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By Roger Highfield on

Planet primed for animal infections to spillover into humans

A study published today shows why changes in land use could pave the way for future disease outbreaks, reports Science Director Roger Highfield.

Viruses that circulate in animals have caused many serious diseases in humans – that was the story of HIV, of SARS, of MERS, of Ebola and now of COVID-19.

The diseases originated from chimpanzees in the case of HIV and bats in the case of Ebola, SARS, MERS and COVID-19. In fact, most pandemics are thought to be linked to these so-called ‘zoonotic diseases.’

That is why scientists have long warned that a pandemic was inevitable and emerging infectious diseases are on the UK National Risk Register.

That is also why, in 2018, the World Health Organisation singled out one hypothetical pandemic candidate, which it labelled Disease X, as ‘a novel coronavirus other than SARS or MERS‘.

Today, in the journal Nature, scientific research led by University College London has shown that changes in global land use, notably the relentless conversion of natural vegetation into agricultural crops, is disrupting animal communities in such a way that it benefits the wild species that carry diseases that are known to infect humans.

The effect is strongest for rodent, bat and passerine bird species, which make up more than half of all bird species, and the team warns that global land use change – to date the human footprint affects around 75% of the earth’s land surface – is creating increasing opportunities for contact between people and species that are potential hosts of human disease.

The global population is expected to rise from 7.7 billion to almost 10 billion by 2050. Today’s findings suggest that the resulting changes in land use will prime the planet for future spill-overs of diseases that originate in animals.

Many changes in land use see the conversion of natural vegetation to cities and to agricultural ecosystems – from the rise of cultivation in West Africa to the expansion of oil palm plantations in south east Asia and the human-set fires used to clear forests in the Amazon.

Although the rise of zoonotic diseases, such as Ebola, Lassa fever and Lyme disease, has been linked changes in land use, there has been debate in recent years among scientists about when, and how, land use change contributes to the risk of spillover.

Close up of Personal Protective Equipment warn during an Ebola outbreak
Close up of Personal Protective Equipment warn during an Ebola outbreak in Sierra Leone and now on display in the Science Museum

To investigate, Prof Kate Jones, Rory Gibb, Dr David Redding, Kai Quin Chin and Tim Blackburn of the UCL Centre for Biodiversity & Environment Research (Prof Jones and Dr Redding are also at the ZSL Institute of Zoology) worked with Prof Christl Donnelly of the Department of Statistics, University of Oxford, and the MRC Centre for Global Infectious Disease Analysis, Imperial College London.

The team began with data from the Projecting Responses of Ecological Diversity in Changing Terrestrial Systems (PREDICTS) biodiversity database of 666 studies, focusing on 184.

They studied close to 7,000 species, 376 of which are known to carry human-shared pathogens – species that are potential reservoirs of human disease – to analyse how zoonotic host animal species change, on average, as landscapes alter from natural vegetation, to agricultural, pasture and urban ecosystems, when some species thrive, and many go into decline.

Lead author, Rory Gibb, said:

‘The way humans change landscapes across the world, from natural forest to farmland for example, has consistent impacts on many wild animal species, causing some to decline while some others to persist or increase.

Our findings, based on this big data set, show that these changes are making the world more hazardous in that the animals that remain in more human-dominated environments are those that are more likely to carry infectious diseases that can make people sick.’

In comparison, most other wild animal species – such as carnivores and non-human primates – are found in lower numbers in disturbed environments compared to natural habitats.

He added that the findings allowed the team to rule out an alternative hypothesis, that we have shared landscapes with species for so long that we share diseases with them too.

When it comes to SARS-CoV-2, the roots of the current pandemic date back to when a cluster of pneumonia cases of unknown cause was reported by health authorities in Wuhan, China, last December.

As outbreaks like Ebola in West Africa, SARS and now COVID-19 have shown, once a new infection spills over into people, it’s now much easier for it to rapidly spread across the globe through air transport links.

Analysis of the genetic sequences of the virus responsible for the outbreak has confirmed the virus is natural.

The virus is a coronavirus, one of the many families of virus that cause the common cold. We lack immunity to SARS-CoV-2 because this particular virus came from another species.

This transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, emerging from the surface of cells
This transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, emerging from the surface of cells.
The image was captured and colorized at NIAID’s Rocky Mountain Laboratories (RML) in Hamilton, Montana.

By reconstructing the evolutionary history of SARS-CoV-2, an international research team of Chinese, European and U.S. scientists found that the lineage that gave rise to the virus has been circulating in bats for decades and likely includes other viruses that can infect people.

A link with bats was also found in the case of  its predecessor, SARS, in the early 2000s. Although SARS-CoV-2 is genetically similar (about 96%) to the RaTG13 coronavirus, which was sampled from a Rhinolophus affinis horseshoe bat in 2013 in Yunnan province, China, the team found that it diverged a relatively long time ago, in 1969.

One of the older traits that SARS-CoV-2 shares with its relatives is a protein on its exterior (the receptor-binding domain on what is called the Spike protein) which enables the virus to bind to and infect human cells.

In other words, commented David Robertson, professor of computational virology, MRC-University of Glasgow Centre for Virus Research, ‘other viruses that are capable of infecting humans are circulating in horseshoe bats in China.’

The details of how the virus spread from bats to people for COVID-19 are unknown, notably exactly where and when and how many transfers to people occurred. ‘It is hard to know what the ultimate drivers of the spill over were,’ said Rory Gibb.

Experts agree that more novel diseases will emerge, though their ability to develop into pandemics can be curbed with sufficient investment in healthcare and early detection.

The study announced today (5 August) was funded by the UKRI‘s Medical Research Council and Natural Environment Research Council, Wellcome and the Royal Society.

The latest picture of how far the pandemic has spread can be seen on the Johns Hopkins Coronavirus Resource Center or Robert Koch-Institute.

You can check the number of UK COVID-19 lab-confirmed cases and deaths along with figures from the Office of National Statistics.

There is more information in my coronavirus blog series (including in German by focusTerra, ETH Zürich, with additional information on Switzerland), from the UKRI, the EUUS Centers for Disease ControlWHO, on this COVID-19 portal and Our World in Data.

The Science Museum Group is also collecting objects and ephemera to document this health emergency for future generations.