Climate change: advance of germs

More than half of known human pathogenic diseases such as dengue, hepatitis, pneumonia, malaria, Zika and more, can be aggravated by climate change. In the coming decades, ecological degradation, rising temperatures, and extreme weather events could intensify the threats to human health posed by viruses.

Tropical mosquito species that can transmit pathogens are increasingly conquering northern latitudes. Warmer waters allow germs to thrive. Drought and forest fires force animals to move close to people. And then there’s the thawing permafrost releasing ancient germs.

climate change

We know from past epidemics that changes in temperature, rainfall, and humidity can have profound effects on the spread of infectious disease. In the summer of 1878, the southern United States was struck by a catastrophic outbreak of yellow fever, a viral disease indirectly transmitted between people via the mosquito Aedes aegypti. Around 100,000 people contracted the disease, and up to 20,000 people lost their lives. Some estimates put the economic cost as high as $200m.

Yellow fever was a regular scourge of cities in the lower Mississippi River basin in the 18th and 19th centuries. “During its brief reign — July to October — its ravages were such as to completely paralyze both the social and commercial interests of a given city, and even an entire section of our country.”

Smoking gun

By 1911, improvements in rainwater storage and sanitation had denied the mosquito many of its former breeding grounds in open rain barrels and cisterns close to houses. But it would take until the end of the 20th century before scientists realized why outbreaks were so much worse in some years than others. Between 1793 and 1905, there were nine devastating yellow fever epidemics. Seven coincided with a major El Niño episode. El Niño is a band of warm water that develops off the Pacific coast of South America every 4 years or so. The phenomenon results in high rainfall, warm springs, and hot summers in southern U.S. states.

A taste of things to come

Predicting how future climate change will influence the spread of viral infections is fraught with difficulty. This is due to the complexity of interactions between climate, nature, and human activity. But annual fluctuations in some viral infections, such as seasonal flu, and historical epidemics, such as yellow fever, provide some clues.

According to the Intergovernmental Panel on Climate Change, human activity has already caused approximately 1.0°C of global warming above pre-industrial levels. If warming continues at its current rate, temperatures will reach 1.5°C above these levels between 2030 and 2052.

As a result, there is likely to be more extreme weather, including more droughts, flooding, and heatwaves. Changes in temperature, rainfall, and humidity will have numerous knock-on effects on the world’s animals and ecosystems.

Among the species affected will be the animal hosts of viruses that also infect humans — or that have the potential to do so — and the insect “vectors” that transmit them.

There is no evidence that climate change played any role in the coronavirus pandemic, but there is intense debate about a possible role of different weather patterns.

Nonetheless, there are lessons to be learned about how future changes in human activity driven by climate change might increase the likelihood of viruses jumping from wild species into our own. As happened with COVID-19, which is the infection caused by the novel coronavirus SARS-CoV-2, the leaps of these viruses between species can create new diseases to which humans have little immunity.

According to a report by the World Health Organization (WHO)Trusted Source, “Climate change, one of the global environmental changes now underway, is anticipated to have a wide range of impacts upon the occurrence of infectious disease in human populations.”

It is possible to summarize the mechanisms through which it might influence the spread of viral disease as follows:

  • Insect vectors
  • Animal hosts
  • Human behaviour
  • The immune system

Insect vectors

Biting insects, such as mosquitoes, ticks, and sand-flies, that transmit viral infections are cold-blooded. This means they are unable to regulate their body temperature, so external fluctuations strongly influence them.

A sudden, large increase in temperature might eliminate an insect vector, but it might benefit from smaller, incremental increases. Warmer conditions might improve breeding conditions, make food more abundant, increase activity, or extend its lifespan.

In theory, increases in temperature due to climate change could potentially increase human exposure to insect vectors, or increase their biting rate. There is a limited range of climatic conditions within which insects can survive and reproduce. A warming climate may, therefore, result in shifts in their geographical range or force them to evolve in some way to adapt. These changes could result in “emerging infectious disease,” defined as an infection that has increased in incidence or spread to new regions or populations in the past 20 years.

A report published in 2008 in the journal Nature found that vector-borne infections accounted for around 30%Trusted Source of all emerging infectious diseases over the previous decade. Worryingly, the increase to 30% represents a significant increase over previous decades.

The authors wrote: “This rise corresponds to climate anomalies occurring during the 1990s, adding support to hypotheses that climate change may drive the emergence of diseases that have vectors sensitive to changes in environmental conditions, such as rainfall, temperature, and severe weather events.”

Changes in precipitation

Experts predict climate change to increase rainfall in some regions and reduce it in others, with complex, unpredictable effects on vectors. Increased precipitation could result in the development of more areas of still, open water. These areas, such as puddles and discarded containers, are perfect for the larval stages of vectors to grow in.

