Infectious Salmon Anemia (ISA) has shaped the salmon farming industry for more than four decades. But it doesn’t usually make headlines until it does.
This year, the virus has re-emerged with force in Northern Norway. Companies have already culled fish. New outbreaks keep popping up. And there’s growing concern about how far (and how fast) the virus might spread in 2025. But ISA isn’t new. It’s been part of salmon aquaculture’s history since the 1980s.
So the question is: What have we learned from the past and are we actually applying it today?
ISA is caused by an orthomyxovirus, the same family of viruses as human influenza. Once inside the fish, it attacks the red blood cells, leading to anemia, weakness, and eventually death. The most severe outbreaks can wipe out more than 90% of a farm’s stock.
But what makes ISA especially difficult to manage is how quietly it moves. Fish can carry and shed the virus well before they show signs. It spreads through water, waste, equipment, and potentially even sea lice. It’s not always visible, but it’s highly contagious.
This is why early detection and rapid response are the cornerstones of ISA control. Most countries now use PCR testing and genetic sequencing to identify the virus early and determine whether it’s a virulent strain. Once confirmed, the response is swift: cull the site, impose movement restrictions, and begin fallowing.
So far in 2025, Norway has confirmed 14 ISA outbreaks, with another 2 sites under suspicion. Most are clustered in Nordland and Troms, but cases have appeared across several regions. If current trends continue, the year could match or exceed some of the worst in recent history.
A few comparisons:
In 2021 (a peak year), Norway had 14 cases by June and ended with 36 total.
In 2024, there were only 8 cases by June and 21 by year’s end.
This year? We’re already at 14 by the end of June.
So while 2025 isn’t breaking records yet, it’s clearly trending toward the higher end of Norway’s usual range, especially concerning given warmer waters and shifting production strategies that may increase risk.
To understand today’s outbreaks, it helps to revisit Norway’s first brush with ISA in the 1980s.
The virus was first detected in 1984, but the real crisis hit in 1990: over 80 farms reported ISA that year. It was a wake-up call. The Norwegian government quickly made ISA a notifiable disease, introduced strict fallowing rules, and required sites to house only a single generation of fish. These early policy moves laid the foundation for the biosecurity systems in place today.
Thanks to these efforts, ISA outbreaks declined significantly over the next two decades. And while cases have never disappeared completely, they’ve mostly remained manageable, until now.
ISA’s impact hasn’t been limited to Norway. Each of the world’s major salmon-producing nations has had its own experience with the virus - some devastating, some instructive.
ISA hit New Brunswick in 1996 and escalated rapidly. By 1998, over 1.2 million fish were culled across 16 farms, and the government stepped in with CAD 10 million in emergency relief. DNA-based vaccines were licensed soon after, and today, outbreaks in Canada are typically small and well-contained. But the virus remains endemic in eastern provinces.
Chile’s 2007 ISA outbreak is still the most severe in history. In just a few years, production fell by half—from nearly 500,000 metric tons in 2006 to just over 250,000 in 2010. Over 150 farms were affected, and the estimated economic loss reached $600 million USD.
This crisis revealed a core vulnerability: rapid industry growth had outpaced disease management. Afterward, Chile overhauled its entire regulatory framework—introducing zone-based stocking, synchronized fallowing, and strict limits on farm density. These reforms took time, but they worked.
Scotland faced ISA in 1998 and again in 2008. Both outbreaks were contained quickly, thanks to aggressive culling and movement controls. Since then, the country has leaned heavily on area-based management and surveillance systems, ensuring it remains ISA-free for most years.
The Faroes suffered a 70% drop in production during their early 2000s outbreak. But they chose a radical path: complete industry restructure. Single-farm fjords, strict boat traffic rules, and powerful veterinary oversight were put in place. Since 2005, the Faroes have had almost no major ISA events. Their turnaround is now held up as a global model.
Compared to where we were 20 years ago, the aquaculture industry now has far better tools to manage ISA:
Diagnostics: PCR and genetic sequencing help detect and trace outbreaks earlier than ever.
Vaccination: Norway vaccinated nearly 200 million fish in 2024. Vaccines don’t eliminate the virus, but they slow its progression and reduce its spread.
Biosecurity: Mandatory disinfection, equipment tracking, and site-level access controls have become standard in many regions.
Area-Based Management: Coordinated fallowing, stocking, and harvesting across zones limit the opportunities for virus transmission.
Still, none of these tools guarantee prevention. They buy time. And when outbreaks happen, it’s often a sign of either biosecurity lapses, operational fatigue, or system stress - things that require continuous attention, not just regulatory compliance.
ISA isn’t going away. It’s part of the biological reality of farming Atlantic salmon. But the past four decades show that it doesn’t have to be catastrophic.
This year’s resurgence in Norway is a reminder, not just of what can go wrong, but of what’s required to keep it contained. It’s about sustaining the hard-won systems that make early detection, containment, and recovery possible.
And right now, execution matters more than ever.
For more aquaculture insights, head to our Fish n’ Bits blog.