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What are the origins of novel human infectious diseases like COVID-19?

By EJ Milner-Gulland, Lauren Coad, Yuhan Li, Kelly Malsch, Munib Khanyari, Hollie Booth from the Interdisciplinary Centre for Conservation Sciencem University of Oxford

Originally published the UKRI website

Sunda pangolin. Photo by Dan Challender and Save Vietnam Wildlife

Several of the earliest reported cases of COVID-19 were linked to a wet market in Wuhan city, China, which sold a range of fresh food produce, including wildlife. Although there is not enough evidence yet to say how the new coronavirus jumped from its original host (probably a bat) to humans, there have been previous examples of viruses originating in wild animals causing disease epidemics in people. Understanding the link between wild and domestic animals and COVID-19 is important for managing the current pandemic and future outbreaks of infectious diseases.

Several of the earliest reported cases of COVID-19 were vendors at a “wet market” in Wuhan, China. It has been reported that the virus was first transmitted from an animal host to humans at this market, though this has not yet been proven.[1]

“Wet market” is a term used across various parts of the world, notably in China and Southeast Asia, for a food market in which individual retailers sell fresh products such as vegetables, fruits and fresh meat, providing an essential food source to many. The “wet” here is historically to distinguish from “dry markets” that sell non-perishable goods such as fabrics, electronics, grains, dried food (e.g. dried mushrooms) and household products, and also because of the use of water for cooling the produce and cleaning floors and surfaces. Chinese “wet markets” include some that sell fruits and vegetables in a setting more like a European farmer’s market, while others sell a wider array of meat and live animals, both wild and domestic, often kept in crowded and unhygienic conditions. “Wet markets” have been implicated in SARS (via civets) and H5N1 influenza (“bird flu”) via domestic poultry.[2]

Beyond “wet markets”, a wide variety of other markets that sell live, wild animals operate across the world. The animals sold in these markets can be wild-sourced or captive-bred, for use as food, medicine, pets and ornaments, and the markets are of varying degrees of legality, biosafety, sustainability and social legitimacy. Wildlife markets range from live bird markets for poultry and pets (e.g. in Indonesia, Vietnam and Egypt), to bushmeat markets for subsistence (e.g. in Cameroon and Ghana) to Traditional Chinese Medicine markets (e.g. in China and Singapore). Markets such as these can make substantial contributions to food security and livelihoods. For example, the Chinese Academy of Engineering stated in 2016 that captive-bred wildlife sold for food in China contributed $14 billion to the economy, and employed six million people. The Wuhan market which is at the centre of the COVID-19 outbreak predominately sold seafood, along with other animal products including live, wild and domesticated species for meat.

One proposed route of transmission for COVID-19 involves bats and pangolins, although it is not known whether pangolins were being sold at the Wuhan seafood market at the time. Bats are natural reservoirs of coronaviruses[3]; a pangolin could have been the intermediate host, although the exact route of transmission of COVID-19 to humans remains unknown.[4][5] Further research is needed to be sure of how the virus got into humans, and to understand the role of wildlife and markets in transmission.

Historically, over two-thirds of zoonotic viruses (viruses that are transmitted between animals and humans) have originated in wild animals, most frequently rodents, bats and primates.[6] The transmission of zoonotic diseases primarily occurs when there is close contact between humans and animals. Even dead animals can pass on diseases to people and other animals if their carcasses are fresh and if people consume the meat or handle the dead animals in an unhygienic way. These risks have led to calls for more stringent bio-safety rules in relation to the sale and trade of live animals, as well as calls to stop wildlife being traded in markets altogether. As a result of COVID-19, the Chinese government has now banned farming and trade of almost all terrestrial wild animals for human consumption; only a few species (such as those on the List of Genetic Resource of Livestock and Poultry) are exempt.[7]

While there are clearly disease risks from the wildlife trade, wildlife markets are only one source of infections from animals. Human health is intricately connected to wildlife and their habitats. The destruction of natural forests brings people and wildlife into contact in a way that can promote the spread of zoonotic diseases, such as Nipah virus infection which is carried by fruit-eating bats in Asia.[8][9] In addition, nearly half of all the infectious zoonotic diseases that have emerged in humans since 1940 have come directly from domestic livestock, even if they originated in wild animals.[10] For example, the 2009-10 swine flu pandemic came from domestic pigs. International food supply chains, the movement of people globally, and unprecedented changes in pathogen life cycles due to climate change, further facilitate the conditions for emergence and spread of diseases.[10]

