On the trail of the origins of Covid-19

While the Covid-19 virus (coronavirus SARS-CoV-2) continues to spread and claim lives around the world, its origins remain unknown. Each scientific community advances its own theory, with some suggesting that the virus may have leaked from a laboratory.

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, posits that an ancestral virus in bats rhinolophus continued to infect wild and/or domestic animals. to humans. Indeed, in these various studies, several viruses with genetic sequences very close to SARS-CoV-2 were isolated in these bats.

A missing link

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

In addition, the host pangolins in which the viruses genetically close to SARS-CoV-2 were found had for the most part been confiscated from the live animal markets, at the end of the supply chain. As a result, they had been in prolonged contact with other animal species. It is very likely that they were infected along this supply chain rather than in their natural environment. Mink farms were also suspected to be an intermediate host in China.

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

Finally, pangolins and rhinolophus bats do not share the same habitat, which makes any contact between the two species in which the virus has jumped from one to the other very unlikely. In contrast, civets 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, including the Ebola virus. The latter is transmitted to humans by wild animals, in particular bats, antelopes and primates such as chimpanzees and gorillas, and then spreads among humans, mainly by direct contact with blood, secretions and other bodily fluids of infected people. The average fatality rate is about 50%.

In 2013, the first cases of Ebola virus disease were detected in West Africa. The increase in these cases has resulted in more than 10,000 deaths, mainly in Guinea, Liberia and Sierra Leone.

The risky habit of eating bushmeat

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

The ZooCov project sought to define and quantify this risk in Cambodia. For nearly two years – and from the start of the pandemic – it has taken a ‘One Health’ approach to exploring whether – and how – pathogens such as coronaviruses can be transmitted to humans from wild animals. which are hunted and eaten.

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

Medicinal use is also widespread. In Vietnam, an analysis of records from Vietnamese authorities confiscating pangolins and related by-products between 2016 and 2020 found 1,342 live pangolins (6,330 kg), 759 dead pangolins or pangolin carcasses (3,305 kg) and 43,902 kg of pangolin scales.

But this consumption also has a cultural and social dimension that is still poorly understood. Among the wealthy – and often in large cities – people sometimes eat bushmeat out of a desire for social status and a belief that eating it gives them the physical or physiological attributes of the animal. They also sometimes eat bushmeat as a rejection of industrially produced meat, which is considered unhealthy. Animals are widely bred to meet this demand and the demand for fur production.

In the provinces of Stung Treng and Mondulkiri in Cambodia, where protected forest areas remain, researchers interviewed more than 900 people living on the edge of these forests to determine the structure of the illegal bushmeat trade. Statistical analyzes are underway to identify the people most at risk of contact with wildlife and therefore with such pathogens. We already know that those at risk are predominantly young, middle-class men, and that some communities are more at risk than others. Sociological studies have also made it possible to better understand the current context: the legal framework, the profiles of the players in the trade, their motivations and obstacles to the trade and consumption of wild animals, and the evolution of the context with each health crisis ( bird flu, Ebola, SARS-CoV-1, etc.).

Which populations are most at risk?

These successive crises seem to have hardly affected the habits of these communities. In addition to eating bushmeat regularly, a quarter of families surveyed said they still hunted or captured wild animals, and 11% said they sold bushmeat or wild animals. In addition, in the same study areas, more than 2,000 samples taken from wild animals trafficked or consumed for subsistence purposes – bats, rodents, turtles, monkeys, birds, wild pigs, etc. – were analyzed. Some of these samples have tested positive for coronaviruses 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, researchers took blood samples from more than 900 people in the same region to find out if they had been in contact with one or more coronaviruses. These analyzes are still ongoing, but what we do know is that these people had not been exposed to SARS-CoV-2 at the time of the survey.

If the Covid crisis has taught us anything, it is the importance of early detection of such emergences in order to nip pathogens in the bud. While many questions remain about how cases emerge, so too do about the tracking 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 the spread of zoonotic viruses, in particular by strengthening the wildlife health surveillance system already operational in Cambodia, which was set up by the Wildlife Conservation Society. (WCS) . Other large-scale research and development projects will help us understand, detect and prevent these emergent case phenomena.

The authors would like to thank the Ministry of Health of Cambodia, its Ministry of Agriculture, Forestry and Fisheries, and its Ministry of the Environment, as well as all the project partners: 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), the GREASE network, International Development Enterprise (iDE), the 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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