Keep it in the ground: climate change could prompt the reemergence of zombie pathogens

There is nothing permanent about permafrost. Models suggest that between 24% and 70% of existing permafrost will thaw by 2100 owing to climate change.1 This is a frightening prospect—permafrost covers almost a quarter of the northern hemisphere and a few areas elsewhere. The thaw is changing the form and function of these frozen ecosystems,2 by releasing much that has been firmly underfoot or safely stored away in ice, including carbon dioxide and methane that will contribute to further global warming; subsiding ground that is destabilising arctic infrastructure; and revealing mummified wildlife, livestock, and people and their pathogens within.

A frequently headlining risk from permafrost thaw is the emergence of long frozen microbes often called zombie viruses, although bacteria, fungi, and other potential pathogens also lie beneath. Together, they are feared for their potential to resurrect and become infectious again, potentially even sparking another pandemic.3 Yet opinions are divided as to whether climate driven, zombie pathogen emergence is a real possibility or an imagined threat, and what action could be taken to prevent it.

Given current knowledge, we should avoid hyperbole and exaggeration when it comes to communicating the risks. Not one case of viral infection from thawed permafrost has yet been reported in humans.4 For that to happen, viruses capable of infecting people would need to have been preserved and then thawed under extremely favourable conditions in order to survive and become infectious. This scenario is very different to what has been described of most viruses from permafrost so far.

Variola virus DNA, for example, has been recovered from a human corpse with smallpox frozen in Siberian permafrost for 300 years, but it was degraded and non-infectious.5 RNA viruses, the cause of most human viral diseases and nearly all pandemics, are even more fragile. Pandemic H1N1 influenza—an RNA virus and cause of the Spanish flu, one of the most devastating pandemics in history—was recovered from the lungs of an individual buried in a mass grave in Alaskan permafrost in 1918, but again it was not viable.6 Natural human infection from viruses of epidemic or pandemic potential thawing from permafrost seems exceedingly unlikely.4

Yet there does remain real cause for concern about pathogen resurrections from permafrost and we must ensure we do not exacerbate these risks. For instance, the degraded and unviable pandemic H1N1 recovered from the Alaskan permafrost has since been reconstructed in a laboratory. From there, it quickly proved its mettle in laboratory infection trials by causing illness or death to mice, ferrets, swine, and non-human primates.7 Although laboratory accidents and leaks of dangerous pathogens are rare, intentional and unintentional breaches in biosecurity resulting in human and animal deaths within and well beyond the laboratory have occurred frequently enough. Permafrost offers a very deep pool of microbes to experiment with, some of which will prove pathogenic as we continue to work out how to resurrect them.

Other groups of pathogens have also proven more resilient than dangerous viruses to permafrost thawing. The bacterial Bacillus genus has very hardy spores that can survive in various hostile environments (hot, salty, frozen) for decades to millennia. Anthrax outbreaks caused by spores of the bacterium Bacillus anthracis have occurred because of exposure to 75-year-old livestock carcasses thawing from permafrost. One such outbreak in Siberia in 2016 infected and killed more than 2000 reindeer and a 12-year-old boy. Around 90 people were admitted to hospital.8 About 1.5 million reindeer infected with anthrax is buried in frozen ground in arctic Russia alone and will need biosafety management when, or before, they thaw.8

Other equally important examples exist where risk is harder to assess. For example, the megavirus Pithovirus sibericum was revived from 30 000-year-old Siberian permafrost by thawing it and offering it today’s version of its natural host, an amoeba.9 While such giant and hardy viruses are predominantly viewed as pathogens of unicellular organisms, some can infect humans and have even been associated with an immune response and illness (eg, Mimivirus and pneumonia), and their role in the human microbiome remains unclear.10

Researchers and public health officials should pay greater attention to permafrost thawing induced by climate change and the risk of emerging pathogens. Equally, we cannot ignore the impact that the exploration and exploitation of fossil fuels, infrastructure development, and growing urban populations in permafrost regions are having. These are all major catalysts for changes in land use, ecosystems, and biodiversity.11 The presence and increase in these major risk factors for disease emergence, combined with permafrost thawing induced by climate change, could potentially transform these literal coldspots into future hotspots of disease emergence.12

We have become worryingly good at disturbing and bringing microbes back from the dead from permafrost. The best mitigation strategy for averting the emergence of zombie pathogens, as for climate change itself, is to limit our impact on these fragile landscapes and keep it in the ground—that is, as much fossil fuel, carbon dioxide, methane, and microbes as possible.