The Viruses infecting Sea Lice

Sea lice are a serious problem for the fish farm sector, but a new study from the University of British Columbia has found there are viruses that prey upon them.

“In theory viruses might be deployed in a way in which they might be able to help control sea ice densities in aquaculture. I certainly wouldn’t be a proponent of that at this stage, and would strongly advise against that because we just don’t know enough about it,” explained senior author Dr. Curtis Suttle.

Image: Dr Curtis Suttle, Department of Earth, Ocean and Atmospheric Sciences, UBC – submitted photo

“I’m a professor at the University of British Columbia, and my group works primarily on different kinds of microbes and different kinds of pathogens.  One of the exciting things that we found recently is that there’s a lot of viruses which infect sea lice, which were previously completely unknown.” 

His team found 30 previously unknown RNA viruses in sea lice. 

“What we also know is that these viruses do appear to be replicating and infecting the sea lice. We could see that the sea lice were mounting a very specific immune response to the individual viruses infecting them.” 

“We were only looking at wild salmon smolts, not at anything to do with fish farms per se, but we would assume that viruses might be prevalent within fish farms when population densities are high. Sea lice are a parasite, their populations will grow really fast because essentially they’re unlimited in terms of the number of hosts that they can exploit. So in those kinds of situations, populations tend to get out of control. You would expect that viruses would be able to spread from sea lice to sea lice relatively quickly.”

His team collected the viruses found in sea lice on five sites, ranging from southern Quadra Island to the Broughton Archipelago. Cortes Currents  was especially interested in their findings in the Discovery Islands, done in collaboration with the Hakai Institute

“You can be sure that these viruses infecting sea lice are the same viruses that are swimming around Cortes,” he said. 

CC: Are any of these RNA viruses that you were studying known to be  fatal to sea lice, or is that unknown? 

CS: “That’s a complete unknown, and a very difficult question to address because the only way that you could really do that would be to be able to culture and grow sea lice. The only way that I know to grow and culture sea lice is to infect a fish. It becomes a very complicated experiment. You would have to have large quarantine facilities, take salmon that have sea lice and put some in with viruses, some without viruses and see what those consequences are.”

CC: What is a virus? 

CS: Viruses are obligate pathogens. In other words, they can’t replicate at all unless they can find a host to infect. On the other hand, viruses tend to be very specific in terms of the organisms that they infect.”

“We don’t usually catch the viruses that our cats or dogs would have, although there’s exceptions. They’re not likely to catch a cold that we have. Viruses have to find the specific host.”

“They disperse incredibly widely. They are usually not dispersal limited. In fact, they aerosolize very easily because they’re tiny, tiny, tiny. If a virus was to be scaled to the size of the head of a pin so that you could actually see it and I was to be scaled by the same size, by the same amount, I would be 50 kilometres tall.” 

“Viruses are incredibly small, but they have this incredibly important effect because once they infect an organism, then essentially they become alive.  They can replicate very rapidly and bring serious health consequences for the organism that they’re infecting. 

CC: How often would they be fatal? 

CS: “It really depends on what kind of organism you are and what kind of virus it is.” 

“We know very little about the viruses infecting fish.”

“In humans there’s about 140 different viruses which make them ill. We have very complex immune systems, which in just about all cases, are able to battle those viruses and prevent them  from causing our death. In some cases though, where  a virus jumps from a different host, you have a mutation or something, and that can happen. Then often our immune systems are not really trained to recognize them, and they can have more devastating consequences, but mostly we fight them off.”

CC: Can you tell us more about how viruses might spread to sea lice?  

CS: “Sea lice start out as free living zooplankton in the water and zooplankton are this important organism in the food chain, which salmon  depend on to eat. They have a stage which we say is ‘free living,’ and it’s that free living stage in which they will actually have a certain amount of time to try to find a host on which to attach and to metamorphose into the form that we know as sea lice.” 

