New Research Reveals How Fish Viruses Spread 

Cardiomyopathy Syndrome (CMS) and Infectious Salmon Anaemia (ISA) are among the most costly and serious diseases for farmed fish in Norway, and CMS causes significant health challenges in Scotland.  Despite this, we still know relatively little regarding how the viruses which cause these conditions actually spread. 

Thankfully, that’s starting to change. 

Pharmaq is part of a research project at The Royal Veterinary College (RVC) in London, where, for the first time, the full genetic material of these viruses has been mapped out on a large scale. Hundreds of genomes are being fully sequenced to catch every variation and mutation. 

The data gives researchers a detailed picture of both where and when the viruses are spreading. The results point to the most important factors for transmission. 

“We’re reconstructing possible chains of infection, both geographically and over time. It gives us unique chances to learn about outbreaks and how we might stop them spreading in future,” says Mingli Zhao, postdoctoral researcher at RVC and one of the scientists behind the study. 

Geographical Closeness is Key 

Geographical closeness is the most important factor for the spread of both the CMS virus (piscine myocarditis virus, or PMCV) and the ISA virus. The closer the sea farms are to each other, the more the virus gets about. This goes, to some extent, for land-based sites too. 

“The virus can spread through shared staff, equipment, and boats. It can also travel with ocean currents,” says Marius Karlsen, researcher and project lead at Pharmaq. 

“When planning new sea farms, it’s wise to think about how close sites are and where they’re placed, to cut down the risk of infection spreading,” advises Dr. Zhao. 

Comparing Boat Movements and Virus Spread 

The analysis also shows that well boats can carry the viruses over longer distances. 

Researchers modelled all boat movements along the Norwegian coast using tracking data from BarentsWatch. They found that farms infected with the same type of virus often had visits from the same well boat. 

“The results suggest well boats help the virus reach new areas,” says Karlsen at Pharmaq. 

Well Boats Can Carry CMS Virus to Scotland 

At least seven times between 2004 and 2022, the PMCV virus spread from Norway to Scotland, the research shows. 

Scientists don’t have a definite answer for how the virus crossed borders, but Karlsen notes that well boats have sailed between Norwegian and Scottish farms. The boats are big and hard to clean properly. That’s why biosecurity during transport is so vital, says the Pharmaq researcher. 

“It’s a bit like having a leaky roof. There’s no point mopping up the living room if the water’s still coming in from above. You need to fix the roof — or in this case, stop the spread via well boats,” explains Karlsen. 

Zhao says it’s wise to test the fish for disease before transport. 

“It’s also crucial to remove all dead fish from the boats and disinfect thoroughly,” Zhao adds. 

Eggs Are Not the Main Route 

Some have wondered if PMCV could spread through eggs — that is, vertically from mother to offspring. The findings suggest eggs aren’t the main route, though it can happen: 

“We found one case with nearly identical viruses at farms far apart, both had eggs from the same batch. The virus can survive on the outside of the egg, so disinfecting roe is important. But most virus movement between fish populations doesn’t happen via eggs,” explains Zhao. 

Not Black and White 

The research project overturns old assumptions about viruses, says Karlsen. 

“Before, folk maybe thought in black and white about how viruses spread: one virus spreads this way, another that way. But viruses don’t stick to just one method,” Karlsen says. 

“It’s like the coronavirus pandemic: most transmission happened between folk close together, but air travel was crucial for spreading it worldwide. It’s the same with fish viruses,” he adds. 

Multiple Infections 

Researchers highlight several complex features of the viruses: 

This complexity wouldn’t have been spotted without full genome sequencing. 

“PCR tests only show if the virus is there or not. Full genome sequencing maps the whole genetic story. Then you see how complex it really is. A virus you thought came from one place might actually have two origins,” says Karlsen at Pharmaq. 

Whole Genome Sequencing is the Future 

Whole genome sequencing became a household term during the coronavirus pandemic: health authorities sequenced the coronavirus to track how different versions spread. 

“Before, we could only see how the virus changed over decades. With the whole genome, we can see exactly how it’s spread geographically over the last couple of years. That’s important, since fish only live two or three years,” says Karlsen at Pharmaq. 

Zhao reckons whole genome sequencing will become the standard for analysing fish viruses in the future. 

“You get a detailed, complete story about the virus and how it arrived at your site,” she says. 

Fact Box: PMCV and ISA Virus 

Piscine Myocarditis Virus (PMCV): 

Infectious Salmon Anaemia (ISA): 

Fact Box: About the Research Project