But do we know why this happened? In other words, was this
pandemic avoidable and are we ready to prevent or contain the inevitable
next one?
SARS-CoV-2 is the perfect example of a zoonosis spilling over from
wildlife and establishing shortly afterwards in human populations. This
type of event has happened many times during human evolution and in the
history of human civilizations10. For example,
diseases such as smallpox, bubonic plague, and cholera, introduced by
Europeans to the Americas and Africa caused extinction of indigenous
communities, but had initially an animal origin. Other diseases such as
yellow fever, HIV/AIDS, dengue and Zika all emerged from non-human
primates. Similarly, measles diverged from rinderpest, a pathogen that
has circulated in livestock for centuries with now humans being the only
reservoir. While this type of emergence is not new, we now live in a
world that is more connected than ever due to the globalization of
trade, advances in transportation systems, a population experiencing
exponential growth and the highest rates of urbanization in history.
Connectivity clearly accelerates disease spread once a spillover event
has occurred, but the frequency and volume of those events is also
increasing due to global environmental changes.
A key contribution of evolutionary ecology is to help understand
pathogen circulation within animal communities. Viruses of pandemic
potential such as avian influenza and coronaviruses circulate within
animal communities, infecting a great diversity of species including
different species of birds, bats, canines, felids, rodents, equines,
camelids, and rabbits11. The structure of these animal
communities and their biological properties (e.g., species
richness or species composition) can dramatically modulate pathogen
emergence, the range of virus circulation and their evolutionary
trajectories. While we are undergoing the 6th mass
extinction on record and the first due to anthropogenic pressures, some
studies (despite controversies) suggest that a loss of biodiversity can
increase transmission of some
pathogens12.
Understanding the underlying drivers of pathogen spillover and the role
of biodiversity in pathogen transmission can help devise proactive
ecological interventions to prevent and reduce spillover events. Indeed,
ecosystem management and other strategies to protect species diversity,
including restoration, rewilding and the management of wildlife
reserves, halting habitat loss, population declines and illegal wildlife
trade, can be a lever to prevent and mitigate pathogen emergence and
transmission. In addition, increasing rates of emergence from domestic
animals can be attributed to the demographic explosion and genetic
homogeneity derived from intensive farming systems, where they can act
as amplifiers and edge-hosts of novel pathogens. Increasing host
diversity within farming systems can moderate the role of domestic
animals in the emergence of new pathogens, as has been shown for plant
pathogens13.
An additional driver of emergence is the rising exposure of human
populations to pathogens due to habitat encroachment and settlement into
natural systems14. With nearly 40% of the Earth’s
land surface already converted to cropland or pasture, most of the land
conversion in recent decades is taking place in forests of tropical and
subtropical areas, which represent some of the greatest hotspots of
biodiversity15. This increased exposure to previously
isolated wildlife and their pathogens is amplified by human activities
such as bushmeat consumption, recreational and agricultural activities,
or wildlife trade. Reducing this animal/human interface can be partly
achieved through law enforcement, education programs for at-risk groups
to reduce their exposure to viral contamination, and by providing viable
alternatives to bushmeat hunting, which is always associated with the
poorest communities, in order to minimize human exposure to wildlife.
However, with roughly two thirds of all agricultural land used for
animal grazing and pasture, and a population expected to reach 9 billion
people by 2050, swift changes in food production and consumption
patterns will be needed to avoid further encroachment and settlement due
to agricultural needs.
We know that this human-animal interface can potentially impact pathogen
adaptation to human populations. So far, most of the medical research
focused on this topic consists in finding genetic markers demonstrating
that an adaptation has taken place and would yield an efficient
human-human transmission16. However, fundamental
research accumulated in evolutionary ecology can bring important
insights to the subject by identifying how such adaptation to a new host
can be promoted17. Such studies could therefore
provide practical knowledge for pandemic risk management if they become
rooted in real-world data (e.g., by considering contact data
between human and animal species) and/or on experimental models that can
mimic closely human/animal contact processes.
While we now have a substantial understanding of the ecological
and evolutionary drivers of pathogen spillover, there is a critical
knowledge gap regarding which ecological interventions will most
effectively reduce human disease burdens and pandemic risk. Empirical
examples exist, such as pilot interventions on prawn farms to control
schistosomiasis via reduction of the snail intermediate host or using
landscape management to modify bat distribution and reduce risk of
bat-borne pathogens 18,19. Nevertheless,
these remain scarce 18,19. In this sense, one
of the greatest opportunities in disease ecology lies in integrating
pathogen data collection and evaluation methods (e.g.,observational, quasi-experimental) with the myriad of ecosystem
management and conservation efforts happening around the world.
Efforts to tackle many of these issues in order to reduce the likelihood
of the next pandemic will face seemingly unsurmountable challenges as
well as conflicting political and economic agendas. Yet, if the COVID-19
epidemic has taught us anything, it is that in a globalized world,
decisive action, widespread behavioral changes and large investments to
avoid catastrophic health consequences are not only possible but can
happen over short time scales. In less than six months since the first
cases were declared, roughly one third of the world’s population has
been forced to self-quarantine, whole economic sectors have been drawn
to a halt and most international and domestic travel has stopped. G20
countries have already committed 5 trillion USD just to keep their
economies afloat and are imposing restrictions that affect most aspects
of their citizens’ daily lives. The global recession is also revealing
how reactive approaches might be orders of magnitude more expensive than
proactive ones. These economic efforts should be redirected towards
developing evidence-based preventive approaches in the post COVID-19 era
through a globally coordinated interdisciplinary and integrated program
focused on the improvement of current systems for pandemic risk
prediction in the short-term, as well as identifying and testing
long-term solutions in current and future socio-ecosystems that can
quantitatively reduce the probability of new pandemics.
The case for investments in drug and vaccine development seems more
straightforward and is certainly justified, especially as a pandemic is
underway, but these should also be complemented with policies and
strategies that address the root causes of disease emergence in global
hotspots such as educational programs to reduce exposure to wildlife by
Asian farmers or biological conservation in Latin America or sub-Saharan
Africa. Though less visible, this preparedness strategy could certainly
be highly efficient and cost-effective, especially if we consider the
health and economic costs averted by a reduction in spillover events and
a reduced likelihood of a new pandemic. Furthermore, investing in
stronger health systems, universal access to healthcare and education in
developing countries could help contain these outbreaks before they
become widespread. It is therefore mandatory to develop integrative
approaches that combine long-term pandemic prevention via ecosystem
management with reinforced health systems for better disease
surveillance and control, in collaboration with a social science
perspective to improve our understanding of the barriers and the
enablers for adoption of such solutions.
Governments are relying on scientists during this time of “war”
against COVID-19, but such reliance is even more important during
‘peace’ time. We should not consider this pandemic as an isolated
phenomenon, but rather as a part of a causal chain of events derived
from our actions, i.e., a systemic crisis. There is an overwhelming
scientific consensus that we are irreversibly deteriorating ecosystem
integrity and the services they provide20,
transgressing planetary boundaries and running out of time to take
decisive action and reverse this trend 21. Only by
investing in scientific knowledge in close partnership with key
stakeholders (e.g., local authorities and citizens, public health
authorities, supra-national organizations, etc.) we will be able to find
actionable solutions, guide policy decisions and avoid future
catastrophes. As detailed previously, the need for ecological and
evolutionary approaches to understand, model and predict pandemics in a
changing world is crucial. Focusing our efforts only on emergency and
mitigation measures against COVID-19 would constitute a major missed
opportunity to combat the broader structural causes of the pandemic. The
downstream consequences would be dramatic.