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13 Aug 2020 | Ecosystem services

How healthy and biodiverse ecosystems safeguard us against zoonotic diseases

Tashi Dorji, Nakul Chettri & Kesang Wangchuk

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Horses graze in Humla, Nepal (2018). Safeguarding biodiversity is essential to prevent future pandemic-scale zoonotic diseases. But even the last repositories of dwindling biodiversity – the protected areas – are facing both internal and external challenges. (Photo: Jitendra Raj Bajracharya/ICIMOD)

Nakul Chettri, Tashi Dorji & Kesang Wangchuk

Horses graze in Humla, Nepal (2018).
Horses graze in Humla, Nepal (2018). Safeguarding biodiversity is essential to prevent future pandemic-scale zoonotic diseases. But even the last repositories of dwindling biodiversity – the protected areas – are facing both internal and external challenges. (Photo: Jitendra Raj Bajracharya/ICIMOD)

 

In the last century, three-quarters of land and two-thirds of the ocean have been changed to such an extent as to determine the birth of a new era: the Anthropocene, or the era of human beings. In doing so, we have created new conditions for human–pathogen interactions and disease emergence. This has been thrown into sharp relief over the past few months, with the world grappling with the reality of a novel coronavirus (COVID-19) outbreak from what initially seemed like a localized outbreak in Hubei Province of China in December 2019.

In the course of investigating this outbreak, some useful facts have emerged that re-orient focus on our incursions into natural systems and wildlife. First, some 75% of past global pandemics and major diseases were zoonotic, meaning the sources were from animals. Second, zoonotic diseases present higher risks to domesticated animals than to wild animals. Third, many zoonotic diseases, including COVID-19, are transferred to humans even from pets and domestic animals, apart from wild animals. Increasing human population, habitat fragmentation and deforestation for agriculture, and expansion of piggery and poultry farming around peri-urban areas all increase animal–human interface, increasing risks for zoonotic disease transmission. Further, the impact of climate change, particularly higher temperature and moisture, increases the seasonality of vector-borne and infectious diseases by prolonging the enabling micro-climate.

So how do we navigate through the quagmire of deeply intertwined animal–human relationships and interactions? How can healthy and biodiverse ecosystems help stave off future pandemic-scale zoonotic diseases?

Zoonotic diseases and the Hindu Kush Himalaya
Yak in Bhutan’s alpine region (2008).
Yak in Bhutan’s alpine region (2008). At higher elevations of the HKH region, transhumance and seasonal movement of yak, goats, and sheep accelerate disease transmission from wildlife and vice versa and make disease reporting and control difficult given the remoteness. (Photo: ICIMOD photo archive)

 

In the HKH, known for its fragile ecosystems and rich biodiversity, some 70% of its original ecosystems have been modified over the last century and more than 85% people still directly depend on natural resources for subsistence. The bulk of this dependence comes from the agro-pastoralist practice, wherein agriculture is linked with domesticated (cattle, goats, sheep) and semi-domesticated (yak and mithun) livestock rearing, driving pressure to modify wilderness areas for such practices.

At higher elevations, transhumance and seasonal movement of yak, goats, and sheep accelerate disease transmission from wildlife and vice versa and make disease reporting and control difficult given the remoteness. In the mid-hills and lowland areas, traditional livestock breeds are being replaced by exotic breeds that are more susceptible to diseases, including zoonotic diseases. All this brings wildlife, livestock, and humans into closer contact with each other and facilitates the spread of diseases, including emerging infectious diseases.

In addition, hunting, bush meat trade, and market-driven illegal trade of live wild animals create dangerous opportunities for contact and disease spillover. The traditions of drinking raw blood of livestock and consuming raw meat or half-cooked meat among some HKH communities could result in the transmission of zoonotic diseases. A global snapshot of the legal trade in live wild animals showed that 11,569,796 individual live wild animals, representing 1,316 different species, were exported from 189 different countries between 2012 and 2016. China was the largest exporter of live mammals (with 98,979 animals, representing 58.7% of global trade). It is no coincidence that many recent outbreaks have originated in markets that sell a mix of wild and domestic mammals, birds, and reptiles, creating the conditions for the development of old and new zoonotic diseases.

Nature’s link to emerging infectious diseases

The high abundance and diversity of viruses in nature, often in the apparent absence of diseases, suggests that they are naturally embedded into global ecosystems at all ecological scales. Interestingly, 99% of the wildlife virome remains undescribed and viral discovery remains a seemingly boundless scientific endeavour. That undiscovered viruses and pathogens are evolving with the potential to infect the human population is not under dispute. However, it is also clear that even the last repositories of dwindling biodiversity – the protected areas – are facing both internal and external challenges. The risk of diseases in wild animals has increased and many protected areas managed through tourism revenue are now facing challenges. Moreover, with the lockdown and limited monitoring, incidents of poaching and trade have also increased on the one hand and human exposure to wildlife due to animals entering human habitation have also increased. This has also led to increased human–wildlife conflict.

Infectious diseases from both wildlife and domestic animals have emerged at an increased pace within the last century and are likely to continue to emerge, given expected increases in population growth and landscape change. Curbing disease emergence will prove challenging until we have a more thorough appreciation of the epidemiologic circumstances that facilitate pathogen spillover, particularly from wild animals, which are the source of the majority of recently emerging infectious diseases and continue to constitute a substantial gap in disease detection efforts worldwide. However, the majority of work on this topic has focused on direct assessments of the relationship between biodiversity and endemic-pathogen prevalence, without disentangling intervening mechanisms; thus study outcomes often differ, fuelling more debate. For example, most research has been observational (66.9%); about 30.8% were review or concept papers, and only few studies (2.3%) were experimental in nature, with most studies focusing on vector-borne and/or multi-host pathogens.

Blue sheep in Bhutan (2009).
Blue sheep in Bhutan (2009). By carefully disentangling the interactions between human activities and pathogen circulation in wildlife, conservation efforts could mitigate disease risks and hazards in novel ways that complement more typical disease control efforts. (Photo: Alex Treadway/ICIMOD)
Conservation to mitigate disease risk

Safeguarding biodiversity is essential to prevent future pandemic-scale zoonotic diseases. But some critical changes are needed in the conventional approach to understanding relationships between infectious diseases (both endemic and emerging) and biodiversity that may help clarify sources of controversy. First, the distinct concepts of hazards versus risks need to be separated to determine how biodiversity and its drivers may act differently on each. This distinction is particularly important since it illustrates that disease emergence drivers in humans could be quite different to the general relationship between biodiversity and transmission of endemic pathogens. Second, the interactive relationship among biodiversity, anthropogenic change, and zoonotic disease risk needs to be recognized and accounted for. By carefully disentangling these interactions between human activities and pathogen circulation in wildlife, conservation efforts could mitigate disease risks and hazards in novel ways that complement more typical disease control efforts. Avoiding consumption of bushmeat or limiting wildlife trade would go a long way. Third, the remaining wilderness needs to be protected to enable species to thrive within their ecosystems and not be compelled to come outside due to lack of food. Fourth, there is a need to emphasize and promote health and sanitation to prevent transmission and spread of zoonotic diseases in urban settlements and markets where interactions occur frequently.

Once COVID-19 is under control, the world cannot return to business as usual. A thorough review of worldviews, lifestyles, and the short-term economic valuation that drives resource exploitation must be carried out. More importantly, governments in the HKH must renew their efforts to protect nature, invest in research to understand pathogen spillover, and educate and involve the public in understanding emerging infectious diseases, past and future.

 

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