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

By EJ Milner-GullandLauren CoadYuhan LiKelly MalschMunib 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 coronavirusesNature. 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 pangolinsNature. 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-2Journal 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 riskProceedings 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 wildlifeXinhua 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 emergenceCurrent 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 productionNature Sustainability. 2019 Jun;2(6):445-456. DOI: 10.1038/s41893-019-0293-3.

Coronavirus: why a blanket ban on wildlife trade would not be the right response

By Dan Challender, Amy HinsleyDiogo VeríssimoMichael ‘t Sas-Rolfes, Oxford Martin Programme on Wildlife Trade

Originally published in The Conservation

The early stages of the COVID-19 pandemic have been linked to a “wet” market in Wuhan, in the Hubei province of eastern China. Wet markets are common in Asia, Africa and elsewhere, selling fresh fruit and vegetables, poultry, fresh meat and live animals, including wildlife. Reports initially indicated that the coronavirus which causes COVID-19 may have been transmitted to people from wildlife at this wet market because of unsanitary conditions.

The pandemic has led to some wildlife conservation organisations to call for blanket bans on wildlife trade on public health grounds. They include bans on commercial trade in wildlife for human consumption and the closure of these markets. More extreme calls from more than 200 organisations include ending the keeping, breeding, domestication and use of all wildlife, which also covers traditional medicine.

But blanket bans are unlikely to benefit people or wildlife, and are unfeasible because they overlook the complexity of the wildlife trade. The COVID-19 outbreak should not be used opportunistically to prescribe global wildlife trade policy. A more appropriate response would be to improve wildlife trade regulation with a direct focus on human health.

fish

A fish market in Seoul, South Korea. Rodrigo Oyanedel

Wildlife is used globally on a daily basis, from medicinal plants and edible fungi, to wild meat in EuropeNorth AmericaSouthern Africa and elsewhere. Wildlife trade enables people in many parts of the world to meet their basic needs and can provide livelihood benefits from harvesting or farming.

Despite the way it is often presented, wildlife trade involves far more than animals harvested in tropical regions and sold in China. It includes species from land, freshwater and marine habitats, including fisheries, in production systems ranging from wild harvesting to captive breeding. It takes place at local and international levels, includes legal and illegal, sustainable and unsustainable components, and is measurable in billions of dollars annually.

Bans are seldom the answer

Unquestionably, wildlife trade regulations require review in response to COVID-19 for public health reasons. However, while bans may appear to be a logical solution, their impact on public health cannot be assumed to be positive. They could also do more harm than good for biodiversity. Typically, prohibition does not deter all traders in marketplaces. This would mean that trade in some products would likely continue illegally. Traders would be motivated by financial profits, with an increased risk of trade being controlled by organised crime.

Bans may not stigmatise consumption either, especially where products are socially desirable, meaning consumer demand for many products would persist. This is a public health concern because, unregulated, such trade would likely be clandestine and, if unsanitary, could pose the risk of transmitting disease from animals to humans. Bans, especially where they remove legal supply options, such as captive breeding, could raise perceptions of scarcity, and drive up black market prices and increase incentives for poaching. This could accelerate the exploitation and extinction of species in the wild.

The outcome for wildlife economies would also be uncertain. For example, the wildlife “breeding economy” in China is estimated to involve 14 million people and be worth more than US$74 billion annually. The fate of animals under human care and the people employed in these industries would require consideration. In China, bamboo rat and badger farmers are to be compensated and given grants for new businesses following the closure of almost 3,000 farms in response to COVID-19.

photo

Scientists have discovered a virus similar to COVID-19 in the threatened pangolin, which is heavily trafficked for its meat and scales (Shutterstock),

To be effective, bans would need to be largely in step with local social norms and well enforced. But this is unrealistic in many parts of the world where law enforcement is cripplingly under-resourced in terms of technology and manpower. Local people may also challenge the legitimacy of any bans. Requiring agencies to enforce comprehensive bans in these circumstances would most likely overwhelm them.

Even where there are strong laws and enforcement, implementation is challenging and illegal trade still occurs frequently, such as the harvesting and trafficking of the European eel in Europe. It is also unlikely that law enforcement would receive the financial investment necessary to enforce bans in the long term, due to political constraints on spending and other more urgent priorities.

Better regulated trade

Banning all wildlife trade is a knee-jerk and potentially self-defeating measure. A more appropriate response would be improving regulation of wildlife markets, especially those involving live animals. This should include full consideration of public health and animal welfare concerns to ensure there is low risk of future animal-to-human disease outbreaks.

This could be achieved by focusing on highest-risk species and improving conditions along supply chains and in markets, such as health and safety and sanitation, and regular animal health checks. These practices could draw on existing standards that apply to regulations for transporting live animals by air.

Like bans, any new or revised regulations would require enforcement. But approaches such as “smart regulation” could be used to aid the process. This could ensure that new measures are culturally appropriate and incentivise local people, traders, buyers and law enforcement agencies to comply. Devising regulations in this way would mean they are more likely to be effective, rather than undermined which a blanket ban would do.

Rushing to indiscriminately ban all wildlife trade in response to COVID-19 would not eradicate the risk of animal-to-human disease outbreaks. It could also have a severe impact on livelihoods and biodiversity. Improved regulations that focus on health, if implemented well, would avoid these effects while ensuring a low risk of future disease outbreaks.

Resources

Evidence to action: research to address illegal wildlife trade

‘Evidence failure’ blights fight against illegal wildlife trade

Cocaine of the sea, ‘epic failure’ and how following the money can limit illegal wildlife trade

Consumer focus – Tackling illegal wildlife trade by reducing demand

London Conference to Tackle Illegal Wildlife Trade

Researchers Explore Ways to Bring Attention to and Inform Policy on the Illegal Wildlife Trade