sign noted. Epizootics can begin as groups of moribund fish that whirl sporadically or exhibit periods of hyperactivity.

Some recovered fish can become covert carriers.

Mortality in yolk-sac fry and fish <2 months old can reach 90%. Mortality is lower in older fish.

Bloodwork of infected fish features leucopaenia, neutropaenia, increased numbers of erythrocytes which may be bilobed, cellular debris, anaemia and low levels of bicarbonate, calcium, phosphorus, chloride and bilirubin.

Diagnosis

Clinical signs and history of previous outbreaks may be suggestive of IHN.

One of the most rapid tests, taking only 15 minutes, is staphylococcal agglutination which is performed upon cell cultures or infected fish tissues using formalin fixed Staphylococcus aureus cells sensitised with rabbit antiserum which caused agglutination of bacterial cells, visible under a standard light microscope. This test is specific but has poor sensitivity.

Virus can be identified by Virus Neutralisation (VN),, Indirect Fluorescent Antibody Testing (IFAT), ELISA, PCR or DNA probe technology. It can be isolated from cell culture using kidney, spleen and sometimes mucus in non-lethal sampling methods. Ovarian fluid should be used if breeding stock are being tested. The approved test is Virus Neutralisation but this is expensive and time consuming, hence the development of more practical, rapid tests, however these are not officially approved.

On necropsy of infected fry, the liver, spleen and kidneys are pale due to anaemia and ascites is common. The stomach will be full of milky fluid but no food. Petechial haemorrhages are common throughout many organs. Yellowish fluid or mucus in the intestines is seen in both young and adult fish. The haematopoietic tissues of the kidney and the spleen in young fish are most severely affected, demonstrating small, pale multifocal areas with macrophage infiltration and degenerating lymphoid cells.

A pathognomic histological feature of IHN is degeneration and necrosis of granular cells in the lamina propria, stratum compactum and stratum granulosum of the gastrointestinal tract.

On cytopathology, cells appear balloon shaped and nuclei are displaced to one side.

Detection of antibodies to IHN is difficult and no test appears to give reliable results at present.

Treatment

Treatment has been and is attempted with drugs such as amantidine, metisazone, ribavirin and benzimidazole, but is ineffective.

Control

Strict control policies and sound hygiene practice should aim to avoid exposure to IHNV. Fertilised eggs should be thoroughly disinfected and egg incubation and fry rearing performed on proven virus-free water supplies separate from any possible virus carrying fish.

Vaccinations are at an experimental stage.

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References

Animal Health & Production Compendium, Infectious Haematopoietic Necrosis Virus datasheet, accessed 06/07/2011 @ http://www.cabi.org/ahpc/

Animal Health & Production Compendium, Infectious Haematopoietic Necrosis datasheet, accessed 06/07/2011 @ http://www.cabi.org/ahpc/