LONDON - Climate change is melting permafrost soils that have been frozen for thousands of years. And as the soils melt, they are releasing ancient, once-dormant viruses and bacteria that are springing back to life, the BBC reported on Thursday (May 4).

In August 2016, in a remote part of the Siberian tundra called the Yamal Peninsula in the Arctic Circle, a 12-year-old boy died and at least 20 people were hospitalised after contracting anthrax, a serious infectious disease caused by bacteria.

Theory has it that more than 75 years ago, a reindeer infected with anthrax died and its frozen carcass became trapped beneath a layer of frozen soil, or permafrost. It stayed there until a heatwave in the summer of 2016 caused the permafrost to thaw.

This exposed the reindeer corpse, releasing infectious anthrax into nearby water and soil, and then into the food supply. Over 2,000 reindeer grazing nearby became infected, which then led to the small number of human cases.

Investigations into the anthrax outbreak in last August also sparked concern that smallpox could return, as Siberia's melting permafrost exposes ancient graves.

While the last known case of the deadly disease was in Somalia in 1977, Russian scientists investigating the anthrax outbreak found the virus's DNA in corpses that were once buried in the frozen ground, the Independent reported.

A major epidemic of smallpox occurred in Siberia in the 1890s, killing up to 40 per cent of the population in one town. The bodies were buried under the upper layer of permafrost soil, on the bank of the Kolyma River. Now, about 120 years later, Kolyma's floodwaters have started eroding the banks. The melting of permafrost has quickened this erosion.

The BBC said that there are fears that the anthrax outbreak will not be an isolated case.

In normal situations, superficial permafrost layers about 50cm deep melt every summer. But global warming is now gradually exposing older permafrost layers.

Bacteria can remain alive in frozen permafrost soil for very long periods of time - possibly as long as a million years.

The temperature in the Arctic Circle is rising about three times faster than in the rest of the world. Other infectious agents could be released as the ice and permafrost melt.

"Permafrost is a very good preserver of microbes and viruses, because it is cold, there is no oxygen, and it is dark," said evolutionary biologist Jean-Michel Claverie at Aix-Marseille University in France, the BBC reported.

"Pathogenic viruses that can infect humans or animals might be preserved in old permafrost layers, including some that have caused global epidemics in the past."

As humans and animals have been buried in permafrost for centuries, it is possible that other infectious agents could be released. Scientists have discovered intact 1918 Spanish flu virus in corpses buried in mass graves in Alaska's tundra. Smallpox and the bubonic plague are also likely buried in Siberia, the BBC added.

Researchers Boris Revich and Marina Podolnaya wrote in their 2011 study: "As a consequence of permafrost melting, the vectors of deadly infections of the 18th and 19th centuries may come back, especially near the cemeteries where the victims of these infections were buried."

In a 2005 study, National Aeronautics and Space Administration (Nasa) scientists successfully revived bacteria that had been encased in a frozen pond in Alaska for 32,000 years.

Two years later, scientists revived an eight-million-year-old bacterium that had been lying dormant beneath the surface of a glacier in the Beacon and Mullins valleys of Antarctica. In the same study, bacteria were also revived from ice that was over 100,000 years old.

The BBC pointed out that not all bacteria can come back to life after being frozen in permafrost. Anthrax bacteria can do so because they form spores, which are extremely hardy and can survive frozen for more than a century.

Some other bacteria which can form spores - and therefore survive in permafrost - are tetanus and Clostridium botulinum, the pathogen responsible for botulism, a rare illness that can cause paralysis and even prove fatal. Some fungi and viruses can also survive in permafrost for a long time.

Global warming does not have to directly melt permafrost to pose a threat.

The melting of Arctic sea ice has made the north shore of Siberia more easily accessible by sea. As a result, industrial exploitation, including mining for gold and minerals, and drilling for oil and natural gas, is now becoming profitable.

"At the moment, these regions are deserted and the deep permafrost layers are left alone," said Professor Claverie, the BBC reported. "However, these ancient layers could be exposed by the digging involved in mining and drilling operations. If viable virions are still there, this could spell disaster.

"Most viruses are rapidly inactivated outside host cells, due to light, desiccation, or spontaneous biochemical degradation. For instance, if their DNA is damaged beyond possible repair, the virions will no longer be infectious. However, among known viruses, the giant viruses tend to be very tough and almost impossible to break open."

Prof Claverie, who also pointed at the possibility that "we could catch a virus from a long-extinct Neanderthal", said stocks of vaccine should be kept as a precaution.

In February 2017, Nasa scientists announced that they had found 10-50,000-year-old microbes inside crystals in a Mexican mine. Even older bacteria have been found in the Lechuguilla Cave in New Mexico, 305m underground. These microbes, which are resistant to natural antibiotics, have not seen the surface for over 4 million years.

How concerned we should be about all this, the BBC article suggests, depends on our perspective.

Since the risk from permafrost pathogens is unknowable, people might well focus on more established threats from climate change such as a rise in outbreaks of diseases like malaria and cholera which thrive in warmer temperatures.

The alternative mindset is that we should not ignore risks just because we cannot quantify them.

Said Prof Claverie: "Following our work and that of others, there is now a non-zero probability that pathogenic microbes could be revived, and infect us.

"How likely that is is not known, but it's a possibility. It could be bacteria that are curable with antibiotics, or resistant bacteria, or a virus. If the pathogen hasn't been in contact with humans for a long time, then our immune system would not be prepared. So yes, that could be dangerous."