IHNv

Infectious Haematopoietic necrosis virus (IHN) is of the genus Novirhabdovirus of the family Rhabdoviridae. Both Pacific and Atlantic salmon reared in fresh water or sea water can be severely affected by outbreaks of IHNv, which can induce severe mortality. The virus can survive in fresh and salt water, in fresh water for at least 1 month, especially if organic material is present.

Historically, the geographic range of IHNv was limited to western North America, but the disease has spread to Europe and Asia via the importation of infected fish and eggs. Once IHNv is introduced into a farmed stock, the disease may become established among susceptible species of wild fish in the watershed.

Salmonid species susceptible include rainbow trout or steelhead (Oncorhynchus mykiss), the Pacific salmon species (chinook [O. tshawytscha], sockeye [O. nerka], chum [O. keta], masou [O. masou], pink [O. rhodurus], coho [O. kisutch]), amago (O. rhodurus), and Atlantic salmon [Salmo salar]). Other susceptible species include including brown trout (S. trutta) and cutthroat trout (O. clarki), some chars (Salvelinus namaycush, S. alpinus, S. fontinalis, and S. leucomaenis), grayling (Thymallus articus), ayu (Plecoglossus altivelis) and non-salmonids including herring (Clupea pallasi), cod (Gadus morhua), sturgeon (Acipenser transmontanus), pike (Esox lucius), shiner perch (Cymatogaster aggregata) and tube-snout (Aulorhychus flavidus) have occasionally been found to be infected in the wild or shown to be somewhat susceptible by experimental infection.

Transmission: IHNv can be spread through infected fish and covert carriers among cultured, feral or wild fish. The virus is shed via urine, sexual fluids and from external mucus, and can transmit horiztonally by aquaculture infrastructure (i.e. nets or boats between farms), invertebrate vectors (such as sea lice) and/or through the water column. It spreads rapidly and can result in large epidemics of infected fish. The kidney, spleen and other internal organs are the sites in which virus is most abundant during the course of overt infection. Virus entry is thought to occur through the gills and at bases of fins.

The age of the fish is extremely important: the younger the fish, the more susceptible to disease. Fish become increasingly resistant to infection with age until spawning, when they once again become highly susceptible and may shed large amounts of virus in sexual products. Survivors of IHN demonstrate a strong protective immunity with the synthesis of circulating antibodies to the virus.

Symptoms and pathology: The disease is typically characterised by lethargy interspersed with bouts of frenzied, abnormal activity, darkening of the skin, pale gills, ascites, distended abdomen, exophthalmia, and petechial haemorrhages internally and externally. A trailing faecal cast is observed in some species. Spinal deformities are present among some of the surviving fish.

Infection with IHNv often leads to mortality due to the impairment of osmotic balance, and occurs within a clinical context of oedema and haemorrhage. Virus multiplication in endothelial cells of blood capillaries, haematopoietic tissues, and cells of the kidney underlies the clinical signs. Affected fish exhibit darkening of the skin, pale gills, ascites, distended abdomen, exophthalmia, and petechial haemorrhages internally and externally. Internally, fish appear anaemic and lack food in the gut. The liver, kidney and spleen are pale. Ascitic fluid and petechiae are observed in the organs of the body cavity.

Diagnosis: IHNv is best diagnosed through a combination of isolation of the virus in cell culture and field observations.

Prevention: IHNv can be controlled with common disinfectants and drying of equipment. Avoidance of exposure to the virus could be achieved through the implementation of strict control policies, surveillance programs and sound hygiene practices (23). The thorough disinfection of fertilized eggs, the use of virus-free water supplies for incubation and rearing, and the operation of facilities under established biosecurity measures are all critical for preventing IHN at a fish production site.

References:

  1. OIE Aquatic Animal Health Code, (2011) Chapter 10.4
  2. OIE Manual of Diagnostic Tests for Aquatic Animals (2011) Chapter 2.3.4
  3. Saksida SM. 2006. Infectious haematopoietic necrosis epidemic (2001 to 2003) in farmed Atlantic salmon Salmo salar in British Columbia. Dis Aquat Org 72:213-223.
  4. St-Hilaire S, Ribble CS, Stephen C, Anderson E, Kurath G, Kent ML. 2002. Aquaculture 212: 49-67.