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Research Document 2017/073

Characterization of Infectious Hematopoietic Necrosis Virus (IHNV)

By Garver, K., and Wade, J.

Abstract

Infectious Hematopoietic Necrosis Virus (IHNV) is a rhabdovirus that can result in the acute systemic disease infectious hematopoietic necrosis (IHN).  The virus is endemic to British Columbia where it has been detected in freshwater and marine life stages of wild Sockeye Salmon as well as in marine cultured Atlantic Salmon.  Through controlled laboratory exposure studies, Atlantic Salmon post smolts were shown to be nearly 100 times more susceptible to IHN disease than the native Sockeye Salmon at a similar life stage.  However, this differential susceptibility is progressively enhanced with increasing age, whereas adult Atlantic Salmon remain highly susceptible to IHNV while Sockeye Salmon become resistant to IHN disease.  Atlantic Salmon with acute IHN disease can shed enormous quantities of virus with levels peaking one to two days prior to the death of the animal.  Once shed into the marine environment, the infectiousness of IHNV is rendered inactive by exposure to sunlight and natural biota present in the seawater; consequently, IHNV has an abbreviated lifespan whereby it can infect another host.  Laboratory studies exposing fish to IHNV via immersion in virus contaminated water or through cohabitation with IHNV infected fish, have demonstrated that IHNV is transmitted and spread through waterborne exposure.  IHNV dispersion from infected ocean-based net-pen Atlantic Salmon farms is dependent upon the number of diseased fish in the farm population (virus shedding fish), the decay rate of IHNV, and the water movement (currents) in the area of the infected farm.  Epidemiological analyses of farm to farm spread of IHN disease during historical outbreaks in farmed Atlantic Salmon when industry-wide disease management practices were not implemented revealed that farming practices, such as boat movements and the use of shared personnel and contractors were leading causes of the dissemination of the disease amongst farms.  Nevertheless, waterborne transmission of IHNV during these historical unmanaged outbreaks cannot be discounted as model simulations of IHNV dispersion performed using numerical particle releases in accordance with laboratory derived virus shedding and inactivation rates, demonstrate that neighboring naïve farms can become exposed to IHNV via waterborne transport from an unmanaged IHN diseased farm.  However, significant advancement in the control of IHNV have been achieved through industry-wide implementation of a viral management plan and universal use of vaccination against IHNV.  Under modern management practices that include movement controls and eradication, no farm to farm spread was detected in 2012 when IHNV was last reported in farmed Atlantic Salmon.  Additionally, the IHNV vaccine has proven highly efficacious.  In laboratory studies, APEX-IHN® prevented an outbreak of IHN disease in a population of Atlantic Salmon exposed to a lethal dose of IHNV.  Furthermore, the vaccinated fish were incapable of transmitting IHN disease to cohabitating Sockeye Salmon.  Since its licensure, over 60 million doses of APEX-IHN® have been administered to Atlantic Salmon in BC and to date there has been no detection of IHNV in an APEX-IHN® vaccinated farmed Atlantic Salmon.

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