The Covid-19 pandemic, with its virus probably born in bats, has highlighted the dangers of increasingly extensive interference between human activities and nature, which promotes the transmission of animal diseases to the man.
But the risk of epidemics can also come from another disastrous consequence of human activities: climate change, which causes the displacement of mosquitoes carrying malaria or dengue fever, and the beginning of the thaw of permafrost where more microbes are frozen. or older.
“In my darkest moments, I see a truly horrible future for Homo sapiens,” blurted Birgitta Evengard, a microbiologist at the University of Umea, Sweden.
“Our greatest enemy is our own ignorance, because nature is full of microorganisms,” including permafrost, “a real pandora’s box,” she told AFP.
These permanently frozen soils, which cover a quarter of the land in the northern hemisphere, in Russia, Canada, or Alaska, are already a climate time bomb: a “significant” part could thaw by 2100, releasing tens or even hundreds of billions of tonnes of greenhouse gases, according to UN climate experts (IPCC).
And that’s not all. “Microorganisms can survive in a frozen space for a very long time,” said Prof. Vladimir Romanovsky of the University of Alaska at Fairbanks.
“As soon as the ground thaws, water begins to flow through, taking with it particles, organic matter or microorganisms isolated for hundreds or thousands of years”, explains the geophysicist.
Science has proven that some of these microorganisms can be awakened.
“When you put a seed in frozen ground for thousands of years, nothing happens. When you warm the soil, the seed will be able to germinate. It’s the same with a virus,” explains the AFP Professor Jean-Michel Claverie.
Mammoths and Neanderthals
With his team from the Mediterranean Institute of Microbiology, he has succeeded in reactivating Siberian viruses dating back at least 30,000 years.
These awakened organisms only attacked amoeba. But in these frozen regions, “Neanderthals, mammoths, woolly rhinos got diseases, died, fell. Probably all the viruses that caused their problems at the time are still in the ground,” he continues.
The number of bacteria or viruses trapped there is incalculable. But the real question is: are they dangerous? Scientists are divided.
“Anthrax proves that a bacteria can sleep in permafrost for hundreds of years and be revived,” says Birgitta Evengard.
In 2016, in Siberia, a child was killed by anthrax disease, which has disappeared for 75 years in the region.
This contamination is sometimes attributed to the thawing of an ancient corpse of reindeer caught in the permafrost. Some experts believe, however, that the carcasses were simply in the surface soil that thaws every year, so this event would not prove that a pathogen frozen for much longer in the permafrost could still kill.
Other known pathogens, such as the influenza viruses of 1917 or smallpox, are also potentially present in arctic cemeteries welcoming the victims of ancient epidemics preserved in the icy layers.
While some, like Vladimir Romanovsky, believe they are “probably deactivated”, others are less certain.
In any case, for smallpox, thanks to the vaccine, “one would get away with it, even if a local epidemic would do damage,” says Prof. Claverie.
For him, “the real danger” is much deeper, in the layers which may date back 2 million years and which potentially contain unknown pathogens.
It is also necessary for an awakened virus, old or not, to find a host. A meeting that climate change would facilitate, by always opening up the roads to the Far North.
“With the industrial exploitation of the Arctic, we have all the conditions for risk met: a potential danger, with the presence of people”, insists Jean-Michel Claverie.
Tiger mosquito invasion
Global warming could also become a powerful ally for much more current viruses, which are already wreaking havoc across the world.
Malaria, dengue, chikungunya, zika … Some mosquitoes that are vectors of “tropical” diseases could be found at home in Europe or North America.
“Mosquitoes extend their reach towards the North and are now able to survive the winter in certain temperate regions”, underlines Jeanne Fair, researcher at the American Laboratory Los Alamos, who is working on models to predict how far they could s ‘install.
The presence of this vector (tick, mosquito, fly) is not enough. Of course, you need a host. And “special temperature conditions so that the pathogen can replicate in the mosquito”, insists Cyril Caminade, epidemiologist at the University of Liverpool.
For example, the tiger mosquito (Aedes albopictus), native to the tropical forests of Southeast Asia but become one of the worst invasive species in the world, is now present in Europe on almost the entire Mediterranean coast, even in Paris, and could continue its advance north.
For now, the European Center for Disease Prevention and Control (ECDC) has only listed a few indigenous cases of the diseases it can transmit: around 40 cases of dengue fever between 2010 and 2019, two cases of zika in France in 2019, and several hundred chikungunya between 2007 and 2017, mainly in Italy.
But the tiger, often pointed out, is not the only one. Another mosquito (Aedes aegypti), the main vector of dengue fever, is also under surveillance.
“An increase in average temperature could lead to seasonal transmission of dengue fever in southern Europe if the virus-infected A.aegypti becomes established there,” warns ECDC.
As for the risk of malaria returning to regions where it was once endemic, in Europe or North America, it is less clear: the prevalence of this disease transmitted by anopheles and for which treatment exists, is largely linked to the conditions. socio-economic.
According to a study published in 2011, cited by the latest IPCC benchmark report, 5.2 billion people could live in 2050 in areas affected by malaria, if we do not limit global warming. But by adding factors of strong economic growth and social development, that number would drop to 1.7 billion.
Despite everything, “recent experiences in Southern Europe demonstrate how quickly the disease can reappear if the health services decline”, insisted the IPCC, referring in particular to the resurgence of cases in Greece after the 2008 crisis.
As for Africa, which in 2018 had more than 90% of the 228 million cases of malaria, it risks seeing mosquitoes rise in altitude, for example on the Ethiopian plateau and in Kenya.
In general for these tropical diseases, for the moment, the signals “are worrying in terms of the presence of vectors, not necessarily of transmission”, summarizes Cyril Caminade.
“It shows the beginnings,” he notes. And “we are only at the aperitif of climate change”.