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
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.
“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.
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; a pangolin could have been the intermediate host, although the exact route of transmission of COVID-19 to humans remains unknown. 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. 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.
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. 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. 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.
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