Introduction
In the four and a half billion years since Earth was first formed, it has been constantly undergoing all sorts of changes. During some points in the planet’s history, Earth has been covered in active volcanoes, yet it has also spent several periods covered in ice. All these changes have culminated in the life and climate on Earth as we know it today.
However, ever since the beginning of the Industrial Revolution, changes to the environment have been happening at an accelerated pace due to human activity. This is largely because, in order to keep up with ever-increasing energy demands, we have been relying on the use of fossil fuels (hydrocarbons formed over millions of years from the remains of dead plants and animals) such as coal, oil, and natural gas. When these fossil fuels are burned for energy, they release gases such as carbon dioxide and methane, both of which are referred to as “greenhouse gases”.
These gases are so named because they contribute to the “greenhouse effect”, a phenomenon where greenhouse gases circulating in the atmosphere trap some of the infrared radiation given off by the sun and reflect it back towards the Earth, warming the planet’s surface (shown in Figure 1). This is not unlike how the glass roof of a greenhouse traps heat to keep the plants inside warm.
Figure 1: A diagram of the greenhouse effect. Infrared radiation from the sun reaches the Earth’s atmosphere, at which point it is trapped by greenhouse gases in the air such as carbon dioxide (CO2), water vapour (H2O), and methane (CH4). These greenhouse gases direct some of this infrared radiation towards the surface of the Earth, increasing its temperature. The figure is taken from here.
Although the greenhouse effect is a normal process - in fact, without it, Earth would be significantly colder. And life, as we know it, would not have been able to exist - the excessive release of greenhouse gases into the atmosphere due to the use of fossil fuels means that more infrared radiation is being trapped, and as a result, the planet is getting even warmer. An increase of even a couple of degrees can have widespread impacts on agriculture and wildlife.
Even though it has only become a hot topic in the past few decades, climate change is rapidly becoming one of the greatest threats humankind has ever faced. We are already seeing some of the effects of climate change in the form of increasingly extreme weather. However, one of the potential consequences that we may not obviously realise is an effect on the spread of infectious diseases.
Since external environmental factors such as temperature and humidity have a significant effect on the viability and transmission of pathogens (disease-causing microorganisms), it is possible that climate change will lead to more favourable conditions for infectious diseases to spread. The speed at which these climate alterations are occurring also means that we are more likely to be overwhelmed by the onset of these infectious diseases.
The relationship between climate change and infectious disease is highly complex, especially since there are so many factors involved in disease transmission. Nevertheless, it is still important for us to understand how the two problems intersect so that we are able to predict how climate change might increase the spread of certain diseases and come up with strategies to mitigate the impact of such outbreaks.
Climate change and waterborne disease
There are a few different kinds of changes occurring in our physical climate as a result of the greenhouse effect, and these have the potential to lead to an increase in the spread of infectious diseases.
One element to consider is how climate change can trigger changes in the water cycle. The rise in the average global temperature leads to an increase in the evaporation of water, as well as an increase in the amount of moisture in the atmosphere (since warmer air is able to hold more water). This means that the average amount of rainfall the world experiences is increasing, and in some places, the heaviest downpours of rain are becoming even more intense.
This increase in extreme downpours and the rise in sea levels due to the ice caps melting are both contributing to a heightened risk of flooding. According to a report by the European Academies Science Advisory Council, the number of hydrological events (such as floods) across the world has doubled since 2004 and quadrupled since 1980.
As well as causing infrastructural devastation to the communities affected, flooding can lead to the increased spread of pathogens that can be transmitted by water, particularly if dirty floodwater contaminates drinking water sources. This can result in the increased incidence of waterborne diseases, which are already an issue in developing countries due to lack of sanitation. Examples of waterborne diseases that could see an increase due to more frequent flooding include hepatitis A and typhoid fever.
In addition to infectious diseases spreading as a result of flooding, the droughts that often follow floods and heavy rainfall could also lead to the spread of waterborne diseases. If there is not enough clean drinking water available, more individuals have to resort to drinking from stagnant water, which may contain pathogens such as Leptospira (which causes a blood infection known as leptospirosis).
Stagnant water can also be a good breeding ground for mosquitoes, which can carry a variety of infectious diseases. Hence, the impact of climate change on animal host behaviour as a whole is a problem in itself that we will examine in the next section.
However, the transmission of many diseases is dependent on more than one environmental factor. For example, cholera, an infectious waterborne disease caused by the Gram-negative bacterium Vibrio cholerae, which can induce diarrhoea and severe dehydration (that can lead to death if left untreated), is often linked to flooding. However, increased outbreaks of cholera have also been linked to other factors such as rising sea temperatures and changes in salinity, showing that there are many different ways in which climate change can lead to the increased spread of disease.
