Humans making wildlife sick

Whether it’s monkeys and AIDS or mosquitoes and the West Nile Virus, we’re used to thinking of wildlife as reservoirs for emerging infectious human diseases. But a Canadian mathematical biologist says that it’s time that we turned the tables – as often as not, it’s humans that are making the wildlife sick, often to our own detriment.

It’s a 180-degree turn in perspective that Dr. Mark Lewis says is critical to our understanding of emerging infectious diseases of both wildlife and humans. And, he says, in the case of at least one ocean-based disease outbreak, biology and math are proving to be powerful allies in helping stem the growing tide of an ocean plague.

“With emerging infectious diseases of wildlife today there’s almost always some human component,” say Dr. Lewis, an NSERC-funded mathematical ecologist in the mathematics and statistics department at the University of Alberta, Edmonton, Canada.

Dr. Lewis’ lab group has used mathematical mapping tools, often in collaboration with other research groups, to document the spread of pests from the West Nile Virus to the Mountain Pine Beetle in Pacific Northwest forests.

Last year, in a landmark paper, he helped document how commercial salmon farms off Canada’s British Columbia coast are a breeding ground for sea lice, a parasite that then infects young wild Pacific salmon. The research was the first to document the parasitic impact of commercial salmon farms on wild salmon in the Pacific Northwest.

Dr. Lewis and University of Alberta doctoral student Marty Krkosek, who led the sea lice research, are co-presenting their latest sea lice and salmon findings as part of a symposium called The Rising Tide of Ocean Plagues, February 17 at the Annual Meeting of the American Association for the Advancement of Science in St. Louis.

Dr. Lewis is a leader in applying mathematical tools to modelling environmental interactions, from carnivore territoriality to risk analysis related to biological invaders, such as the zebra mussel in the Great Lakes.

When it comes to emerging infectious diseases of wildlife, Dr. Lewis says that public perception and policy needs to move beyond seeing “special cases” to seeing the constant role that people play.

“The way that people often think about emerging infectious diseases is that there are just a lot of special cases. That this happened here and that happened there, without any commonalties,” notes Dr. Lewis. “But there’s a growing sense that emerging infectious diseases are really important as a group. So we need the quantitative tools and mathematical theory to be able to study them, including being predictive and diagnostic.”

In the case of sea lice, Krkosek, Dr. Lewis and biologist Dr. John Volpe at the University of Victoria, Canada used an innovative live-sampling technique to document the transfer and spreading impact of parasite transmission from a fish farm to wild salmon. “There’s a long and beautiful history of mathematical models for parasite transmission that goes back to the 1970s,” Dr. Lewis says. “But the thing that was really unusual here was the spatial structure.”

The researchers analyzed the sea lice infection rates of more than 12,000 juvenile wild chum and pink salmon as they headed out to sea from their natal rivers. The infection rates were measured in intervals before and for 60 kilometres after they passed a commercial salmon farm.

“Our research shows that the impact of a single salmon farm is far reaching,” says Krkosek. “Sea lice production from the farm we studied was 30,000 times higher than natural. These lice then spread out around the farm. Infection of wild juvenile salmon was 73 times higher than ambient levels near the farm and exceeded ambient levels for 30 kilometres of the wild migration route.”

The researchers are now extending their work to assess how this increased parasite load affects the health of the young fish. There’s already initial evidence that this human-induced parasite boost kills many fish. Dr. Rick Routledge from Simon Fraser University and his collaborators recently showed that infection rates similar to those documented by Dr. Lewis will kill juvenile pink and chum salmon.

But, says Dr. Lewis, there’s evidence that some British Columbian salmon farmers aren’t waiting for the final wildlife forensics report to take action. They’re taking the researchers’ sea lice numbers to heart and moving their salmon farms. In an unprecedented agreement, Marine Harvest Canada, a major fish farming company, has agreed to move adult salmon from its farm at Glacier Bay in British Columbia’s Broughton Archipelago to another site further away from a major migration route of emerging wild juvenile salmon.

Says Dr. Lewis: “Ours is basic research, but the mathematical biology clearly gives key results about the contentious issue of fish farm impact on sea lice and wild salmon.”

Both Dr. Lewis’ and Marty Krkosek’s research is funded by the Natural Sciences and Engineering Research Council of Canada.