The diminishing boundaries between humans and wildlife may be leading to more zoonoses, such as Covid-19. Picture: 123RF/GEZA FARKAS
The diminishing boundaries between humans and wildlife may be leading to more zoonoses, such as Covid-19. Picture: 123RF/GEZA FARKAS

Covid-19 has become a global pandemic, affecting lives and markets all over the world. Developing countries are the hardest hit with weak economic resources to deal with the pandemic. It is worse than the 2008 financial crisis and the world will have to adjust and learn quickly from its mistakes.

Prof Kate Jones, a British ecology and biodiversity expert at the University College London in London, has recently appeared on a number of international media outlets, including CNN, discussing zoonotic diseases such as that caused by the new coronavirus. Jones is attached to the department of genetics, evolution and environment of the university’s Centre for Biodiversity and Environment Research. 

She was asked about the relationship between climate change, infectious disease, habitat loss, biodiversity and live wildlife markets, and what can be done about them.

Is there a scientific relationship between climate change and zoonotic diseases?

This is a really broad question, and difficult to answer in general terms. Because of how important climate is to the lifecycles of vectors, such as ticks and mosquitoes, prevailing climatic and weather patterns are important in forecasting seasonal patterns of many vector-borne diseases, such as dengue fever and malaria.

There’s also much evidence that changes to the climate are likely to affect vector populations and how effectively they can transmit certain pathogens, but that these changes may vary geographically and over time.

Areas that become warmer and wetter may become more suitable for dengue, chikungunya virus or malaria vectors, putting people in those areas at greater risk. But other areas may become too hot or too dry for those vector species to persist, or for their role in transmission to be as effective.

The evidence of how climate change will affect other zoonoses (diseases that can be transmitted to humans from animals) that directly spill over into humans and so are not transmitted by vectors is less well defined, due to the wide variety of animal species that carry human diseases.

A desert-adapted animal species would likely respond differently to a general increase in temperature, compared with a neighbouring animal species that is more adapted to temperate forests. Many animal-borne diseases, however, do appear to be strongly sensitive to the climate. We study Lassa fever, a rodent-borne disease that people tend to catch through contact with rodents in homes or in fields; for Lassa, human cases tend to show very strong seasonal surges, that appear to be linked to how hosts respond to seasonal climate patterns.

So, it’s a complex picture, and climate change is unlikely to affect all diseases and areas consistently, and will also interact with other factors such as biodiversity changes to affect disease risk. But it’s not solely about exposure to emerging infections, it’s also about how severe their impacts might be once they reach human populations.

In many regions, climate change is likely to affect food security, poverty, and other socio-economic factors that affect individuals’ and societies’ vulnerability to disease outbreaks.

It’s a holistic, systems-wide challenge, where all of these factors interact to create vulnerabilities to outbreaks and epidemics.

Scientific evidence has established that animal-borne infectious diseases, such as Covid-19, are on the rise. What are the major drivers?

Evidence suggests that overall infectious diseases are emerging at an increasing rate, though it is not easy to measure this. The majority of these are zoonoses. There are a number of drivers, many of which are ultimately linked to the escalating global rate of human effects on the environment and on ecosystems.

Among the most important of these ecologically is land-use change, which is occurring at rapid rates in many parts of the world. Deforestation, agricultural land conversion and intensification drive losses of wildlife diversity, and bring people into closer contact with wildlife; there is some evidence that this favours the kinds of wild species that are more effective at transmitting infection to humans.

Alongside large-scale, industrial livestock-rearing, this can also create increasing opportunities for livestock to come into risky contact with wildlife and, in some cases, act as intermediate hosts that then transmit pathogens onward to people — this was the case with Nipah virus when it emerged in Malaysia.

Crucially, these ecological and agricultural changes are also occurring in a socio-economic world that is much more urbanised and much more connected than before. Global trade both drives long-distance impacts on habitats and biodiversity, and moves hosts, pathogens and vectors around the world — such as invasive Aedes mosquitoes, which are responsible for transmitting infections such as dengue, Zika virus and chikungunya ,which have massively increased in incidence in recent years.

Lastly, a key driver is increasing human populations, with more people coexisting in higher densities alongside animals that carry diseases both new and old. And as outbreaks such as Ebola in West Africa, SARS and now Covid-19 have shown, once a new infection spills over into people, it’s now much easier for it to reach heavily populated, urban centres and be moved around the world rapidly through air transport links.

As a result, we have a global system of interconnected drivers that both often facilitate the spill over of new infections from wildlife, and that increases the potential for those emergence events to become much larger regional and global pandemics.

Will the loss of biodiversity directly lead to an increase in the spillover of these diseases from animals to humans, because the natural barriers are removed?

There has been an ongoing debate for several years in the scientific community about whether loss of biodiversity has consistent effects on disease risk.

At the moment, the evidence does broadly suggest that human-transformed ecosystems with lower biodiversity, such as agricultural or plantation landscapes, are often associated with increased human risk of many infections, though this is not necessarily the case for all diseases. For example, the kinds of wildlife species that are most tolerant of human disturbance, such as certain rodent species, often appear to be more effective at hosting and transmitting pathogens.

This means that biodiversity loss can create landscapes that increase risky human-wildlife contact and increase the chances of certain viruses, bacteria and parasites spilling over into people. Human-disturbed ecosystems do, however, also have other characteristics that can enhance or reduce the risk of disease spillover. For example, increasing standing water — such as in rice paddies or ponds — can increase the breeding habitat for mosquitoes, while better sanitation infrastructure in cities can reduce the risk of contracting many water-borne infections.

Also, more urban and intensively used habitats support fewer individuals and types of species, so host-human-pathogen contact rates might be overall lower. So, it’s a complex picture that needs more research, and lots of factors are at play.

What should we do to decrease the risk of pathogens spreading into humans? Is this crisis an opportunity in this respect? What makes you hopeful and what makes you less hopeful?

Understanding the drivers of pathogen-sharing and pathogen spillover on human-wildlife interfaces will help us predict and prevent these events, therefore increasing and continuing research into these mechanisms is important.

More broadly, and as has already been said, maintaining intactness of landscapes and biodiversity reduces human-wildlife interaction, while also being beneficial to other aspects of (both human and planetary) health.

• Antwi is a sustainable finance and climate-change specialist, and founder and CEO of Nochua International.

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