References

  1. Cohen J. Wuhan seafood market may not be source of novel virus spreading globally. Science. 2020 Jan. DOI: 10.1126/science.abb0611.
  2. Webster RG. Wet markets—a continuing source of severe acute respiratory syndrome and influenza? The Lancet. 2004 Jan;363(9404):234-236. DOI: 10.1016/S0140-6736(03)15329-9.
  3. Maxmen A. Bats are global reservoir for deadly coronaviruses. Nature. 2017 Jun;546(7658):340. DOI: 10.1038/nature.2017.22137.
  4. Lam TT, Shum MH, Zhu HC, et al. Identifying SARS-CoV-2-related coronaviruses in Malayan pangolins. Nature. 2020 Mar. DOI: 10.1038/s41586-020-2169-0.
  5. Li X, Zai J, Zhao Q, et al. Evolutionary history, potential intermediate animal host, and cross-species analyses of SARS-CoV-2. Journal of Medical Virology. 2020 Feb. DOI: 10.1002/jmv.25731.
  6. Johnson CK, Hitchens PL, Pandit PS, et al. Global shifts in mammalian population trends reveal key predictors of virus spillover risk. Proceedings B. 2020 Apr;287(1924):20192736. DOI: 10.1098/rspb.2019.2736.
  7. Xinhua News Agency. China’s legislature adopts decision on banning illegal trade, consumption of wildlife. Xinhua Net. 2020 Feb.
  8. Allen T, Murray K, Zambrana-Torrelio C, et al. Global correlates of emerging zoonoses: Anthropogenic, environmental, and biodiversity risk factors. International Journal of Infectious Diseases. 2016 Dec;53(Suppl):21. DOI: 10.1016/j.ijid.2016.11.057.
  9. Epstein JH, Field HE, Luby S, Pulliam JR, Daszak P. Nipah virus: impact, origins, and causes of emergence. Current Infectious Disease Reports. 2006 Jan;8(1):59-65. DOI: 10.1007/s11908-006-0036-2.
  10. Rohr JR, Barrett CB, Civitello DJ, et al. Emerging human infectious diseases and the links to global food production. Nature Sustainability. 2019 Jun;2(6):445-456. DOI: 10.1038/s41893-019-0293-3.

The COVID-19 response and wild meat: a call for local context

By Stephanie Brittain, Interdisciplinary Centre for Conservation Science, University of Oxford

Originally published on the Interdisciplinary Centre for Conservation Science

Coronaviruses are a family of viruses that cause disease in animals. Covid-19 is one of several such viruses that have made the jump from animals to humans. So far, the evidence suggests that the virus was first transmitted to humans at a seafood market in Wuhan, China. The Wuhan seafood market also sold wildlife, but despite extensive experimental work, scientists are still not confident about the transmission chain from animal to human, with links drawn to snakes, bats and, more recently, pangolins. What is clear, however, is that there is now unprecedented global attention focused on wild meat trade and consumption globally.

The ongoing covid-19 crisis may be the first time many people have heard of wild meat, or “bushmeat” as it’s commonly known. Wild meat is meat from wildlife species that are hunted for human consumption. It constitutes a primary source of protein for many rural communities globally, particularly in tropical forests, where it contributes to the food security of millions of people globally.  Across Central Africa, species hunted in rural areas are also sent to cities and large towns to satisfy the growing urban demand for wild meat. There, wild meat caters to both the growing number of upper-middle class customers who wish to consume high-value and sometimes rare species as a status symbol, as well as the urban poor who consume small species such as rodents which are often still less expensive than farmed animals. The drivers and impacts of hunting and consumption of wild meat are not homogenous, and should not be treated as such.

The impact of wild meat trade bans on people and conservation

In recognition of the importance of wild meat for food security and livelihoods, global conservation policy has focused on the need for sustainable use of species hunted for wild meat, and many experts agree that using wild species sustainably may still be the best way to save them. Yet, following the covid-19 outbreak, we are seeing a growing number of calls to ban the trade and consumption of wildlife globally.

The covid-19 outbreak has shown us that there is clearly a need to readdress our balance with nature. But, what lessons can we learn from past attempts at banning wild meat, and what are the possible realities for both conservation and for those who depend on wild meat for food and income?

  • While some of the proposed bans recognize the important role that wild meat plays for the food security and nutrition of indigenous peoples and local communities globally, wild meat also constitutes an important financial backstop across Africa, in particular during low seasons or when agricultural commodity prices fluctuate. The Ebola crisis and subsequent ban on the trade of wild meat markets across West and Central Africa resulted in unemployment for thousands of women, the primary traders of wild meat. The potentially unequal effects such bans could have on the financial security of different groups of people must be properly considered. Livelihood alternatives must be co-designed and led by those who will be affected in order to mitigate such effects and ensure alternatives are locally relevant.

    Women at the Moutuka Nunene bushmeat market in Lukolela, Democratic Republic of Congo.

