Pangolin. (Used under creative commons license) Photo credit: Adam Tusk

Charlotte Milbank is an interdisciplinary Ph.D. candidate in epidemiology and geography at Britain’s University of Cambridge. Any views expressed are her own.

The COVID-19 pandemic has thrown deserved spotlight on the complex interactions and interdependencies between environmental health and human health in relation to the risk of zoonotic disease emergence.

This piece was motivated by a virtual panel convened by the University of Cambridge Conservation Institute (UCCI) in July, which asked whether healthy ecosystems can prevent pandemics. This piece,  discusses key insights from the panel, arguing that truly interdisciplinary approaches are needed if we are to effectively mitigate zoonotic risk and protect both ecological and human health in the future.

The panel included:

• Shailaja Fennell (Development Studies, Department of Land Economy)
• Kate Jones (Chair of Ecology and Biodiversity, Division of Biosciences, University College London)
• Rosalind Parkes-Ratanshi (Cambridge Institute for Public Heath)
• James Wood (Department of Veterinary Medicine, Cambridge Infectious Diseases)
• Bhaskar Vira, (Panel Chair; Geography and founder of the Conservation Research Institute).

The SARS-CoV-2 pandemic has forced change upon the way we live as humans in 2020. At time of writing, over 20 million people have been infected with COVID-19 across the world, with no sign of any immediate abatement.

Understanding the zoonotic causes of COVID-19’s emergence and the conditions that might lead to similar future pandemics is of urgent concern. Increasing evidence suggests that conditions within degraded ecosystems are ripe for zoonotic spillover and spread. Drawing on expertise from diverse disciplines housed in the UCCI, this panel was well-placed to discuss this topic and broach what the implications of this might be for future research and effective policy response.

Zoonotic spillover

Zoonotic diseases (or “zoonoses”) are diseases that normally exist in (non-human) animals but can “jump” to infect humans in so-called ‘spillover’ events. Transmission from animal to human may be airborne, through food, close or direct contact with animals, their feces or saliva, or vector-borne. Sixty percent of existing infectious diseases are of zoonotic origin or have transmission pathways involving animals, and 75 percent of future emerging diseases are expected to come from animals. Zoonotic spillover is common and the majority of spillover events go unreported. Many neglected zoonotic diseases (such as bovine tuberculosis, brucellosis and plague) remain in regular circulation in some human populations, although it is rare that such diseases go on to cause pandemics

Spillover from animals to humans requires the infectious pathogen to overcome a succession of several disease-related, animal, human and environmental barriers, namely the simultaneous co-existence of an animal host species, sufficient prevalence of pathogen and infection intensity within that animal host, animal exposure to a susceptible human population, and pathogen survival within humans.

With new anthropogenic pressures and behaviors, animals and humans are being brought into closer contact than ever before, and bringing with it new risks of zoonotic emergence. If we can understand the pathways to spillover events, we may be better placed to intervene at several points along these pathways, and mitigate the risk of spillover occurrence.

Spillover in degraded ecosystems

While the animal hosts of zoonotic disease are often subject to acute scrutiny, they are just one part of the spillover story. Human behavior is inextricably linked to spillover events. A recent review of all 183 documented zoonotic pathogens since 1940 found that 31 percent of reported zoonotic spillover events could be linked to anthropogenic land use change, such as deforestation, land clearance for human settlements, industrial agriculture, mining and other infrastructure.

The degradation of natural environments associated with these changes influences the distribution and breeding patterns of both pathogens and animal hosts, and often brings humans and animals into closer contact. This creates new and increased opportunities for spillover.

Citeable examples include findings that increased deforestation and land use modification in the Amazon significantly increased the biting rate of malaria-carrying mosquitoes, Anopheles darlingi.

Landscape fragmentation has also been associated with pathogen prevalence and zoonotic spillover of tick-borne encephalitis in Latvia and West Nile Virus in southern France. As record losses of ecosystems and biodiversity are observed, understanding how these changes may influence patterns of contact between pathogens, animal hosts and humans will be important to limiting the risks of spillover.

