This is a guest post by Professor Emma Despland

Zoonotic diseases, or diseases that jump from animals to people, are not a new phenomenon.  Many well-known human diseases first originated in animal populations. In some cases, animals are the main sources of human infection and human-to-human transmission is rare or null (e.g. rabies); other diseases persist in animal populations and occasionally jump to humans, seeding a human outbreak (e.g. plague), and yet others jumped from animals to people long ago and have been circulating in human populations ever since (e.g. measles).  However, novel zoonotics have been appearing with disturbingly increasing frequency.  

The most recent global pandemic of a novel zoonotic disease was HIV/AIDS, which was first detected in humans in 1981 and is still on-going, with no vaccine but effective treatment that makes the disease manageable. Since then there have been SARS, MERS, swine flu (H1N1), bird flu (H1N5) and Ebola, not to mention innumerable local outbreaks of zoonotic diseases (e.g. Nipah virus).  These occur mostly in remote, tropical, often impoverished areas, and are substantially under-reported, meaning that they probably occur more frequently than we know. 

The appearance of a novel human disease prompts scientists to search for an animal host from which the disease could have jumped, and these searches have turned up a wide range of animals that can transmit diseases to humans, from domesticated pigs and chickens to wild mice, apes and, surprisingly often, bats. 

So Why Bats?

Research into bat viruses took off following the SARS epidemic in 2002-2004, when antibodies to SARS were found in three horseshoe bats in a cave in China.  Since then, Chinese researchers have found and sequenced hundreds of bat coronaviruses, and shown that dozens of these bat viruses can infect human lung cells in a petri dish, suggesting they could cause human respiratory disease if they met the right condition.  When Covid-19 struck, Chinese researchers went back to this databank of bat coronaviruses and found one whose genetic sequence is very similar (96% the same) to the human-disease causing SARS-CoV2 virus. 

Yet 96% similarity still means 4% difference. This implies decades of separate evolution, in which time the original virus might have jumped from bats to another intermediate host (a pangolin?) before jumping to humans and becoming SARS-CoV2. Thousands of SARS-CoV2 genomes from human patients have now been sequenced, and it is clear that they are all descended from a single animal-human transmission event and that the disease has spread by sustained human-to-human transmission.  The amazing diversity of coronaviruses, and viruses generally, that infect bats suggests that bat immune systems are truly unique among mammals, and this might be linked to the other big way in which bats are unique among mammals: flight.  This is still poorly understood, but what is clear is that bats are uniquely able to tolerate heavy viral infection loads with generous ‘shedding’ (further production of the virus) without succumbing to disease, but without clearing the infection either; this makes them excellent super-spreaders. 

Host shifts have always been part of disease ecology, because disease agents (viruses, bacteria and other pathogens) are subject to natural selection, just like any other organism. Those that survive and replicate become more numerous, and those that do not, disappear*.  When a virus comes in contact with a host (for example, when a person breathes in a noseful of infected bat respiratory droplets while cleaning out a bat-infested storage shed), the virus either infects the host if the host is susceptible, or simply gets destroyed  by the immune system.  Like all living organisms, viruses have genetic instructions that can mutate, and these mutations can change the virus’ ability to affect different hosts.  So that noseful of bat respiratory droplets might contain hundreds of virus particles that cannot bind to human cells, and a few that can.  If those few virions successfully invade human cells and replicate inside them, they have caused a novel human disease. If that first patient then sheds virions (in their own respiratory droplets), the disease can spread to other people and cause a local outbreak, a broader epidemic or even a global pandemic.  

Why Are Pandemics Increasing?

The type of host shift I’ve just described is not a novel phenomena by any means, but is now occurring more frequently because human activity has generated risky landscapes.Since the emergence of AIDS as a novel zoonotic disease from wildlife in the 1980s, researchers have predicted that destruction of natural habitats would cause an increase in emergent infectious diseases. This prediction was reiterated more strongly after the SARS epidemic raised the alarm internationally in 2004, with experts stating clearly that the question to ask about a global pandemic was not if, but when.  Sadly, we now have the answer: 2020.   

A seminal paper published in 2000 outlined mechanisms explaining the rapid increase observed in novel diseases affecting wildlife, livestock and people, and linked it to changing transmission pathways created by human activity.  The authors outlined how emergent infectious diseases are increasing with deforestation, intensification of agriculture, globalization and the wildlife trade, as diseases jump back and forth between wildlife, livestock and people, and spread around the globe. 