“it may be, for example, that viruses are extremely important in regulating the population of the sea lice when it’s free, but again, this is a very difficult question to address because when an organism in nature tends to get sick, even a little bit sick, the virus itself might not kill the organism, but certainly there’s all kinds of predators out there. If the organism’s a little bit slower or just not feeling so well, it doesn’t last very long. Something else will take it out of the system.” 

“These are very complicated questions to address specifically,  but very important ones.” 

  • If you had a system in which we know all these viruses, can we isolate the viruses specifically? 
  • Can we take viruses from one sea lice and can we transfer them and infect another sea lice? That’s the first thing that you do to see if the virus transmissible? 
  • How is it transmitted? 
  • And then if you can set up a scenario  or some sort of system in which you find, if the viruses are present, do the sea lice survive less well?

“One thing that people often don’t understand very well is that viruses are absolutely part of the natural system.”

“I always say that we couldn’t exist if it wasn’t for viruses and we wouldn’t even be here if it wasn’t for viruses.  The viruses in our  ancient ancestors, for example, formed very close associations with the organisms they infected.  So for example, if you look at some of the major proteins in the placenta of mammals, those proteins are actually viral proteins that got co-opted by our ancient ancestors and got repurposed to  form the placenta of mammals.”

“If you look at some of the major proteins in our own nervous systems, again, those proteins are actually viral proteins that we picked up in our long-term evolution with the organisms that we are derived from. In fact, about 8% to 10% of our DNA is actually viral DNA which has been integrated into our genomes over time.”

“About 95% of the living material in the ocean, by weight not number, is microscopic. Those microscopic plankton produce half the oxygen on the earth, and viruses kill about 20% of those organisms every day.” 

“So viruses are incredibly important in terms of turning over populations and if you took the viruses out of the system, what happens is things just stop growing.”

“I always say there’s no life, if there isn’t death. Viruses are absolutely essential for turning over  those populations in nature, and again, because they’re  very host specific, viruses tend to only target a single species.”
“One of the major things that viruses do is they maintain biodiversity on the earth. So when populations get really dense, really abundant , then viruses spread through that population very quickly because the host densities are very high and they knock that down and allow other organisms to grow up and fill that space.”

“That’s what we call ‘kill the winner.’ So viruses have this role in nature of what we say, killing the winner. So when populations grow really fast, they take over. When you have a large phytoplankton bloom or something like that, viruses are very likely going to take that bloom out and allow other organisms to come in and help nature find its balance again.”

CC: Do sea lice have beneficial viruses?

CS: “That’s  a really interesting question, and an area that we know very little about in terms of beneficial viruses overall. There’s some amazing stories where we know that viruses can have a positive impact. There’s a plant species that grows near the hot springs in Yellowstone. People noticed that just certain plants would grow near the hot springs and other ones couldn’t tolerate the heat. When they looked at it, they found the ones that grew near the hot springs have a fungus. The fungus is giving them some ability to tolerate the heat. It was actually a virus that infected the fungus, which gave the fungus properties, which allowed the plant to tolerate the high heat.” 

“We know very little about the potential beneficial effects of viruses, but we have co-evolved with viruses for billions of years, and so we have a very tight ecological relationship with them.”

“The only reason that we’re here now is that we have been able to find a way to coexist with viruses. Most of the things that viruses infect are actually bacteria and if you look at what we call the microbiome in humans, for example, we have about five to 10 kilograms of bacteria in our own bodies.”

“If you look at the viruses that are infecting it, they’re regulating our microbiomes as well.” 

“So there’s lots of research that  is going on, for example, for things like Crohn’s disease where your microbiome gets way out of whack. Do viruses have a role in terms of regulating those bacterial populations and controlling them?”

“They’re the original genetic engineers. That’s why we have a significant amount of our own nucleic acids. Our own DNA is made up of viral DNA because viruses move genetic information around  from genomes of one organism to genomes of another virus. So it’s back and forth.”

“One of the things that we always forget is that viruses are a natural part of the process of life. For example, there are more viruses in a teaspoon of seawater than there are people in North America.” 

Top photo credit: Sea Lice (Caligus clemensi) Credit: Hakai Institute

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