Conditions for transmission

Spillover events are comparatively commonplace compared to the actual take-off and widespread transmission of zoonotic disease within human populations. Take-off depends on a whole other suite of pathogenic, human, environmental factors that create opportunities for transmission.

For example, does the pathogen have high environmental stability? Is it highly infectious? And how does it spread? Are humans coming into sufficient, regular contact to allow transmission? And do important environmental factors exist, such as those associated with environmental degradation, that mediate these relationships?

One panelist used the initial spread of human immunodeficiency virus (HIV) through Africa in the late 1900s as a useful example, illustrating again the need to look beyond animal and disease factors, and towards human behavior.

A zoonotic disease of primate origin, HIV/AIDS is thought to have spilled-over to humans in western Africa, spreading initially through human populations via train, truck and trade routes across the continent. Such travel routes bring otherwise-distant populations “into contact” with each other. In an ever-globalized world, the spread of COVID-19 – a highly transmissible virus with moderate environmental stability – has also been facilitated by widespread human movement. Understanding human behaviors and the conditions that are ripe for emerging zoonotic diseases to take off with onward human transmission is as important as understanding the biology of spillover when it comes to designing effective strategies of prevention.

Separating humans and nature?

Given that we know the degradation of ecosystems is associated with zoonotic disease emergence and transmission, what can we do to reduce these risks? The obvious solution might seem to target the separation of humans and nature. When broached with this question, panelists suggested that total separation was not the answer going forward, first and foremost because it is an impossibility – there is no intact ecosystem in the world that does not have some degree of human interference.

Furthermore, exclusionary policies often have unintended negative consequences that often disproportionately affect already-marginalized populations. In India, the Ministry for Environment and Forests has instructed all states to seek to reduce human-wildlife interaction by placing restrictions on access to National Parks, sanctuaries and Tiger Reserves.

This  directive applies to 3 to 4 million (mostly tribal) people who live proximate to these areas, and who often rely on these areas for natural subsistence resources. Elsewhere in Asia, legislation similarly trying to reduce opportunities for human-wildlife contact, including bans on wildlife trade and consumption, have been enacted.

If separation is not the answer, we need to develop better understanding of the ways that humans interact with nature in spaces at risk of spillover. And given that spillover is more likely in degraded spaces, efforts should be made to restore these landscapes.

Compounding/stacking of disasters

Attempts at solutions must also pay heed to the heterogenous needs and vulnerabilities of communities that may be adversely affected by COVID-19 and future emerging zoonoses. While COVID-19 has certainly been a shock, one panelist drew important attention to parts of the world experiencing a “stacking of shocks,” citing the swarms of locusts currently plaguing dozens of countries, including Kenya, Ethiopia, Uganda and India, as an example. In those communities, the threat of COVID-19 compounds the ongoing threat that communities face to their daily food security, as cropland, grazing land and home environments are ravaged by the insects. The pandemic has also increased the vulnerability of displaced populations, who may already inhabit areas that are unsafe, lack basic health infrastructure and coping mechanisms, and are now subject to COVID-19 restrictions. There is need to affirm our understandings of the diverse impacts of COVID-19 on communities worldwide, and recognise that a one-size-fits-all policy response is rarely appropriate.

Adopting a “One Health” approach

All this points to the need for decisive, interdisciplinary action. The “One Health” approach defined by the World Health Organization recognizes the need for collaboration of expertise spanning multiple disciplines to drive effective research and policy responses. Such collaboration would enable recognition and understanding of the complex entanglement of pathogenic, human, animal and the environmental factors that influence the emergence, spread and heterogeneous impacts of zoonotic disease.

That the degradation of environments increases the risk of zoonotic spillover and transmission, and pose threat to human health, is now widely accepted. But we cannot hope to mitigate these risks and threats by continuing to look at any one component (human, animal, pathogen, environment) in isolation. The emergence and ongoing threat of COVID-19 calls for a united response from ecologists, veterinary scientists, epidemiologists, public health practitioners, social scientists – as well as a response that is grounded in understanding of the needs and vulnerabilities of diverse local communities.