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Published Date: 2012-06-14 16:35:00
Subject: PRO/AH/EDR> Infectious hematopoietic necrosis, salmon - Canada (02): (BC)
Archive Number: 20120614.1168492

INFECTIOUS HEMATOPOIETIC NECROSIS, SALMON - CANADA (02): (BRITISH COLUMBIA)
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Date: Wed 13 Jun 2012
Source: FIS.com [edited]
http://www.fis.com/fis/worldnews/worldnews.asp?l=e&country=0&special=&monthyear=&day=&id=52994&ndb=1&df=0

Wild salmon are to blame for an infectious haematopoietic necrosis (IHN) virus outbreak on a Vancouver Island fish farm, fisheries experts say.

Research suggests the virus was carried by sockeye salmon and entered the farm as the wild fish migrated into the farm's area, said an official at the Centre for Aquatic Health Sciences in Campbell River.

The IHN outbreak thus appears to be a reversal of the argument often presented by critics of the aquaculture sector: in this case, rather than wild salmon being infected by diseases carried by salmon raised in open-net cages in the ocean, wild salmon are infecting their farm-raised counterparts.

Gary Marty, a veterinarian and fish pathologist for British Columbia's (BC) Ministry of Agriculture, noted that although wild Pacific salmon have built up a resistance to the virus -- which means they can be infected but not show any signs of infection or even die -- Atlantic salmon have not been so lucky.

"It basically kills the blood-forming cells in the fish, and that includes white blood cells, so the fish cannot fight disease, and red blood cells which carry oxygen," said Marty, according to The Canadian Press. "Both things essentially shut down. The fish dies."

More than 500 000 salmon were killed to prevent the spread of IHN after the outbreak at Mainstream Canada's Dixon Bay farm near Tofino.

The BC Salmon Farmers Association (BCSFA) late last week [8 Jun 2012] announced that independent tests for the virus on all active Atlantic salmon farms in the province have now come back negative.

Still, fish farms will keep monitoring and testing their fish, the association announced.

"Any infectious agent has the potential to cross from fish to fish, and some of those fish might be outside the pen, whereas some of those fish might be inside the pen," said Marty. "We have to assess each infectious agent individually."

Vaccines for IHN are available, but Mainstream Canada did not vaccinate fish at the company's Dixon Bay farm, he stated.

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[At the wildlife-human interface, pathogens might flow both ways. Given the circumstances of the event in question, it was expected that the caged fish became infected with a virus that normally circulates in the ecosystem where they are farmed.

IHN virus is a rhabdovirus that causes acute, systemic disease in salmonid fish and also occurs in asymptomatic fish hosts. The virus is currently endemic throughout the Pacific Northwest of North America, with a contiguous range extending from Alaska to California and inland to Idaho. Because farmed salmon are kept at high densities, stress and high pathogen density may act in synergy, predisposing the fish to develop clinical disease. In the previous post, we mentioned that as the virus is native to that region, farmed salmon infected with IHNV do not represent a major concern for native wild populations.

Portions of this comment were extracted from Kurath et al. (2003; J Gen Virol 84:803-814). A ProMED-mail HealthMap of the affected area can be accessed at http://healthmap.org/r/2zMN. - Mod.PMB]

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