Mosquitoes are on the move due to climate change, and they could bring diseases

Click to play video: 'The world’s deadliest predator'
The world’s deadliest predator
Watch: The world's deadliest predator – Aug 25, 2019

Sharks have nothing on mosquitoes, the most dangerous predator humanity has ever known.

That’s the central message of historian Timothy Winegard’s new book, The Mosquito: A Human History of our Deadliest Predator.

“She has driven the course of human history across her existence right from the very beginning,” he said. “Wars have been won and lost as armies caught mosquito-borne diseases, genes like the one that causes sickle cell anemia have been introduced to protect people against malaria, and the bugs continue to spread misery around the world today.

Although relatively harmless in itself, the mosquito carries a wide variety of diseases as it goes from one bloody meal to the next. Malaria alone killed about 435,000 people in 2017, according to the World Health Organization.

Dengue fever, Japanese encephalitis and yellow fever annually kill tens of thousands more.

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Canada is relatively lucky: our biggest mosquito-borne disease, West Nile virus, typically has only a few hundred confirmed cases each year, and so far has not been identified as the primary cause of any deaths, according to the Public Health Agency of Canada.

But that might change. Scientists are watching for signs of new mosquitoes and new diseases, helped along to Canada by climate change.

After all, we’ve had them before.

Somewhere between 500 and 1,000 people died of malaria during the construction of the Rideau Canal near Ottawa in the 1820s and 1830s – and those estimates may not include women and children. The epidemic was so bad that construction was delayed for months and new cemeteries had to be founded to deal with the dead, according to the City of Ottawa Archives.

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Better medical care, housing design and lots of insecticide helped to eventually defeat malaria in Canada, where it was eradicated only in the 1950s.

Experts say it’s unlikely Canada will see another malaria outbreak. But they’re looking to see what comes next.

Ingredients for an outbreak

You need three things for a mosquito-borne disease outbreak, according to Robbin Lindsay, a research scientist at PHAC’s National Microbiology Laboratory.

First, you need mosquitoes. This is the easy part, he said.

Warmer weather means a longer mosquito season, he explained, and the bugs might be more likely to survive over the winter. Mosquitoes also tend to be more active when it’s warm, meaning they’re more likely to fly around seeking things to bite. Warm weather means their eggs mature faster, making more mosquitoes. And any diseases they might be carrying also tend to mature more quickly when it’s warm out – so are more likely to be transmitted.

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“Essentially, the longer the growing season, the more (high-)degree days that we have in a given year, the greater the potential for spill-over of virus transmission to humans,” said Manisha Kulkarni, a medical entomologist and associate professor at the University of Ottawa’s School of Epidemiology and Public Health.

A recent study estimated a billion more people could be at risk of mosquito-borne disease due to climate change, she said.

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Not every mosquito carries disease, though, and some diseases are specific to only some species of mosquitoes.

That’s why it makes headlines when Canadian mosquito-watchers notice new species showing up in their traps, like the Asian tiger mosquito that seems to now be established around Windsor, Ont. It’s a known vector for chikungunya virus, dengue virus and dirofilariasis.

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There are other exotic mosquitoes appearing in Canada, too.

“One that we captured was called Ochlerotatus japonicus,” said Curtis Russell, a senior program specialist at Public Health Ontario. “Over the years we’ve seen those in the traps. And now every health unit has captured that mosquito.”

Health organizations have expressed concern that this mosquito could help transmit West Nile, chikungunya and dengue.

“The one thing we do know is slowly the distribution of mosquitoes is changing,” Lindsay said. “And we do see events and we see species here that we haven’t seen before.”

But you need more than just mosquitoes. You need diseases, too.

These can be brought in from outside. For example, West Nile virus, as its name would suggest, was primarily found in North Africa and the Middle East before it arrived in New York City in 1999.

Since then, it has established itself on the continent with the last ingredient for an outbreak: hosts.

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For West Nile, the hosts are usually birds. For malaria, it’s people. There have to be enough hosts easily available for a disease to breed and for a transmission cycle to get established, Lindsay said.

The real trick is getting all three elements (mosquitoes, hosts and disease) together at the same time, he said.

“The mosquito populations really have to build up and be available at the time when there’s a pathogen in the environment.”

That’s hard, but not impossible.

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The best way to prevent the spread of mosquito-borne diseases due to climate change would be to prevent climate change, Kulkarni said.

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“If we can really mitigate these temperature increases, we can mitigate a lot of these impacts that we’re going to see in terms of expansion of mosquito-borne diseases.”

Mosquitoes affect the whole world, Winegard argues, so countries need to work together to control disease.

“It is a global concern and it requires a global solution, especially with increasing mobility and travel and trade within the human biosphere.”

When faced with diseases, public health agencies can try to control mosquito populations and encourage people to avoid getting bitten through measures like bed nets, wearing long sleeves and pants, and using insect repellent containing DEET.

In other parts of the world, scientists are also experimenting with novel techniques to try to control malaria, like altering the genes of mosquitoes so they can no longer reproduce.

For now, in Canada, the focus is on disease surveillance: things like watching for trends in the U.S., monitoring mosquitoes through trapping programs, running risk models and scenarios, and keeping an eye out for anything new and unusual, Lindsay said.

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“Right now the risk is fairly low, but we’re seeing sort of the beginning or we’re at the beginning of something that might have a much bigger health impact in the future.”

Russell stresses the importance of staying on alert.

“I think it’s always good to be vigilant,” Russell said. “We didn’t anticipate West Nile coming in. From its introduction in 1999, it went all the way across the U.S., through Canada and it’s gone all the way down to South America.

“You just need to be vigilant because you never know what is going to come in.”

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