Dengue fever, Zika and chikungunya were once considered 鈥渢ropical鈥 diseases. But new research shows climate change is pushing the mosquitoes that carry these viruses northward, putting temperate cities like 海角社区官网at risk.
According to , the city has between a 39 per cent and 61 per cent probability of becoming host to the Aedes albopictus mosquito within the next decade. Commonly known as the 鈥淎sian tiger鈥 mosquito, it is one of two related species that carry the virus that causes dengue fever 鈥 which infects about 400 million people and kills 40,000 globally every year 鈥 and the Zika virus, which can cause birth defects when it passes from infected pregnant women to their fetuses.
The new research is unpublished and has not yet been peer-reviewed. Outside experts stress that hosting the mosquito does not necessarily mean that cities like 海角社区官网are at imminent risk of becoming tropical-disease hotbeds.
The Asian tiger mosquito is already found as far north as Windsor, Ont., yet the closest outbreak of dengue fever is more than 2,000 kilometres south in Florida, notes infectious disease physician Dr. Isaac Bogoch.
鈥淲ith time, we鈥檒l probably see more of these mosquitoes and we鈥檒l see the diseases start to creep north. But if we look at where the diseases are right now, they鈥檙e still really far away,鈥 said Bogoch, who has collaborated with BlueDot in the past but was not involved in this research.
鈥淚t鈥檚 on the horizon, but it鈥檚 a much more distant horizon.鈥
Lyme disease already climbing
But mosquito-borne diseases are already popping up in unexpected places, a shift scientists have attributed in large part to climate change. Last year, , outbreaks that were introduced by infected travellers and then spread by Aedes albopictus.
In 2019, the Public Health Agency of Canada published in this country, and also increasing the risks that Canadian travellers could acquire these diseases abroad.
Huaiping Zhu, a professor of mathematics and director of the Canadian Centre for Disease Modelling at York University, has analyzed the spread of Aedes mosquitoes himself, and thinks BlueDot鈥檚 model overestimates the risk 鈥 his own research suggests the city won鈥檛 be habitable for them until at least 2050.
鈥淚 would agree there is increasing risk due to climate change,鈥 he said, later adding: 鈥淭en years, 20 years, 30 years does not really make much of a difference. It鈥檚 coming.鈥
Bogoch notes that Canada is already facing a dramatically increased burden of Lyme disease, which is spread through a species of tick that was once unable to survive harsh Canadian winters. In 2009 and 2010, fewer than 150 cases of Lyme were reported to Canada鈥檚 public health agency; the past two years saw reported case counts of between 2,000 and 3,100.
鈥淎s we see climate-change-related phenomena occur across the planet, we do see the expansion of many insects that are capable of transmitting various infections as well,鈥 said Bogoch.
Zika鈥檚 U.S. arrival predicted
BlueDot describes itself as a company that offers global infectious disease threat intelligence; it was founded by Dr. Kamran Khan, an infectious disease doctor. In 2016, Zika sparked a swell of global concern after a rash of outbreaks in Brazil. Khan, Bogoch and a group of collaborators published a paper in the Lancet predicting that Zika would emerge in Florida; six months later, the U.S. reported its first case there.
Lauren McKenzie, a mathematician and data scientist with BlueDot who led the new research, says the company wanted to model the spread of Aedes mosquitoes because it was seeing an increase in mosquito-borne diseases worldwide, especially dengue, Zika and chikungunya, known to cause fever, joint pain and headaches.
鈥淭hey鈥檙e progressively becoming a more dominant public health threat and economic burden as they continue to emerge in locations that were previously unaffected,鈥 said McKenzie. 鈥淕oing forward, climate change is just amplifying this risk.鈥
Warmer weather has a multitude of effects on disease-carrying mosquitoes, McKenzie said: it creates better conditions for mosquitoes to breed, extends the season when they are able to transmit diseases, and even increases their biting activity.
BlueDot鈥檚 model looked at two species of mosquitoes: Aedes aegypti, which is a more efficient transmitter of diseases, whose range did increase under warmer conditions but less dramatically, and Aedes albopictus, which transmits viruses less readily but is much more sensitive to warmth and precipitation.
According to their research, Aedes albopictus could spread up to 400 kilometres northward in North America and Europe. Under the worst-case climate change scenario, New York City has a 78 per cent probability of hosting the species by 2032, while London and Beijing are both in the 50 per cent range. Along with Toronto, these cities are home to 40 million people, BlueDot calculated.
Little immunity in local population
Dr. Andrea Thomas, the company鈥檚 director of epidemiology, notes that the introduction of diseases won鈥檛 happen overnight.
鈥淲hat these models show is where we might expect to potentially have these mosquitoes present down the road. It still can take maybe some more time for the diseases to be imported in to a location,鈥 she said.
But she also noted that countries where these diseases haven鈥檛 circulated locally aren鈥檛 likely to have much immunity at a population level, potentially worsening their impacts. Perhaps even more significantly, countries already overburdened by these diseases are seeing new outbreaks in areas that have been previously unaffected; Brazil had a particularly bad year, Thomas said.
Bogoch says that public health units, primary care providers and other health-care professionals need to be on the alert for many different diseases that are either now appearing locally because of climate change or may appear soon, including other diseases borne by ticks. (海角社区官网Public Health confirmed that it has not detected any Aedes albopictus mosquitoes; an official with the Windsor-Essex County Health Unit said that none of the Aedes mosquitoes they trapped had tested positive for these diseases of concern.)
鈥淲e haven鈥檛 published the model yet, but when I saw the results, my eyes widened,鈥 said BlueDot鈥檚 Thomas. 鈥淚 think we鈥檙e in for some interesting times ahead.鈥
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