Salmon, aquaculture, and the spread of Infectious Salmon Anemia

Coho salmon - public domain image

Coho salmon - public domain image

In 2008, a deadly virus decimated Chilean aquaculture facilities, causing $2 billion in damage and crippling an industry. This week, preliminary reports suggest that this same disease may have infected wild salmon in the north Pacific. The internet has been blowing up with news reports of Infectious Salmon Anemia (ISA) detected in wild salmon populations. Reports range from balanced – Deadly Fish Farm Virus Found in Wild Pacific Salmon – to hyperbolic – B.C.’s salmon feedlots need to be closed – but all hinge on the fact that ISA, a lethal salmon-infecting virus previously resigned to aquaculture facilities, has been detected in wild salmon populations in British Columbia. This has the potential to be a very big deal. ISA is 90% lethal and mortality occurs in 10 days or less. The virus is waterborn, but can also be transmitted through handling with contaminated equipment. There is no treatment once a fish is infected.

Before I go on, a couple points need to be clarified:

  1. ISA does not infect humans, though as it threatens a fishery and a major agricultural industry, it most certainly affects humans.
  2. ISA was isolated from 2 wild sockeye salmon. It has not been confirmed from independent test yet, although one statement indicates that the current infection is from a non-infectious strain of ISA (which raises some interesting questions about who currently knows what about this outbreak).

What is ISA?

ISA is caused by an Isavirus in the Orthomyxoviridae family of RNA viruses. This family also includes the influenza viruses. The virus originates from a non-lethal strain found in Atlantic salmon. It made the jump into aquaculture facilities, where it mutated into a lethal strain. The first reports of ISA began in Norwegian fish farms in the mid-1980’s. Since then, ISA has appeared in several aquaculture facilities around the world. While it predominantly affects Atlantic Salmon (the most common farm raised salmon, even in Pacific aquaculture facilities), it is also known to infect Atlantic herring, Atlantic cod, Sockeye Salmon, Coho salmon, rainbow trout, brown trout, and arctic char.

Fish farms, especially off-shore floating pens, are breeding ground for diseases. Concentrated livestock makes vector transmission much more likely, and factory farmed fish, bred or engineered for rapid growth, tend to have weakened immune systems, making them more susceptible to disease than their wild counterparts. Off-shore pens and fish escapes provide a mechanism for introduction of disease vectors into wild populations. Whether or not the disease takes hold in wild populations depends on the biology, physiology, and behavior of the fish. It may be that a disease which wreaks havoc on farmed fish may not be able to take hold outside of the farm.

ISA thrives at 15 degrees C, which the waters around British Columbia reach in the summer. If ISA really has jumped into wild salmon populations, it may not become epidemic until the waters warm.

The future of Salmon

Beyond being an incredibly profitable industry that supports many communities on the Pacific coast, salmon fulfill vital ecosystem processes both offshore, where wild adult salmon mature, and inshore, when they return to their natal rivers to reproduce. An ISA epidemic in wild populations would have major repercussions for the entire north Pacific marine ecosystem and could dramatically destabilize west coast fisheries.

For now, we’re waiting on independent confirmation that ISA has made the leap from farm-raised to wild salmon, and that it is the lethal strain. Currently, fish farms in British Columbia have not reported any incidents of ISA, so if the case is confirmed, it is unlikely that the source of the outbreak was from those farms. It’s possible that low levels of ISA have always been present, undetected in fish stocks, or that the virus was transported from other fish farms through shipping. Also possible, though less likely, is that the newly opened northwest passage has allowed infect Atlantic salmon to migrate through the arctic to the Pacific ocean. Were that the case, this would be only the beginning of large-scale ecologic changes that will accompany the warming north.

If this truly is an outbreak of the lethal strain, this may be the beginning of the end for open-ocean salmon farms.

  1. As you’ve surmised, this could go either way. On the one hand, ISAV already infects wild salmon in the Atlantic, and has apparently evolved to be nonlethal in that population. If higher virulence gave the virus a selective advantage in wild Atlantic salmon, ISAV would have become a killer long ago. It seems pretty clear that the salmon farms were what selected for virulence, probably because of the inherently shorter life cycles and greater crowding there. If the same pattern holds in the Pacific, then there’s little to worry about.