Changes in host behaviour
One of the main ways that some infections can spread is via contact with animals. Diseases caused by pathogens that can make this jump between animals and humans are known as zoonotic diseases (or zoonoses), and it is thought that around 75% of emerging diseases originate in animals.
These zoonotic diseases can be directly transmitted from animals to humans. For example, some types of influenza viruses that originate in birds or pigs can spread to humans. However, these viruses can also be spread indirectly by an intermediate organism that carries the virus without getting ill, and this kind of organism is known as a vector. Most vectors tend to be insects, but there are some examples of mammals acting as vectors, particularly bats.
Since the movement and lifestyles of animals are greatly influenced by their environment, climate change has the potential to indirectly lead to a greater spread of disease by impacting the habits and migration patterns of animal hosts. For example, climate change could change the distribution of diseases carried by birds, such as avian influenza, by influencing the locations to where disease-carrying birds migrate.
Mosquitoes, in particular, are known for carrying a variety of dangerous diseases such as malaria, dengue fever, and Zika virus, which they then pass onto humans when they bite to feed on human blood. Since mosquitoes cannot internally regulate their own body temperature in the same way warm-blooded animals can, they rely on warmer temperatures to survive. This means that the diseases they carry are typically confined to regions closer to the equator. However, if on average the world gets warmer, then this will allow mosquitoes to survive further away from the equator, and thus hundreds of millions of more people will be vulnerable to these mosquito-borne diseases.
An increase in temperature can also have an impact on the life cycle of the pathogens. For example, the temperature is known to shorten the extrinsic incubation period (EIP) of some pathogens, which is the length of time between a vector coming into contact with the pathogen and the vector being able to spread the disease. One demonstration of this was in a 2012 study, which showed that in dengue viruses, a shorter EIP was predicted at temperatures of around 30°C compared to 25°C. This is significant because a shorter EIP in this context could mean that the mosquito is infectious for a greater period of time and is, therefore, more likely to spread the dengue virus to humans when it feeds, leading to a greater spread of dengue fever.
Permafrost
One of the biggest potential pathogenic threats to us is not yet a reality, but if the planet continues to heat up, then it is possible that we could end up facing, long-frozen pathogens that are hiding in permafrost.
Permafrost is any sort of ground that stays frozen for at least two consecutive years and is generally found either in high, mountainous regions or polar regions. In fact, around a quarter of the northern hemisphere is covered in this permafrost. Some of this permafrost has already begun to thaw, leading to the release of greenhouse gases that have been contained inside it, further contributing to global warming and climate change. Additionally, there is also concern among scientists that if this permafrost melts due to climate change, then it could unleash dormant pathogens which are lying under the permafrost.
Although extreme cold is generally not good for the survival of microorganisms, it is possible for some pathogens to withstand these conditions and instead be preserved. In 2014, a 30,000-year-old virus that had previously been trapped in Siberian permafrost was unearthed by scientists and became infectious once it had thawed. Although this particular virus was not deemed dangerous to humans, it is possible that other, more lethal, pathogens could be unearthed.
The potentially deadly consequences of this thawing of permafrost were witnessed in the summer of 2016, where an anthrax outbreak in a remote part of the Arctic Circle that killed one person and hospitalised several others was traced back to a frozen reindeer corpse. This reindeer corpse had since thawed due to a heatwave in the area, infecting thousands of reindeer and eventually spreading to humans.
If climate change continues, it is possible we will see other incidents like this with deadly viruses. For example, it is possible that ancient strains of the smallpox virus, the only disease we have ever completely eradicated, could make a return if corpses of those who have previously died of the disease are uncovered. Furthermore, it is possible that diseases which we have not encountered before (and therefore may not know how to fight) could be released into the world. At present, the exact scale of this threat remains unknown.
Hope?
But it’s not all bad news.
Although the potential impact of climate change on the spread of a variety of infectious diseases is a cause for concern, having an awareness of these issues has led researchers around the world to develop systems which will help us predict disease outbreaks based on climate data.
The main solution, however, is to do all that we can to prevent further climate change from ravaging our planet. We are seeing an increased proportion of individuals who are concerned with climate change and are taking steps in their own lives to reduce their carbon footprint. More importantly, we are seeing countless countries committing to reducing their energy emissions with initiatives such as the Paris Climate Agreement.
Hopefully, with such massive global efforts to reduce humanity’s environmental impact on the planet, we will be able to mitigate some of the consequences of climate change, and by extension, prevent the spread of some infectious diseases.
Here are some resources providing more information on how the threats of climate change and infectious diseases intersect:
Author
Emma McCarthy
MSc Health Data Science
University of Manchester