According to the WHO, wet, humid conditions may have caused past outbreaks of yellow fever and Dengue fever, both spread by the A. aegypti mosquito.

In some places, droughts could also increase opportunities for vectors to breed, as river beds dry up to leave stagnant pools, and as humans try to collect and store more rainwater in butts and reservoirs.

Experts think that a warm winter followed by a hot, dry summer in 1999 led to outbreaks of mosquito-borne West Nile virus in mid-Atlantic U.S. states through a complex web of ecological changes.

In addition to the increased availability of stagnant water for breeding, the ecological changes may have skewed the natural balance of nature in other ways. For example, there may have been fewer frogs and dragonflies around to eat the insect larvae. Birds are the virus’s primary host, and their higher concentrations at the shrinking waterholes may have made them an easy target for biting insects.

Animal hosts

Infectious diseases that people catch from animals are known as zoonoses. The authors of an article in the journal Annals of the American Thoracic Society point out that if climate change displaces wild animals, they will bring their zoonoses with them.

They write: “Climate change may shift habitats and bring wildlife, crops, livestock, and humans into contact with pathogens to which they have had less exposure and immunity.”

Changes in rainfall and temperature, for example, can affect the availability of food eaten by animal hosts, such as bats, chimps, pangolin, and deer. The resulting changes in the size and range of their populations may bring them into closer contact with humans. There is some evidence that this has happened in the past.

In late 1999 and early 2000, scientists in Los Santos in Panama identified the first ever cases in Central America of hantavirus pulmonary syndrome.

This potentially fatal lung disease is a zoonosis caused by a virus shed in the saliva, urine, and feces of rodents.

A report in Emerging Infectious Diseases pins the blame for the outbreak on a two- to three-fold increase in rainfall in Los Santos in September and October 1999, which led to an explosion in rodent numbers.

Excess rainfall may also indirectly promote the spread of enteroviruses that affect millions of people worldwide every year. Humans transmit enteroviruses, including poliovirus, coxsackie, and echovirus, to other people via the fecal-oral route.

For example, climate change can cause flash floods on land and sweep human sewage into the sea. When this happens, some of these viruses might contaminate shellfish, for example, leading to higher levels of disease in humans.

Human behaviour

It is estimated that three out of every four new or emerging diseases come from animals. Experts have linked the earliest cases of COVID-19 to the Huanan “wet” market in Wuhan province, China, where people sold wild animals for meat.

A new study published in Nature has confirmed that novel coronavirus was not made in a laboratory, as some conspiracy theories had suggested. Rather, its genome bears a striking resemblance to bat coronaviruses, and it is similar to coronaviruses that infect pangolins. This is consistent with the theory that the virus spread to humans from bats via the pangolins sold in the Huanan market.

While there is no suggestion that climate change played any role in the emergence of COVID-19, it may have a knock-on effect on the type of human activity that brings wild animals and people into closer contact, particularly when food is in short supply. For example, if crops fail and livestock perishes due to increased flooding, droughts, heatwaves, or pests, hunger may drive people to hunt and eat more wild animals.

Something similar may have led to the emergence of Ebola, a particularly infectious and deadly virus, in a village deep in the Minkebe Forest in northern Gabon in 1996. Experts believe that the outbreak was due to the villagers killing an eating a chimpanzee. Scientists linked a later outbreak that began in 2007 in West Africa to eating fruit bats. The destruction of pristine forest ecosystems by logging and other human incursions may also increase the risk that other viruses will leap from wild animals into people.

According to another study published in Nature, degraded habitats harbour more of the viruses that can infect humans. This may be because biodiversity loss “amplifies” viral infections in the remaining species.

The scientists write: “In principle, loss of biodiversity could either increase or decrease disease transmission. However, mounting evidence indicates that biodiversity loss frequently increases disease transmission.”

Human immunity

In northern latitudes, influenza epidemics tend to occur between October and May, peaking in January and February. In general, warm weather reduces the spread of flu, possibly because people are less likely to gather indoors in large groups. Alternatively, warmer and more humid conditions may reduce the viability of respiratory viruses. So, climate change may push seasonal outbreaks northward, where it’s cooler and drier. There is no scientific consensus on whether warmer conditions in the coming decades will result in more or less severe flu epidemics. Climate change may have more subtle effects, however.

An analysis of influenza in the U.S. between 1997 and 2013, for example, found that warm winters were followed by earlier, more severe flu seasons the next year. The paper in PLOS Currents: Influenza suggests that mild winters may reduce “herd immunity” because fewer people are contracting the virus. This makes it easier for the virus to spread the following year, resulting in worse outbreaks.

