A climate for microbes: how the dynamics of infectious diseases will change on a warming planet
Microbes are shaping the planet’s atmosphere.
The number of bacteria on our planet is estimated to be a mind-boggling 5 x 1030 (a 5 with 30 zeroes) with viruses further outnumbering bacteria ten-fold. This incredible diversity of bacteria and viruses is the result of four billion years of evolution. During this time, microbes have shaped Earth in a myriad ways. Perhaps most importantly, bacteria have changed the atmosphere of our planet due to their production of oxygen, which then allowed the emergence of complex lifeforms.
In the 21st century, human activity has become a driver of change in the planet’s atmosphere as we are now facing the global challenge of a warming planet due to the increasing levels of the greenhouse gasses, most prominently carbon dioxide (CO2) and methane (CH4). Perhaps surprisingly, microbes are again contributing to these processes. An important source of methane are the bacteria that inhabit the gut of livestock. A dangerous positive feedback loop of climate change is formed by the thaw of permanently frozen soils (permafrost), which activates methane-producing bacteria and thus can lead to significantly increase production of this greenhouse gas in the Arctic regions of Russia, Alaska and Canada.
A warmer planet means more infections
The changing climate is also changing how bacteria and viruses behave and spread across the planet. The warmer winters that we are already seeing across Europe is leading to a more northwardly spread of mosquitoes that can potentially carry viruses, including those that cause yellow fever, West Nile fever, Zika and Chikungunya. In a ‘business-as-usual’ scenario for climate change, nearly a billion individuals will be newly exposed to these mosquitoes and the viruses that they can transmit. Current scenarios do not foresee a spread of these mosquitoes to the United Kingdom, but the mosquitoes are likely to become endemic across popular southern European destinations for tourism, thus potentially leading to increasing numbers of travel-related infections across the continent.
However, these mosquito-borne viruses might not be the only risks that threaten us on a warming planet. An estimated 10,000 viruses that are currently circulating in wildlife are thought to be able to infect and spread among humans. These viruses are entirely unknown to science and humanity will probably only first know of them after they cross the species barrier to start causing disease in humans, similar to what SARS-CoV-2 (the virus that causes COVID-19) and HIV have done in the past. Due to the effects of climate change, many of the ecosystems on our planet will undergo dramatic changes and this can lead to the movement of animal species into new habitats. When animal populations move from their original habitats to new sites, there is an increased risk that they come into contact with humans, thereby increasing the risk of animal-to-human transmission of the viruses they carry. This is particularly relevant for bats, who are broadly recognised as an important reservoir for novel viruses. The capacity of bats to fly allows them to travel hundreds of kilometres within their lifetime, thus potentially contributing to the dissemination of novel viruses on continental scales after their original habitats have been disrupted by climate change.
While viruses continue to form an acute threat to our health, bacterial infections also remain an important cause of disease and death globally. Since the mid-20th century, bacterial infections have been treated with antibiotics. Worryingly, bacteria that have evolved resistance to these drugs have become increasingly widespread. Infections caused by these antibiotic-resistant bacteria are becoming increasingly difficult to treat due to a lack of development of novel antibiotics. Antibiotic resistance can be regarded as a ‘slow pandemic’ that takes decades to spread, but can result in 10 million deaths per year by the year 2050 if not acted on. It may make medical interventions that are currently considered routine very risky as we would no longer be able to rely on antibiotics to suppress and cure infections. Recent research has shown that increasing temperatures in Europe and North America are associated with higher levels of drug-resistant bacteria. The mechanisms that are driving increased levels of antibiotic-resistant bacteria at higher ambient cultures are currently unknown. A simple explanation is that bacteria, including those that are resistant to antibiotics, just grow faster at higher temperatures. It is, however, also possible that bacteria can more efficiently share DNA containing the genes that code for antibiotic resistance with each other at higher temperatures. Global warming may thus directly contribute to the rapid spread of antibiotic-resistant bacteria across the planet.
A global response to infectious diseases in the age of climate change
To mitigate the impact of climate change on the spread of infectious diseases, we can build on the experience of the COVID-19 pandemic, which has made it abundantly clear humanity always needs to be prepared to go to battle with its microbial enemies. Rapid deployment of non-pharmaceutical interventions and the development of novel drugs and vaccines will remain cornerstones for an effective response to infectious diseases. In addition, global networks for surveillance and monitoring of infectious diseases will need to be expanded to rapidly flag and identify new outbreaks of infections. On a warming planet, a truly global effort is thus urgently needed to stem the emergence and spread of new and existing infectious diseases.