Data from thousands of studies now makes it clear that biodiversity loss and increased human footprint on environments lead to increases in disease outbreaks. Deforestation weakens and stresses wild animals, making them more susceptible to disease, and increases contact between humans and wildlife, facilitating the inter-species jump. For instance, bats naturally roost in caves or hollow trees during the day, but as their habitats are destroyed and hollow trees disappear from the landscape, bats move into buildings instead.  Fruit-eating bats swarm into cities at night to feed on garbage as fruit-tree filled tropical forests are cut down. Numerous local disease outbreaks in Asia, Africa and Australia have been traced to bats inhabiting human settlements. Intensive industrial agriculture, where livestock animals are overcrowded in often unhygienic rearing pens, generates perfect incubators for novel diseases that not only cause economically disastrous veterinary outbreaks (e.g. the Swine acute diarrhoea syndrome coronavirus outbreak in 2016-2017), but can also jump to wildlife (e.g. brucellosis decimating bison herds) or to humans (e.g. Swine flu including the 1918 pandemic, bird flu). 

Globalization, shipping and travel mean that novel diseases move to new populations that can be more susceptible (e.g. European colonizers brought diseases novel to the Indigenous people of the Americas that killed the majority of the population).   The international wildlife trade, in which wild animals are hunted, often illegally, and shipped around the world in overcrowded and unhygienic conditions for food, pets, traditional medicine or luxury products, increases the risk that disease jump between stressed and weakened animals of different species, and then to people, potentially far from their place of origin.  Detection of a coronavirus similar to SARS-CoV2 in three captive pangolins seized by custom agents raised the possibility that pangolins might be the intermediate host that transferred the virus from bats to people; however, subsequent coronavirus tests of hundreds of wild pangolins came out negative, suggesting that if pangolins do carry coronaviruses, they catch them in captivity.

Related: Scientists call for pandemic investigations to focus on wildlife trade

Recent research has also suggested one more insidious mechanism by which ecosystem disruption increases zoonotic disease spillover: a loss in biodiversity usually results in a few species replacing many — and these species tend to be the ones hosting pathogens that can spread to humans. Scientists now use this information to build effective risk maps for known disease like Ebola; but we cannot predict for as-yet-unknown diseases, like Covid-19 was until recently.  The human cost of zoonotic diseases is now sadly clear to everyone; however, focusing efforts only on tracing, containment, treatment and vaccine development is like treating the symptoms without addressing the underlying cause: habitat destruction and biodiversity loss. 

A purely financial analysis of the effects of novel disease outbreaks leads to the conclusion that curbing deforestation would have an excellent return on investment even if the only consequence was a decrease in emergent infectious diseases.  Healthy ecosystems provide many basic services necessary for human life: food, clean air and water, climate regulation to name a few obvious ones.  It’s becoming increasingly clear that disease regulation is one more important benefit humans get from intact ecosystems, one that is financially important enough to ensure that halting deforestation would pay for itself, even without considering all the other ecosystem services a healthy forest provides.  

We have entered the Anthropocene: the geological epoch in which humans are the primary cause of permanent global change.  Human activity has profoundly modified global biogeochemical cycles and transformed all ecosystems. Species extinctions ae accelerating at a rate not seen since the dinosaurs disappeared. This has a huge impact on human well-being, and new diseases are just the tip of the iceberg.  Indeed, the 2019 report from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services shows that environmental degradation has reached a point where loss of biodiversity and destruction of ecosystems is undermining progress on most Sustainable Development Goals related to poverty, hunger, and health. One can no longer pit environmental protection against human development; instead we need to protect ecosystems in order for them to continue to support human life. The IPBES report concluded that: “The essential, interconnected web of life on Earth is getting smaller and increasingly frayed” – one consequence of that fraying is that new diseases are slipping through the gaps. 

Professor Emma Despland is a biologist working on species interactions at Concordia University, Canada. You can read more about her work at this link or follow her on Twitter @EmmaDespland, and listen to her talk on environmental and the coronavirus at the link below, and register for her upcoming talk on Insects as Indicators of Global Change here.

Emma Despland and Liz Miller on Environmental Degradation and the Pandemic

*It’s questionable to use words like ‘survive’ when talking about viruses, because it’s debatable whether viruses are truly alive, but they certainly replicate and evolve like living organisms, so I’ve used this language for simplicity’s sake.

Title Image Credit: Katharina N, Pixabay licence, Image Cropped