Ecosystem health and preventing future pandemics

In mid-December last year, in Wuhan, China, a new strain of coronavirus was contracted by a human for the first time. The virus was most likely transmitted from another animal species at one of the city’s notorious ‘wet’ markets, where a huge number of non-human animals are either sold live or killed to order. The animals for sale in markets such as this one, are kept in cramped and unhygienic conditions, making them susceptible to disease, which can then potentially be passed on to humans.

What is a zoonotic pandemic?

Diseases that cross from non-human animals to humans are called zoonoses, and they are far from rare. It is estimated that zoonoses made up half of all the new infectious diseases that affected humans between 1940 and 2005 (1). Even over the past two decades, the world has seen numerous novel zoonoses emerge, including dangerous viral outbreaks such as: SARS, MERS, swine flu and avian flu, and the re-emergence of the Ebola virus. To become successfully established in humans though, a virus must be able to spread not only to people, but also between them. 

The novel coronavirus, Covid-19, has proven extremely transmissible between people. So much so, that even though its case-fatality rate is lower than that of both SARS and MERS (approximately 2-3% of people who contract Covid-19 die from it (2,3), which compares to 10% for SARS (4) and 34% for MERS (5), it has caused far more deaths than both of them combined (2), because it has infected so many more people. These figures show the critical importance of preventing and containing viral outbreaks.

Humanity’s best defences against viral outbreaks are vaccination, isolation of confirmed cases and basic hygiene (i.e. handwashing and covering coughs and sneezes) (6,7). In the case of emerging viruses, however, vaccines and formal tests take time to develop, produce and deploy at scale. The viruses that caused the SARS and MERS outbreaks were extremely dangerous but were largely contained through hygiene and case isolation measures. This was possible because SARS is only contagious once there are symptoms, so infected people could be detected before they passed on the virus to many more people (4), and MERS does not spread easily between people (8). The Covid-19 virus, on the other hand, can spread before symptoms present, which makes it much more difficult to prevent transmission (3). Because of this, drastic physical distancing measures have been required in an attempt to curb its spread and prevent health services of major, developed nations being completely overwhelmed (3). The implications of a SARS outbreak on the scale of the current Covid-19 pandemic are unthinkable.

The potential for novel zoonoses to cause extreme disruption and suffering, combined with the difficulties in producing vaccines to protect against them, means that there is a clear need to increase measures to prevent, or at least reduce, their emergence in the first place. To do this, it is necessary to understand the underlying factors that contribute to their emergence and to investigate ways they can be mitigated or eliminated. 

Risk factors for zoonotic emergence

Zoonoses rely on close contact between human beings and non-human animals in order to emerge. Fundamentally, an increase in the frequency and proximity of encounters leads to an increased risk of transmission (9,10). Often though, the specific interactions are complex and depend on a variety of factors such as the local environment and the nature of human activity (1,11–13).

Studies indicate that the greatest risk of cross-species disease transmission exists in regions where there is high human population growth and overlap between human and wildlife populations1,11. This can occur for several reasons, such as incursion into tropical forests for hunting or mining, or deforestation for either expansion of human settlements due to population increases or agriculture (11,12). In the cases of hunting and mining the association with zoonotic risk is clear, as they put people in regular close proximity to wild animals. Deforestation though, introduces additional factors that make the pathway more complex.

Deforestation

Deforestation is particularly associated with increased risk of zoonotic emergence in tropical regions, where predictive models suggest that disease emergence is most likely (10–13). What is more, land clearing is often linked to new human settlement in close proximity to wild animal populations, and also creates habitat for insects to reproduce (9). This is important because some insect species can transmit disease between humans and non-human animals (14), for example ticks are known to spread Lyme disease and mosquitos can spread a range of diseases including malaria and dengue fever. Pathways of transmission, such as these insects, are called vectors, and an increase in their numbers may increase the likelihood of disease transmission.

Deforestation for animal agriculture is particularly problematic, because along with nearby human settlement and providing habitat for known disease vectors, it also introduces a new species in monoculture. This provides a potentially novel host living in a concentrated environment, so viruses can establish a reservoir population from which they can then potentially jump to humans (11,13,15,16). People working in close proximity to these animals are at heightened risk not only from insect vectors but also from direct transmission, as is anyone involved in slaughtering, butchering or handling these animals (including preparation of the raw meat for cooking). 

This is an important consideration because the SARS, MERS, avian flu, swine flu and Covid-19 outbreaks are all thought to have originated from close human contact with farmed animals or food markets (17–19). It is clear that the conditions in ‘wet markets’ expose both traders and consumers to a high level of risk (20), and while biosecurity measures can reduce this risk, both new and resistant diseases remain a threat (21). In any case, as both the human population and global demand for meat products increase, tropical forests will continue to be destroyed to make way for pasture and animals will continue to be raised in high density conditions, perpetuating an elevated risk of zoonotic emergence.

Biodiversity loss

It is clear that deforestation and biodiversity loss due to human activity negatively affect global health, and it has been suggested that increasing biodiversity can reduce this impact. A theory called the “dilution effect” proposes that if one animal species hosts a potentially dangerous disease, then an increase in the total number of species sharing the environment (i.e. biodiversity) will reduce, or dilute, the proportion of ‘host animals’; thus, there is reduced risk of vector species feeding on both the host species and humans, and thereby transmitting disease (22,23). The evidence for the dilution theory though, relies on studies of particular sites and particular species relationships within those sites (24–26). When a broad range of studies are considered it seems that increasing biodiversity can actually have positive, negative or neutral effects on zoonotic emergence (27), supporting the notion that the relationship is complex. 

Increasing forested area and biodiversity may reduce the risk of viral outbreaks indirectly though, through carbon sequestration. Climate change is projected to have primarily negative implications for human health, including by potentially increasing the habitable range of disease vectors, such as mosquitos, as well as the extent and duration of climatic conditions that are favourable to disease outbreaks (28). It is not clear that mitigating climate change impacts through reforestation would reduce the rate of new zoonoses emerging, but it clearly remains beneficial for both the environment and human health.

Ecosystem restoration and preventing epidemics

The importance of reducing the emergence of new zoonotic diseases is obvious, and restoring ecosystems, protecting existing forests and halting biodiversity loss are critical in order to achieve this. Deforestation to provide land for areas of high human population growth and high-density animal agriculture presents particular risks. Furthermore, while restoring forests on previously degraded lands might not directly reduce the emergence of new zoonoses, it offers obvious co-benefits to humanity and the natural environment through sequestering carbon-dioxide and mitigating the effects of the climate crisis. No individual can single-handedly solve these issues, but the cumulative effect of many people making small changes to their daily lives can have a significant impact and benefit both the environment and our health.

References

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