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Tracing the origins of Covid-19

As the Covid-19 virus (Coronavirus SARS-CoV-2) continues to spread and claim victims worldwide, its origins remain unknown. Each scientific community has its own theory, with some suggesting that the virus may have leaked from a lab.

Another theory, based on recent studies of the Wuhan wet market in China, as well as others conducted in Cambodia, Laos, Japan, China, and Thailand, suggests that an ancestral virus in Rhinolophus bats was derived from infecting wild and/or pets went out to people. In fact, in these different studies, several viruses with genetic sequences very similar to SARS-CoV-2 have been isolated in these bats.

A missing link

Although some bat species have been shown to naturally harbor these coronaviruses, the wild or domestic animal (or animals) that acted as a bridge between them and humans – the missing link – remains unidentified. Pangolins were suspected at first, but now appear to have been collateral victims rather than one of those much-discussed missing links. A coronavirus genome sequence detected in pangolins was indeed related to that of SARS-CoV-2, but the rest of the genome was too distant genetically to support the hypothesis.

In addition, the pangolin hosts in which viruses genetically related to SARS-CoV-2 were found had largely been confiscated from live markets at the end of the supply chain. As a result, they had long-term contact with other animal species. It is very likely that they were infected along this supply chain rather than in their natural environment. In China, too, mink farms were suspected of being intermediate hosts.

It is unlikely that pangolins were the intermediate host of the virus.
Arief Budi Kusuma/Shutterstock

Finally, pangolins and rhinoceros bats do not share the same habitat, making it highly unlikely that there was contact between the two species where the virus jumped from one to the other. On the other hand, civet cats and raccoon dogs could be an intermediate source of SARS-CoV-1). Rodents or primates could also carry pathogens with zoonotic potential, such as hantaviruses – which can cause hemorrhagic fever with renal syndrome – or filoviruses, which include the Ebola virus. The latter is transmitted to humans by wildlife, particularly bats, antelopes, and primates such as chimpanzees and gorillas, and then spreads among humans, primarily through direct contact with the blood, secretions, and other bodily fluids of infected individuals. The average mortality rate is around 50%.

In 2013, the first cases of the Ebola virus were detected in West Africa. The surge in these cases resulted in over 10,000 deaths, mostly in Guinea, Liberia and Sierra Leone.

The risky habit of eating bushmeat

Activities such as hunting, handling animals or eating wild animal meat therefore create the conditions for the transmission of viruses from animals to humans – a potentially devastating phenomenon known as “spillover”.

The ZooCov project has attempted to define and quantify this risk in Cambodia. For almost two years – and since the beginning of the pandemic – she has been using a “One Health” approach to investigate whether and how pathogens such as corona viruses can be transmitted from hunted and consumed wild animals to humans.

In fact, wildlife is regularly traded in Southeast Asia and bushmeat is commonly eaten. These eating habits are often opportunistic. In some communities, it supplements a low-protein diet. It can also be frequent and targeted. In Cambodia, out of 107 families surveyed as part of the ZooCov project, 77% said they had eaten bushmeat in the past month.

It is also widely used for medicinal purposes. In Vietnam, an analysis of Vietnamese authorities’ records seizing pangolins and related by-products between 2016 and 2020 revealed 1,342 live pangolins (6,330 kg), 759 dead pangolins or pangolin carcasses (3,305 kg), and 43,902 kg pangolins.

However, this consumption also has a cultural and social dimension that is not yet properly understood. Among the affluent – and often in large cities – people sometimes eat bushmeat out of a desire for social status and a belief that eating it bestows on them the animal’s physical or physiological characteristics. They also sometimes eat bushmeat out of rejection of processed meat, which is considered unhealthy. Animals are raised on a large scale to meet this need and the demand for fur production.

In Cambodia’s Stung Treng and Mondolkiri provinces, where protected forest areas still exist, researchers interviewed more than 900 people living on the edge of those forests to determine the pattern of the illegal bushmeat trade. Statistical analyzes are underway to identify the people most at risk from exposure to wildlife and thus to such pathogens. We already know that those exposed are mostly young, middle-class males and that some communities are more exposed than others. Sociological studies have also helped to better understand today’s context: the legal framework, the profiles of the actors in the trade, their motives and deterrents in trading and consuming wildlife, and how the context compares to any other health crisis (bird flu, Ebola, SARS -CoV-1 etc.).

Which population groups are most at risk?

These successive crises seem to have had little impact on the habits of these communities. In addition to regularly eating bushmeat, a quarter of the families surveyed said they still hunted or captured wild animals, and 11% said they sold bushmeat or wild animals. In addition, over 2,000 samples of wild animals that were traded or consumed for subsistence – bats, rodents, turtles, monkeys, birds, wild boar, etc. – were analyzed in the same study areas. Some of these samples have tested positive for coronavirus, and scientists at the Institut Pasteur du Cambodge (IPC) are currently sequencing their genome to learn more about their origin, evolution and zoonotic potential. Finally, the researchers collected blood samples from over 900 people from the same region to find out if they had been exposed to a coronavirus or coronaviruses. These analyzes are still ongoing, but what we do know is that these individuals were not exposed to SARS-CoV-2 at the time of the interview.

If the Covid crisis has taught us anything, it is the importance of detecting such phenomena early in order to nip the pathogens in the bud. While many questions remain about how cases emerge, there are just as many questions about the surveillance systems that should be put in place to track them. The results of the ZooCov project will be used to develop a system for early detection of zoonotic virus outbreaks, in particular by strengthening Cambodia’s already existing wildlife health surveillance system set up by the Wildlife Conservation Society (WCS). . . Other major research and development projects will help us to understand, detect and prevent these emerging case phenomena.

The authors would like to thank the Cambodian Ministry of Health, its Ministry of Agriculture, Forestry and Fisheries and its Ministry of Environment, as well as all partners of the project: Institut Pasteur du Cambodge (IPC), the Wildlife Conservation Society (WCS), Flora and Fauna International (FFI), Institut de Recherche pour le Développement (IRD), Hong Kong University (HKU), GREASE network, International Development Enterprise (iDE), World Wildlife Fund (WWF), Elephant Livelihood Initiative Environment (ELIE ), BirdLife International, Jahoo and World Hope International.

Translated from French by Thomas Young for Fast ForWord.

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