The authors of a study published this year in IOPscience warn that rapid fluctuations in temperature — a characteristic of global warming — impair the immune system’s ability to fight off respiratory infections.

They found that rapidly changing weather in the fall has associations with more severe outbreaks of flu in the ensuing winter months.

The scientists write: “[Climate models suggest] that the rapid weather variability in autumn will continue to strengthen in some regions of northern mid-latitudes in a warming climate, implying that the risk of an influenza epidemic may increase 20% to 50% in some highly populated regions in the later 21st century.”

The immune systems of young children and older adults seem to be particularly vulnerable to rapid temperature changes. Doctors write in Annals of the American Thoracic Society that spikes in childhood pneumonia in Australia are associated with abrupt falls in temperature.

Cause for optimism?

There is concern that a changing climate will bring more viral disease outbreaks. However, although outbreaks may become more frequent, science is in a better place to counter them.

Recent technological advances mean that scientists can develop and manufacture diagnostic tests and vaccines at a speed that would have been unthinkable just a decade ago.

However frustratingly slow the response to COVID-19 may feel at the moment, a situation such as this would have been worse a decade ago, when it could take 10-15 years to develop a vaccine. Now, scientists are hopeful of having a vaccine against SARS-CoV-2 within the next 12-18 months.

An analysis of infectious disease outbreaks published by Journal of the Royal Society Interface in 2014 concluded:

“Our data suggest that, despite an increase in overall outbreaks, global improvements in prevention, early detection, control, and treatment are becoming more effective at reducing the number of people infected.”

Pathogens on a world tour

Even today, travellers still bring diseases with them, such as the monkeypox virus, which probably spread rapidly from a traveller to Africa due to a super spreader event in Spain. Or exotic mosquito species arrive as stowaways with deliveries of goods in Europe’s ports.

The Asian tiger mosquito, which can transmit potentially dangerous diseases such as dengue, chikungunya or yellow fever, is considered a particular risk.

The Bernhard Nocht Institute keeps an eye on that. Here, the contents of insect traps set up throughout Germany are regularly evaluated, but dead birds are also examined for pathogens that could infect the mosquitoes. The feathered carcasses are sent in by finders all over Germany.

Only a matter of time

“At the moment, exotic mosquitoes are not a big concern for us,” says mosquito expert Lühken in an interview with NetDoktor. The populations in Germany are small, the outbreaks in southern Europe are very limited. “We are not yet seeing any transmission of pathogens – but that is only a matter of time,” says the scientist. In Italy, it took about 20 years for populations to grow large enough. “That will also be the time horizon for us.”

The expert warns that the more urgent problem is currently caused by native mosquitoes. “They sit here in every rain barrel and can definitely transmit the West Nile virus.”

Learn from Greece

For the future you have to think about how to fight mosquitoes. “We can learn something from Italy and Greece.” For example, the larvae can be specifically controlled with special agents that do little to other insects. The Upper Rhine has many years of experience, but other warm regions in Germany still have no plan and no structures. “We are only at the beginning,” says the scientist.

Another example of vector-borne diseases spread by changing climatic conditions are those passed on by ticks. With warmer temperatures, the bloodsuckers can reproduce more successfully. They can now even be found in the northern regions of Norway and Sweden. Ticks transmit tick-borne encephalitis ( TBE ) and Lyme disease. The number of regions that are considered risk areas for these diseases is increasing year by year.

What are zoonotic diseases, and how are they affected by climate change?

Zoonotic diseases are those which can jump between different species. In general, the more closely related two species are, the more likely they are to jump between them.

Some species are more likely to harbour zoonotic diseases than others. Among mammals, bats carry the highest proportion of these diseases, as they have particularly strong immune systems that allow them to tolerate, rather than succumb to, these infections.

Their ability to act as disease carriers, as well as being able to cover large distances in flight, makes them a significant vector for zoonotic events.

In the past, this has been less of a concern for the spread of viruses between different species, as most mammal species only share limited parts of their range with another. A 2020 study estimated that 93% of mammal species pairs shared less than 5% of their ranges, while only 0.5% shared more than half.

However, as Earth’s temperature increases, it is likely that this will change. The study estimates that the vast majority of mammal species will overlap with at least one new species somewhere in their future range in any climate scenario, doubling the current level of contact.

Bats in particular will have some of the greatest range changes, with the study estimating that 88% of the new encounters between mammals will be between a bat and another species. Of these new meetings, around 8,500 will be between bats and primates, resulting in around 110 instances of viruses crossing between these species for the first time. This could mean that more diseases could spread from animals to humans.

