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A killer combo Illustrated by Laura Steel Pascual

A killer combo

How climate change fuels the spread of infectious diseases

The 2020s have only just begun and have already been a busy time for people studying infectious diseases. In the last three years, the COVID-19 pandemic has shaped peoples’ daily lives, put a strain on the economy and continues to cause deaths and long-term health problems. Since the beginning of 2022, the world has witnessed the emergence of another virus: Monkeypox. At the same time, the climate crisis becomes more urgent every day. Is there a connection between climate change and more frequently occurring infectious diseases? A new study has answers.

Monkeypox, as well as COVID-19, are so called zoonotic diseases, meaning they spread between humans and animals. In a study published in April 2022, Colin J. Carlson from the Center for Global Health Science & Security at Georgetown University in Washington D.C., USA, and a team of researchers found that such diseases are more likely to emerge in response to habitat destruction, population growth and global warming. The key finding of the study predicts that roughly 4,000 diseases may be transmitted from animals to humans in the next 50 years, leading to an increased risk of global pandemics.

While COVID-19 is a completely new disease first detected in China in 2019, Monkeypox has been known for a long time. Researchers discovered the virus in monkeys in 1958 and in humans in 1970. The disease previously caused problems in Africa, where it was largely contained until a recent outbreak in Nigeria. Cases in Europe occurred every now and then in the past, mostly in people travelling back from Africa, but it never led to significant secondary spread. Until now.

In May 2022, researchers discovered a cluster of cases in the UK and shortly after in Portugal, Spain, Canada, and the US. By now, more than 65,000 cases have been confirmed worldwide. The World Health Organization has declared the epidemic a public health emergency of international concern, meaning the outbreak is significant and requires international cooperation to contain.

Carlson’s study shows that there are multiple explanations for why such animal-borne diseases emerge. One is the destruction of animal habitats due to deforestation and warmer global temperatures, which cause unnatural processes. Many species are forced to leave their natural habitat and interact with other species they were geographically isolated from in the past. This facilitates the transmission of more viruses among these animals. Additionally, with this change of species composition in certain habitats, some species become dominant and form a kind of monoculture. When this happens, viruses find ideal conditions to spread and develop. Another reason is that humans expand more and more into previously uninhabited areas and come into closer contact with wild animals. Thereby making diseases more easily transmitted from animals to humans.

Carlson and his co-authors also found that any climate mitigation efforts could not prevent these processes from happening. Even if humanity manages to keep global warming below two degrees within the 21st century, it will not change anything, as the ecological transition is already ongoing, and viral sharing has become unstoppable. The researchers highlight an urgent need to closely monitor the emergence of new viruses, especially in rapidly warming, tropical regions that contain a lot of animal-borne diseases.

As far as other measures are concerned, Prof. Dr. Josef Settele, biologist and professor at the University of Halle, Germany, stresses the importance of biodiversity in an interview. According to Settele, the protection and preservation of highly biodiverse habitats like tropical forests and coral reefs is crucial since they act as a shield against the spread of diseases and prevent the emergence of monocultures. Consumers can also do their part in preserving and restoring nature by reducing meat consumption, buying organic products, and allow diversity in their home gardens.

As far as the Monkeypox outbreak is concerned, new cases have started to decline in most countries. Vaccines and behavioral changes have been effective in the containment of the disease. Experts assume that Monkeypox immunity lasts as long as smallpox immunity, the better-known sibling of Monkeypox, responsible for millions of deaths during the 20th century, until its eradication in 1980. This immunity should protect people who have recovered from the virus for quite some time, maybe even forever, which also helps controlling the spread of Monkeypox.

According to these observations, it looks like Monkeypox will not evolve to be the next global pandemic. However, as the study by Carlson and colleagues suggests, this will not be the last virus spreading around the world. The next one might be more dangerous and climate change is one of the key drivers of these potential diseases.

Usually, when people read the news about the effects of climate change, they learn about extreme weather events like heat waves and floods, which will become more frequent in the future. The findings of Carlson and his co-authors challenge the world to also look at it from a human health perspective which, if further neglected, could become very costly.