    Women at the Moutuka Nunene bushmeat market in Lukolela, Democratic Republic of Congo.  Photo by Ollivier Girard/CIFOR

  • If alternative sources of food and income are not provided for those who need it, bans on wild meat trade and consumption could result in malnutrition among the young and most vulnerable, or push the trade of wild meat underground and worsen contributing factors to the spread of disease. Following the 2013-2016 Ebola outbreak, a universal ban on wild meat markets was imposed across West Africa. Rather than being enforced, it pushed many wild meat markets underground, rendering regulation more complex and worsening food hygiene conditions, a key driver of disease spread. Further, past attempts to limit wild meat sales in Equatorial Guinea, were only transiently effective as the hunting ban was again not enforced and was quickly followed by a marked increase in wild meat hunting compared to hunting rates prior to the ban.  Poorly considered bans resulted in the erosion of food security and health and unintended consequences for conservation, ending in calls for a more nuanced understanding of how future spillovers could be prevented.
  • There is a common assumption that people in rural areas eat wild meat because they have no other alternatives. The Darwin Initiative project “Why Eat Wild Meat”, looks to understand individual level differences in wild meat preferences, hunting, and consumption and the reasons for doing so, in rural villages in southeastern Cameroon. The project is finding that yes, people eat wild meat because it is readily available to them, but also, and more importantly, because they like the taste. As such, it’s important that conservationists keen to devise protein alternative projects first understand why people eat and hunt wild meat. Alternatives are best when locally conceived, and when projects are co-designed, to ensure that subsequent alternatives meet local needs and have longevity.
  • It is also often assumed that livestock or poultry present culturally viable alternatives to wild meat. However, the “Why Eat Wild Meat” study identified greater concerns by some local people in rural Cameroon over the health of domestic animals than they did over many species hunted for wild meat. This is due in part to several outbreaks of disease relating to domestic animals, including most recently avian influenza, and personal experiences of livestock or poultry falling ill. Additionally, animals such as chickens and goats are kept for sale or for large celebrations in these villages, and are not routinely consumed for subsistence purposes. As such, encouraging a shift away from hunting and towards a dependence on livestock and poultry would require a deep cultural shift for some rural communities.
  • The impact of a shift to livestock or poultry raises concerns for many conservationists about deforestation; researchers estimate that if livestock such as cattle were to replace wild meat in the Congo Basin, 25 million hectares of forest would be converted to pastureland. Realistically, pigs or chickens would be seen as more viable replacements to wild meat, but again this would require an estimated additional 4.5 million tons of pigs or chickens, again unlikely to happen any time soon in the Congo Basin. We must consider the potential adverse impacts that calling for a shift to livestock could have, and whether such calls are realistic in the timeframe required.

Deforestation

Deforestation: A bird’s eye view shows the contrast between forest and agricultural landscapes near Rio Branco, Acre, Brazil.  Photo by Kate Evans/CIFOR

We need local understanding

Much of the recent work on wild meat and disease has focussed on the international convention scale. If we are to change behaviours to protect against the emergence of further zoonotic diseases and encourage a shift away from any unsustainable or illegal behaviours, we must also have a local understanding of why people make the choices they do, as well as the pathways of trade and transmission, to help ensure that policy changes regarding wild meat are driven by evidence, and that subsequent actions are locally relevant and realistic.  As the “Why Eat Wild Meat” project is discovering, the reasons behind wild meat hunting and consumption, the types of biodiversity involved in trade and the pathways for disease transition are complex and varied. So then, should be the proposed effective ways to tackle the spread of new diseases such as covid-19.

Discussions on whether and how bans on the wild meat trade should occur will surely continue over the coming months. Dialogue between wildlife trade specialists, virologists, conservationists and public health experts are needed to assess the likely impact of policy responses for people and for nature and to try to fill existing knowledge gaps.  In the mean time, wild meat remains a vital source of protein and income for millions in rural areas, and banning trade will not be a silver bullet solution to the challenges raised by this virus. Rather, bans risk oversimplifying the reality of covid-19 and other coronaviruses that are sure to emerge in future, diverting attention away from the greater challenges, such as poverty, a lack of locally and culturally relevant and viable alternatives, and the external forces that incentivize people in rural areas to hunt to meet the additional demand.

 

A woman prepares duiker meat with chili sauce

A woman prepares duiker meat with chili sauce in Cameroon. Photo: Stephanie Brittain

We must learn from previous crisis responses to climate shocks, disease outbreaks and humanitarian disasters, where hurried response interventions led maladaptation and further damages to lives, livelihoods, and the environment. We need greater efforts to understand these local contexts and drivers of wild meat hunting and consumption, if we are to change patterns of use.

[With thanks to Emilie Beauchamp and EJ Milner-Gulland for their helpful inputs.]