More germs cavort in warm water

Germs also live in the water and multiply suddenly at higher temperatures. They include bacteria such as cholera and vibrio, but also pathogenic amoebas. So far, the North and Baltic Seas, rivers and bathing lakes have only been slightly polluted. Corresponding outbreaks of disease were unlikely in our latitudes.

That is also likely to change: the temperature in the world’s oceans is rising. For example, measuring stations have recorded warming in the North Sea in the range between 0.5 and 2 degrees over the past three decades. This is already causing problems for the world’s sea creatures, from krill to blue whales. In the shallower shore waters, however, temperatures rise significantly more in hot summers. Best conditions for pathogens.

When the permafrost thaws

One threat that few people have on their radar in terms of health hazards is the warming of the Arctic regions. Not only are the polar ice caps melting, the permafrost is also thawing faster and faster. In doing so, they expose not only well-preserved mammoths but also pathogens. Unlike the woolly giant elephants, some of these germs are highly active again after thawing.

In 2016, a 12-year-old boy died of anthrax in north-eastern Siberia . The highly dangerous bacterium that caused the disease came from a thawed animal carcass that infected grazing reindeer. Seventy other people required hospital treatment. Unlike viruses, which usually perish quickly in the air, some bacteria sometimes prove to be amazingly viable even after thousands of years of hibernation.

Pandora’s Box

“The emergence of such frozen pathogens could open a Pandora’s box,” warn Mora and colleagues. A large pool of pathogens could have accumulated that could be completely unknown to modern humans – and which encounter a correspondingly unprepared immune system.

What to do? There is only one answer to this question. Lühken says: “The quintessence of the Hawaiian study is: Crucial to all these problems is the fight against global warming, that’s the absolute key.”


Climate change is a recognized global challenge that threatens the economic and social stability of nations. However, climate change does not affect all countries equally, but some are more targeted than others due to their specific characteristics, such as geographic location, population density and urbanization, structure of the economy, and access to natural resources.

The cost of making fast and radical change to existing economies and societies isn’t small. However, current calls for such change are framed in the argument that they will be a major economic boon in the long run, as well as an actual imperative to protect from unmitigated disaster. The Commission estimates that a $1.8 trillion investment made over the course of the decade to improve climate resilience would yield up to $7.1 trillion in net benefits.

With growing pressure from green lobbying groups, youth activists, scientific institutions and visionary business leaders, governments across the world are waking up to the fact that climate adaptation is no longer an option, it’s a necessity.

Examples of leading countries and cities adapting to climate change

The United Arab Emirates: The UAE is one of the 6 countries in the world most vulnerable to the effects of climate change, due to the “extreme risk” of water stress. However, due to its high level of resources and long-term desire to diversify its economy, the UAE is better placed than most to adapt and become more resilient to the threat. Under the umbrella strategy of the National Climate Change Plan of the UAE 2017–2050, the country is already forging ahead with significant green energy investments, plans to develop new freshwater supplies, vast tech-driven improvements in desalination and sweeping reform of agriculture to reduce water wastage.

The Philippines: With a long history of vulnerability to hurricanes, tsunamis and other natural disasters, the Philippines is on the frontline of climate change-related struggle. The country has experienced an increase in political enthusiasm and tangible action on the subject, focusing most intently on Manila, after a 2009 flood submerged a full 80% of the city. Climate adaptation plans created by the country’s Climate Change Commission include modernising of the Manila metro and infrastructure around waterways, as well as long-term prioritisation of water and food security, renewable energy, smart infrastructure and other essential sustainability improvements nationwide.

Germany: Germany has already achieved a great deal in the field of climate action. In 2019 about 43 per cent of electricity was generated from renewable sources, such as wind and solar power. Under the Climate Action Programme 2030 and the new Climate Action Act (Klimaschutzgesetz) the German government has made a binding undertaking to reduce greenhouse gas emissions by 55 per cent by 2030.

United states: While overall US policy on climate change is a mixed bag to say the least, individual states and cities are forging ahead with plans to protect themselves from climate-related disasters. Aiming to emulate the success of London’s Thames, New York and New Jersey are developing plans for a $119 billion seawall to protect against storm surges and hurricanes like Hurricane Sandy that ravaged the East Coast in 2012, causing $70 billion in damages.

A global imperative

Emerging research from international organisations regarding overall climate change adaptability readiness paints a somewhat uncertain picture, as some countries tackle the highly challenging issue head on while others skirt around it or largely ignore it. One important takeaway from a recent IISD (International Institution for Sustainable Development) report is that these differences among countries cannot be attributed to their development status. Instead, progress appears to be driven more by the impact and energy of government leadership figures, as well as the impact of various advocate groups.

Progress, then, appears to depend on the right people making the right impact at the right time.