In recent decades, zoonotic diseases–those transferred from animals to humans–have gained international attention. Ebola, avian influenza (or bird flu), H1N1 flu virus (or swine flu), Middle East respiratory syndrome (MERS), Rift Valley fever, sudden acute respiratory syndrome (SARS), West Nile virus, the Zika virus–and now, the novel coronavirus COVID-19–have all either caused or threatened to cause major pandemics, with thousands of deaths and billions in economic losses.
Researchers have yet to identify the exact point at which the SARS-CoV-2 virus was transferred from animals to humans and presented itself in the form of COVID-19. However, one thing is clear: COVID-19 will not be the last pandemic.
In 2016, the United Nations Environment Programme (UNEP) flagged a worldwide increase in zoonotic epidemics as an issue of concern. Specifically, it pointed out that 75 per cent of all emerging infectious diseases in humans are zoonotic and that these zoonotic diseases are closely interlinked with the health of ecosystems.
Human activity and ecosystems
According to the UNEP Frontiers report, zoonoses are opportunistic and thrive where there are changes in the environment, changes in animal or human hosts, or changes in the pathogen, itself.
In the last century, a combination of population growth and reduction in ecosystems and biodiversity has culminated in unprecedented opportunities for pathogens to pass between animals and people. On average, one new infectious disease emerges in humans every four months, the report says.
Changes in the environment
Human activities have resulted in major changes in the environment. By altering land use–for settlement, agriculture, logging, extractive or other industries and their associated infrastructure–humans fragment and encroach into animal habitats. They destroy the natural buffer zones that would normally separate humans from animals, and create opportunities for pathogens to spill over from wild animals to people.
Climate change–primarily the result of greenhouse gas emissions–exacerbates the situation. Changes in temperature, humidity and seasonality directly affect the survival of microbes in the environment; and evidence suggests that disease epidemics will become more frequent, as the climate continues to change. Rapid climate change is challenging to those with fewer resources for responding quickly, leaving them more vulnerable and amplifying their risk of harm from the spread of zoonotic disease.
Changes in pathogen hosts
Changes in human and animal populations that serve as hosts for certain pathogens are also often the effects of human activities. They may be related to migration, urbanization, changing dietary preferences, trade demands, and travel.
In many developing countries, economic growth and demographic shifts from rural to urban areas have stimulated consumer demand for dairy and meat products in cities. This has led to the expansion of cropland and more intense livestock farming near and around cities, increasing opportunities for exposure.
Livestock often serve as an epidemiological bridge between wildlife and human infections, such as in the case of avian influenza. Pathogens first circulated in wild birds infected domestic poultry, and were then passed to humans.
Proximity to different species though wet markets or consumption of wild animals can also facilitate animal to human transmission. Early cases of SARS were associated with contact to caged civet cats, being sold in wet markets; and some cases of Ebola in Central Africa are believed to have been transferred from animal to human hosts when infected gorilla meat was consumed.
Incubation–the time between human infection and the time when that human presents signs of infection–may last days or weeks; but millions of people, under normal circumstances, travel every day, from one country to another, in just hours. A disease that originates in one country can quickly spread to others, regardless of the distances between them. This is particularly visible in the rapid spread of COVID-19, which affected almost every country in the world within three months of the first reported case.
Changes in pathogens
Pathogens change genetically (mutation) as they evolve which allows them to exploit new hosts and survive in new environments. One example of this is the emerging resistance of pathogens to antimicrobial drugs–such as antibiotics, antifungals, antiretrovirals and antimalarials–often resulting from the misuse of the drugs, either by people or in veterinary medicine.
Ecosystems integrity and human health
Ecosystems are inherently resilient and adaptable and, by supporting diverse species, they help to regulate diseases. The more biodiverse an ecosystem is, the more difficult it is for one pathogen to spread rapidly or dominate. Human action, however, has modified wildlife population structures and reduced biodiversity at an unprecedented rate, producing conditions that favour particular hosts, vectors and/or pathogens.
For example, genetic diversity provides a natural source of disease resistance among animal populations; whereas intensive livestock rearing often produces genetic similarities within herds and flocks, making them susceptible to pathogen spillover from wild animals.
Similarly, biodiverse areas enable disease-transmitting vectors to feed on a larger variety of hosts, some of which are less effective pathogen reservoirs. Conversely, when pathogens occur in less biodiverse areas, transmission can be amplified, as has been shown in the case of West Nile Virus and Lyme Disease.
UNEP Executive Director, Inger Andersen has observed that, “We are intimately interconnected with nature, whether we like it or not. If we don’t take care of nature, we can’t take care of ourselves.”
What can be done
Addressing zoonotic disease emergence requires addressing its root cause–primarily, the impact of human activities on ecosystems.
This means recognizing the close relationships between human, animal and environmental health. It means increased monitoring of human and wildlife health in landscapes that are at the beginning of transformation process to develop baselines, improve understanding and preparedness for potential outbreaks, and inform development to minimize risks to both humans and nature. And it calls for collaborative, multisectoral, transdisciplinary and international efforts, as encapsulated by the One Health approach.
With a global population nearing 10 billion, Andersen is emphatic that 2020 is “a year when we will have to fundamentally re-shape our relationship with nature.”
UNEP, the UN Food and Agriculture Organization, and hundreds of partners across the planet are launching a 10-year effort to prevent, halt and reverse the degradation of ecosystems worldwide. Known as the UN Decade on Ecosystem Restoration 2021-2030, this globally-coordinated response to the loss and degradation of habitats will focus on building political will and capacity to restore humankind’s relation with nature. It will be a direct response to the call from science, as articulated in the Special Report on Climate Change and Land of the Intergovernmental Panel on Climate Change, and to the decisions taken by all UN Member States in the Rio Conventions on climate change and biodiversity, and the UN Convention to Combat Desertification. UNEP is also working with world leaders to develop a new and ambitious Post-2020 Global Biodiversity Framework and bringing emerging issues (such as zoonotics) to the attention of decision makers.
As the world responds to and recovers from the current pandemic, it will need a robust plan for protecting nature, so that nature can protect humanity.