Infectious Hematopoietic Necrosis (IHN): A Disease Guide

Infectious hematopoietic necrosis virus causes high mortality in young salmonids. There is no treatment, so control relies on virus-free water, eggs, and biosecurity.

Overview

Infectious hematopoietic necrosis (IHN) is a serious viral disease of salmonid fish. The causative agent is infectious hematopoietic necrosis virus (IHNV), a bullet-shaped, negative-sense single-stranded RNA virus of the genus Novirhabdovirus, family Rhabdoviridae. Virions are roughly 45-100 nm in diameter and 100-430 nm long. The disease causes acute systemic infection with necrosis of the hematopoietic tissue of the kidney and spleen, and it is a notifiable disease of the World Organisation for Animal Health (WOAH).

Affected species

IHNV mainly infects salmonids, including rainbow and steelhead trout (Oncorhynchus mykiss), Pacific salmon, and Atlantic salmon (Salmo salar). Fry and fingerlings are the most susceptible stage, and mortality is very high in young fish, whereas older fish are generally more resistant. The virus is most common along the Pacific coast of Canada and the United States and has also been reported in Europe and Asia.

Clinical signs

Lethargy alternating with bouts of aberrant, hyperactive swimming
Skin darkening and exophthalmia (bulging eyes)
Pale gills and anaemia
Distended abdomen with ascites
Petechiae and haemorrhages, including at the bases of the fins and behind the head
Trailing mucoid faecal casts; haemorrhage of the yolk sac in fry

Transmission

IHNV is shed in faeces, urine, sexual fluids, and external mucus, and spreads horizontally through the water by direct or close contact, often entering fish at the base of the fins. The virus is also associated with eggs, so the movement of infected fish, contaminated water, and reproductive products are key routes of spread between facilities.

Diagnosis

Diagnosis is confirmed by virus isolation in cell culture with virus neutralisation, supported by ELISA and RT-PCR. Because the gross signs overlap with other systemic viral diseases of salmonids, laboratory confirmation is required before management or trade decisions are made.

Treatment, control and prevention

There is no effective treatment. Control depends on strict isolation, hygiene, and testing, and on preventing exposure of susceptible young fish to the virus.

Use a virus-free water supply and avoid sources shared with infected populations
Source eggs and stock from tested, IHNV-negative broodstock and disinfect egg surfaces
Screen broodstock and incoming fish; quarantine new arrivals
Disinfect equipment and facilities and separate year classes
Where available and appropriate, use vaccination, and follow WOAH surveillance and reporting protocols