As covid-19 continues to bring unprecedented damage to the lives of millions around the world, Tom Llewellyn questions whether the actions of humanity have amplified the risk of global pandemics.
Are we inviting global pandemics by exploiting our planet? New research from the University of California, Davis is helping us answer that question, and their conclusions don’t bode well for the future of our public health.
Animal to human virus transfer
The consensus within the scientific community is that the SARS-CoV-2 virus disease (covid-19) first originated in bats and pangolins before being transmitted to humans.
Unfortunately, infectious diseases that originate in animals compromise the majority of recurrent and emerging virus threats, from the 2003 outbreak of SARS to the 2014 outbreak of Ebola in west Africa.
These outbreaks occur when there is physical contact between the local human population and infected animals in what is known as a ‘disease spillover’.
‘Spillover of viruses from animals is a direct result of our actions involving wildlife and their habitat. The consequence is they’re sharing their viruses with us’
Christine Kreuder Johnson, Professor of Medicine & Epidemiology at UC Davis
Recently, researchers from the University of California, Davis’ One Health Institute have shed light on this issue by comparing data on the viruses that inhabit terrestrial mammals with IUCN data on the human-driven threats affecting the same species.
They first identified a positive relationship between the conservation status of a species and the number of viruses shared with humans. The more abundant an animal is, the greater the number of viruses they are able to transmit.
This raises concerns, as abundant and widespread animals are more likely to have interactions with humans, increasing the risk that they will transmit highly infectious diseases.
Unfortunately, we are changing the natural world in such a way that human-animal interactions are occurring more than ever before, increasing the chance of disease spillover.
Animal trade
When thinking about the wildlife trade industry, the majority of us will immediately think of lucrative private zoos that have recently been portrayed in Netflix’s hugely popular Tiger King series.
However, the bulk of wildlife trade occurs in the name of ‘art’, religious beliefs or perceived medical qualities.
Whatever the absurd reason is, animal trade requires close human contact with different species from around the world. Many of these animals will lack any form of veterinarian examination.
The UC Davis researchers also found that species threatened because of human exploitation (hunting and trade, for example) possess more than twice as many virus diseases as those threatened for other reasons.
Pangolins – identified as just one suspected source of covid-19 – are the most trafficked animal in the world and represent 20 per cent of all illegal trade due to claims their scales “stimulate lactation and relieve skin diseases”.
Urbanisation and disease
Rapid expansion of our cities is transforming huge areas of habitat into concrete jungles. While this may increase a region’s economic growth and development, it also supports the disease-carrying populations of rodents.
In UC Davies’ study, 61 per cent of the sampled viruses resided in rodents – the house mouse and the black rat, with 16 and 14 viruses respectively. Of the ten species studied, rodents were the only two wild species present.
The scientists add that overlapping distribution of hosts was “highly correlated with cross-species transmission among rodents”. The more urbanised an area is, the more likely human-rodent interactions are and ultimately the risk of disease transmission.
The researchers also found a high proportion of animal-residing viruses occurred in bats (30 per cent), a species widely believed to be the originating animal for a variety of infectious diseases, including SARS, Ebola, Marburg, Nipah and Hendra viruses.
As rapid urbanisation across the globe attracts more bat populations within urban habitats, concerns are growing surrounding what the consequences of this will be.
Agriculture
In addition to unprecedented urban development, rural landscapes around the world are significantly altered by the domestication of animals and intense farming infrastructures.
The role of livestock in the transmission of viruses to humans is well documented, with industrial farming lending itself to dense animal populations and regular human contact.
With regards to global pandemics, the issue with agriculture principally rests with the scientists’ findings that “domesticated species harboured an average of 19.3 viruses compared with wild species with a mean of 0.23 viruses” and shared eight times the number of viruses with humans than wild species.
Agriculture may indeed be a vital part of the global economy, but the close contact between livestock and humans, in addition to their high virus count, could lead to farms becoming ground zero for the next global pandemic.
To mitigate this risk, we should begin to move away from intensive farming to a veterinarian surveillance programme, helping to identify infected livestock quicker.
What next?
Infectious diseases that originate in animals have “emerged at an increased pace within the last century and are likely to continue to emerge, given expected increases in population growth and landscape change”.
Today, we are destroying huge areas of rich, biodiverse habitats at an alarming rate to develop incredibly dense urban communities that currently contain about half of the world’s population.
Critically, species that are able to adapt to new environments, while still maintaining close contact with the local human population, could be “an important source of pathogen transmission” in the future.
‘We need to be really attentive to how we interact with wildlife and the activities that bring humans and wildlife together. We need to find ways to co-exist safely with wildlife, as they have no shortages of viruses to give us’
Christine Kreuder Johnson
It’s time to reconsider the human activities that drastically change the environment and learn that our exploitation of the planet will continue to cause immeasurable pain for people when disease spillover does occur.
Far from suggesting that limiting either urbanisation or agriculture should be used as a preventative measure for the outbreaks of infectious diseases, instead they should be continuously evaluated as potential vectors for outbreaks.
On the other hand, there are some human activities that should be limited as preventative measures in the future.
At the time of writing, more than 150,000 people had died from covid-19. Would half the world be under lock-down had we restricted the illegal hunting and trade of pangolins?
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