opinion: climate change and predicting the next pandemic
with global warming, mosquito-borne diseases that thrive in warm temperatures are expected to shift to higher latitudes and be transmitted for longer periods.
5 minute read
climate change is bringing a higher risk for the spread of disease. in this june 11, 2021, file photo, activists pose as dead bodies during a climate change protest in cornwall, great britain, during the summit of g7 leaders from canada, france, germany, italy, japan, the uk and the united states.
daniel leal-olivas
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afp via getty images
by: anne jarvis
a study in the scientific journal nature climate change this spring documented how covid-19 pandemic lockdowns caused a record seven per cent decrease in global carbon emissions in 2020.
but the real story is the other way around: how global warming caused by carbon emissions is unleashing more infectious diseases.
as the third wave of covid-19 was rising this spring, colin carlson, a biologist at georgetown university in washington, d.c, was presenting a seminar at the university of toronto called “global change and the next pandemic.” the subtitle was ominous: “or how we survive the next century.”
carlson studies the link between climate change and emerging infectious diseases and what that means for health care. he works with a team of scientists from different disciplines who try to predict which viruses could infect humans, where they could emerge and what health-care systems everywhere should be preparing for.
it’s like medical geography or infectious disease cartography, he says.
the covid-19 pandemic is the latest pandemic of a non-influenza zoonotic disease. a zoonotic disease is an infectious disease caused by a pathogen that has jumped from an animal to a human.
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most of the initial research in this field studied the impact of climate change through vector-borne diseases, for example, malaria and dengue, transmitted by mosquitoes. everything about mosquitoes — how long they live, how many eggs they lay, how many times they bite people — is all driven by temperature, said carlson.
he recounted a project proposed by one of his students to study shifts in elevation among mosquitoes in madagascar. scientists knew that species seek higher elevations because of changing temperatures. it was logical to expect that vectors of human disease also move.
they were right. mosquitoes are moving quickly, they discovered, tens of kilometres a year.
we see the result in windsor and essex county, where public health officials now routinely monitor mosquitoes for west nile virus because climate change has increased the risk of exposure to that virus.
another aedes albopictus mosquito, which can carry the zika virus, was found in windsor, public health officials announced monday. also known as the asian tiger mosquito, it’s normally found in the tropics but has been found here since 2016. this one tested negative for the virus. an aedes aegypti mosquito, the most common carrier of zika virus, was also found here in 2017. it was the first time one had been captured in canada.
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“this is pretty significant from a risk perspective since this mosquito type normally doesn’t prefer our cooler climate,” local medical officer of health dr. wajid ahmed said at the time. “it also brings the new risk of diseases.”
the study showing how fast mosquitoes are moving “is a big deal,” carlson said. “we know that this has consequences for human health.”
when carlson and his team studied temperature and the prevalence of malaria in africa, they saw the correlation they expected.
there was an optimal temperature for transmission of the disease, and above and below that temperature, there was less transmission.
if scientists know the optimal temperature at which malaria is transmitted, and what temperatures are expected to be in 2100, they can map the future of the disease.
“that’s … the core idea of how we approach these problems,” he said.
all of this is part of a broader body of research over the last decade on temperature, vector-borne diseases and what could happen in the future. with global warming, mosquito-borne diseases that thrive in warm temperatures are expected to shift to higher latitudes and be transmitted for longer periods.
dengue virus, which causes dengue fever, is also carried by mosquitoes. also found in the tropics, dengue tolerates temperatures between 17 and 34 degrees celsius.
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between 1950 and 2018, the suitable climate for transmitting dengue through the aedes aegypti mosquito has increased by 8.9 per cent and through the aedes albopictus mosquito by 15 per cent, according to the lancet countdown 2020 report, which tracks the impact of climate change on health.
“we estimate that there will be about a billion people who will be in the path of dengue as it leaves the tropics by mid-century,” said carlson.
but if we cut carbon emissions and reduce the magnitude of warming, we can cut that number in half, he said.
“we can protect a lot of people,” he said. “mitigation is up-front the most important thing there is.”
the other 500 million people — that’s what the health-care system must prepare for.
“we need mitigation, vaccines and other measures “to have any chance of having a livable world over the next 100 years,” said carlson.
there are millions of viruses and thousands of mammals, so the impact of climate change is much bigger than the spread of dengue, carlson said. he and his colleagues also study the potential for viral sharing among other species because of climate change to predict the possible threat of new pathogens.
for example, they look at where a species is today, where its range could shift to in the future because of warming temperatures and which different species could meet for the first time because of these shifts and share a pathogen that could then jump to humans.
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this work is based on assumptions, he said, “but the broader pattern — that species are on the move, they’re taking their viruses with them and that’s probably consequential — is something you don’t even really need a model to show.”
bats are the key players in this because there is evidence that their range is shifting very quickly, and they carry viruses, including coronaviruses, which caused sars and covid-19, and the virus that causes ebola.
“we’re worried about the human health impacts from this for reasons i don’t really have to explain anymore,” carlson said.
bats are going to meet primates, he said, “and that’s probably a decent stepping stone for viruses to reach us.”
people tend to settle in areas with stable climates, and so do many species, so “all of this is going to be happening in our backyard,” he said.
if you look at species’ ranges historically and consider that the world is already one degree warmer, “there’s a reasonable chance that this has already kicked in and that species are already moving and sharing viruses,” he said.
in fact, carlson said, research in the arctic shows that as sea ice melts, species are meeting for the first time — and sharing viruses — “which gives us a reasonable indication this is probably something we want to worry about.”
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so we need health-care systems that can withstand pandemics, he said.
“covid has shown us that everything we’ve built, even universal health-care systems like canada has, like the nhs in the uk, all of these are prone to collapse, so it’s our job to think about things that we can do at scale, not just as a moral obligation because we live in a society, but as climate change adaptation tools, as ways to make our society more prepared for the next century.”
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