    However, there’s no guarantee the same pattern will hold. Wild Pacific salmon are not Salmo salar, so perhaps the fish farms have allowed ISAV to jump to a new species. Viral species-jumping is a scary and unpredictable thing, particularly given the natural history of orthomyxoviruses in humans. This could become the piscine equivalent of a new flu pandemic. As you say, though, it’s important to wait for the tests to be confirmed before we go assuming worst-case scenarios.

    Now for some shameless self-promotion: we talked a little bit about ISAV on Episode 41 of This Week in Virology (http://www.twiv.tv/2009/07/19/twiv-41-fish-flu/), the weekly podcast about viruses. That was back in ’09, though, so maybe we should revisit it sometime soon.

  2. Hi there, nice breakdown of the situation and the reporting. I work in the industry and would like to add a little bit here.

    The virus was never in farmed salmon in B.C. Nearly 5,000 tests have been done on farmed salmon samples since 2003 for ISA, and they all tested negative.

    Also, farmed salmon are most at risk from this virus, which has been shown in studies to not affect Pacific wild stocks.

    I take some issue with this statement: “Fish farms, especially off-shore floating pens, are breeding ground for diseases. Concentrated livestock makes vector transmission much more likely, and factory farmed fish, bred or engineered for rapid growth, tend to have weakened immune systems, making them more susceptible to disease than their wild counterparts. Off-shore pens and fish escapes provide a mechanism for introduction of disease vectors into wild populations. ”

    This is not true. Our fish are healthy. Our survival rates are around 90 per cent from the time we enter smolts into the water. They are not genetically engineered in any way. Those smolts are all vaccinated by hand against common diseases in the ocean. Their health is monitored by professional veterinarians. Escapes are rare. There is no evidence our farmed fish pass disease on to wild fish; in fact, it’s often the opposite scenario.

    Finally, let’s all remember these reports on ISA are based on two test results of two juvenile sockeye smolts. These are very small fish. If the results are true positives, it stands to reason they were infected by their parents. If the virus was that prevalent to be transmitted vertically, we would be seeing signs of it in our fish. We are not.

    • Hi Grant, and thanks for commenting.

      For the most part we’re in agreement, and I tried to make it as explicit as possible in the article that it’s very unlikely that this current outbreak originated within BC salmon farms. Indeed, if there really is an epidemic, they’ll be the most at risk.

      I do have to disagree with you on the following statement, though:

      This is not true. Our fish are healthy. Our survival rates are around 90 per cent from the time we enter smolts into the water. They are not genetically engineered in any way. Those smolts are all vaccinated by hand against common diseases in the ocean. Their health is monitored by professional veterinarians. Escapes are rare. There is no evidence our farmed fish pass disease on to wild fish; in fact, it’s often the opposite scenario.

      First off, unless you’re actually representing all salmon farms worldwide, you seem to be taking my argument that…

      Fish farms, especially off-shore floating pens, are breeding ground for diseases. Concentrated livestock makes vector transmission much more likely, and factory farmed fish, bred or engineered for rapid growth, tend to have weakened immune systems, making them more susceptible to disease than their wild counterparts. Off-shore pens and fish escapes provide a mechanism for introduction of disease vectors into wild populations.

      …rather personally. Of course there are salmon farms with exceptional health records. Of course there are many that don’t use genetically engineered fish, and of course some farms have very low escape rates, but I’m talking about the industry as a whole, not specific companies or facilities, and I’m talking about the issues that can and do emerge in this industry.

      Furthermore, the principle that concentrating livestock increases disease susceptibility is a fundamental concept in agriculture, and not limited to fish farming. Whenever you increase the density of livestock, you make it easier for diseases to spread and persists. It’s great that your fish are healthy, but you’re vaccinating them by hand and carefully monitoring their health precisely because they’re more susceptible to disease. I can’t understand why you would argue otherwise. The original lethal ISA virus emerged from fish farms and spread though fish farms because the conditions that make farming salmon profitable also increase disease susceptibility.