A zombie is a fictional creature that comes back to life through a pathogen that reanimates a dead body, and it is typically found in horror and fantasy stories. In these fictional stories, a microorganism, such as a virus, attacks the brain of an individual, triggering a biochemical reaction that revives the deceased body. Although it is unlikely that science can create zombies through viral infections like those depicted in fiction, recent evidence suggests that some types of viruses can remain viable for extended periods. Pandoraviruses are examples of giant viruses that can survive in a dormant state in hostile environments like sea ice and permafrost. This essay will explore the possibility that significant diseases, once thought to be extinct, could be resurrected as a result of melting sea ice and permafrost, and the consequent release of Pandoraviruses. In addition, this essay will identify possible concerns of the scientific community regarding the resurgence of these pathogens, and evaluate whether the resurgence of these pathogens could lead to a disease outbreak in the future.

The term ‘permafrost’ refers to soil that has been frozen for a prolonged period of time throughout the year. Permafrost covers around one-fifth of the Northern Hemisphere and provides an ideal environment for preserving biological material. However, climate change has caused a shift in the world’s permafrost, leading to the melting of upper layers in Canada, Siberia, and Alaska. This thawing process has the potential to release hazardous pathogens, such as viruses, that could pose a threat to human health. Evidence that viral pathogens can be conserved in ice repositories such as ice lakes, ice sheets, and glaciers supports this hypothesis [1]. Furthermore, in 2015, a team of esteemed scientists from France and Russia, led by Jean-Michel Claverie and Chantal Abergel, discovered new viruses that, even after being frozen in a dormant state for thousands of years, could still infect hosts once thawed.

One of these viruses, known as Pandoravirus yedoma, had been frozen for over 48,000 years but became infectious again due to the melting of permafrost in Russia [2]. This makes it the oldest virus to become active again. P. yedoma is distinguished by its exceptional size, genome complexity, and genetic content. It contains a significant number of proteins whose functions are yet unknown; this lack of knowledge might be because this virus is relatively new to science, and studies to discover mechanisms of a virus usually take years alongside significant financial investments. However, though current knowledge is limited, scientists have speculated that the adaptations of P. yedoma for survival in hostile low-temperature environments may involve specific structural protein mechanisms that allow the virus to remain stable for extended periods in a latent stage [3]. For instance, these proteins may provide the virus with a highly resistant capsid or outer shell that prevents ice crystal formation in low-temperature conditions [4].

Additionally, scientists have discovered that P. yedoma is a DNA virus [5]. Based on this discovery, researchers have been working to verify the hypothesis that P. yedoma would require specific mechanisms to safeguard its genetic material from degradation due to freezing and thawing cycles in the permafrost. These mechanisms might involve the packaging of DNA in a protective protein coat, or the presence of DNA repair mechanisms that can mend the damage caused by freezing conditions [6].

Scientists have also put forth a hypothesis that P. yedoma is capable of enduring harsh conditions for extended periods due to environmentally protective advantages, specifically protection against UV radiation. While permafrost is usually shielded from direct sunlight, UV radiation can still permeate the upper layers [8]. The virus may contain mechanisms that protect it from UV- induced damage, such as pigments or proteins that absorb or deflect harmful UV rays. Furthermore, P. yedoma may have developed specific tactics to exploit this long-term preservation, including the formation of aggregates or biofilms that shield it from environmental stressors [7].

Indeed, P. yedoma is an intriguing subject for scientists because this virus challenges our current understanding of viruses, and studying it helps to expand knowledge of viral biodiversity and evolution. It also can help scientists to gain insight into the diversity and complexity of viruses in the biosphere. Although P. yedoma is an ancient virus, it is harmless to humans. It has been documented to infect amoebas (specifically, the Acanthamoeba genus), which are unicellular eukaryotic organisms found in aquatic environments. The virus gains entry into the amoeba via phagocytic vacuoles, which penetrate the amoeba’s vacuole membrane, resulting in the release of viral particles into the host amoeba’s cytoplasm [8].

Though P. yedoma is unable to infect humans, in 2008, scientists isolated Acanthamoeba amoebas carrying an unknown endosymbiont from the inflamed eye and contact lens of a patient suffering from keratitis in Germany. Several years later, genome analysis of this endosymbiont confirmed its viral nature, and it was subsequently identified as a Pandoravirus [9]. However, the study did not determine whether the presence of Acanthamoeba and Pandoravirus on their eye lens was the source of the patient’s eye problem [9]. Although P. yedoma is often referred to as the “zombie virus”, it is doubtful that it could cause a zombie outbreak as depicted in fictional stories as this virus only infects amoebas. However, the scientific community remains concerned about the potential resurgence of these pathogens in the future if they adapt and infect humans. An unknown disease outbreak is a cause for concern. This pattern is observed in other types of viruses that can survive in harsh environments, such as the influenza A and B viruses [10]. Influenza A and B viruses cause highly contagious respiratory illness, affecting a significant percentage of the population each year and sometimes causing death [10]. Zhang et al. [10] found that three ice lakes in northeastern Siberia indicated that influenza A virus RNA was more frequently detected in the lakes with higher concentrations of migratory waterfowl and is conserved in higher concentrations in lake ice than lake water due to the freezing cold temperatures. This study revealed that ice acts as a reservoir for influenza A viruses, suggesting a potential long-term survival mechanism for the virus [10].

Another virus that can survive long-term is the variola virus, which causes smallpox, a highly devastating disease that has caused epidemics in different parts of the world for thousands of years. This illness may have started and disseminated from the East of Egypt or the Indus Valley about 3000 to 4000 years ago [11]. Due to modern human vaccination campaigns, smallpox is the only human infection that has been considered globally eradicated [11]. However, in 2004, a group of French and Russian archaeologists found frozen mummies in northeastern Siberia, dating back to the late 17th to early 18th century, with evidence suggesting death from variola virus. The analysis of lung tissue from the mummies has historical and biological significance, as it serves as a reservoir of DNA fragments from ancient pathogens such as variola virus, posing a potential threat to the resurgence of smallpox among humans [12]. Consistently, the discovery of variola DNA fragments and P. yedoma in Siberia highlights the importance of considering the possibility of other dormant pathogens in unexplored regions and the need for further research to identify and understand these hidden threats, as they could pose significant risks to human health if revived.

To conclude, the melting of sea ice and permafrost poses a significant threat by potentially releasing ancient pathogens that have been dormant for millennia. The case of P. yedoma highlights the possibility of such pathogens re-emerging and adapting to current environmental conditions. While this virus does not infect humans, its revival underscores the broader risks of climate change and the thawing of frozen environments. The discovery of other long-term surviving viruses, such as variola virus, further emphasises the need for vigilance and preparedness in the scientific community to mitigate the risks of potential outbreaks. As scientists’ knowledge about these ancient pathogens are limited, continued research is essential to understand their mechanisms and develop strategies to protect human health against possible future threats.