Climate change will affect diseases in widespread and varied ways

Is climate change set to cause a sicker future? It turns out that we don’t really know.

A warmer world will change weather patterns, alter habitats and shift the distribution of many animals. This will bring different populations of humans and wildlife together – raising the risk of zoonotic disease jumping between them.

However, a new analysis of hundreds of scientific studies shows that the exact impacts are hard to predict. The effect of climate change has only been investigated in around 6% of the 816 zoonotic diseases that affect humans.

Even in diseases where the climate risks have been studied, there’s a lot of variability. While higher temperatures have been linked with a higher risk of disease, that’s not always the case. For rainfall and humidity, the results are even less clear.

Many of the links between climate change and disease are also obscured by differing research standards, which make the outcomes of many studies difficult to compare. Artur Trebski, the study’s lead author, says that we need to do a much better job of researching climate-disease interactions.

“It’s sometimes suggested that climate change will animal-borne diseases worse for humans, but our research shows that it’s much more complex than that,” Artur says. “We see so much variation, even within the same disease, that we need much more nuance in how we summarise the future health impacts of climate change.”

Dr David Redding, who leads our research into biodiversity and health, adds that public health research needs to move away from “a one-size-fits-all approach.”

“Climate change is an all-encompassing process that will affect nearly every single living thing on the planet,” David says. “The fact that there’s not a consistent way to examine how different animals and the diseases they carry are affected by this process is really surprising.”

“I hope that this study will be the start of moving people towards a common research framework that allows us to act in a more co-ordinated way. By better understanding the nuance of these relationships, we will be in better place to make effective control measures.”

The findings of the study were published in the journal Proceedings of the National Academy of Sciences.

How does the climate affect zoonotic diseases?

While the research found that zoonotic diseases were generally sensitive to the climate, they found that the infections responded to change in a variety of different ways.

The strongest links were between animal-borne diseases and temperature, with higher temperatures almost twice as likely to raise the risk of infection than they were to decrease it. However, it’s not as straightforward as one disease having a single response to temperature because it can vary depending on the exact situation.

“Think of the plague as an example,” David says. “Plague is caused by a bacterium that circulates between rodents and the fleas that feed on them. It’s strongly influenced by temperature. Warmer conditions can boost rodent populations in some regions and speed up flea development, which can increase opportunities for transmission.”

“However, this relationship is not linear. At higher temperatures, plague is less efficiently transmitted because the conditions that allow the flea to become infectious break down. Beyond a certain point, further warming actually reduces the spread of plague.”

The link between zoonotic disease and temperature also depends on the animal that it’s spread by. While warmer conditions tend to lead to more mosquitoes, and thus disease, there wasn’t as good an understanding how this would impact bats and rodents, probably because the mechanisms that dictate their spread are more complex.

It was also less clear how disease and other climate factors, such as rainfall and humidity, are linked. While temperature changes tend to affect animals in more predictable ways, changes in rainfall and humidity lead to more varied impacts.

“Rainfall and humidity can affect organisms in multiple ways,” Artur says. “On one hand, flash flooding can create watery environments that are perfect for mosquito breeding, but it might also wash out the existing breeding habitats they already have.”

“In rats, rainfall can lead to greater food availability, thus increasing their population size, but excessive rain can damage or destroy their burrows. There are so many different ways that rainfall and humidity affect wildlife that they obscure each other, making it hard to identify a consistent pattern.”

Looking to the past and future

Having examined how the impact of different diseases is affected by different aspects of the climate, the team used this to look to the future.

They found that zoonotic diseases linked positively with temperature are likely to become more prominent, as the majority are found in areas which will see substantial temperature increases in the coming decades.

While the future impact of increasing rainfall is more uncertain, many areas where it’s linked to disease are likely to get wetter. This means that the risk of zoonotic disease could increase in these specific areas.

To improve our understanding of how climate change will affect these diseases further, the team have called for researchers to report a consistent set of data that can be more easily compared. They’re also hoping to start building up an idea of how these diseases have already changed by delving into the past.

“We’re hoping to start sequencing museum specimens to help us build up information about how pathogens have already adapted to the changing climate of the past century,” David adds. “By understanding their history, we’ll be better able to anticipate the more complex ways these diseases might change in the future.”