On the trail of the origin of Covid-19

While the Covid-19 virus (coronavirus SARS-CoV-2) continues to circulate and claim victims around the world, its origin remains unknown. Each scientific community advances its hypothesis. Some suggest the possibility of a virus leak from a laboratory.

Another hypothesis, which is based on recent studies related to the Chinese market in Wuhan and others carried out in Cambodia, Laos, Japan, China and Thailand, is the evolution of an ancestral virus present in bats, of the horseshoe bat family in particular, in domestic or wild animals, then the transmission of the virus from these animals to humans. In fact, during these various studies, several viruses with genetic sequences very close to SARS-CoV-2 were isolated in these bats.

a missing link

Although it has already been proven that certain bat species naturally harbor these coronaviruses, the identity of the domestic or wild animal(s) that would have acted as a relay between them and humans – missing links – remains. a mistery. The pangolin, initially suspect, now appears more like a “collateral victim” than one of those famous missing links. In fact, a sequence of the coronavirus genome that was detected in pangolins was very similar to that of SARS-CoV-2, but the rest of the genome was genetically very far from it.

On the other hand, pangolins in which viruses genetically close to SARS-CoV-2 were isolated were most often confiscated from live animal markets, at the end of the commercial chain, and thus were in prolonged contact with others. animal species. It is very likely that they were contaminated along this route and not in their natural environment. Mink farms are also suspected in China.

It is unlikely that the pangolin was the intermediate host of the virus.
Arief Budi Kusuma/Shutterstock

Finally, pangolins and horseshoe bats do not share the same habitats, which makes contact between the two species very unlikely, during which the virus would have passed from a bat to a pangolin. Civets and/or raccoon dogs may constitute an intermediate reservoir for SARS-CoV-1). Rodents or primates can also carry pathogens with zoonotic potential, such as Hantaviruses, which can cause hemorrhagic fever with severe renal syndrome, or Filoviruses, including Ebola disease virus. The latter is transmitted to humans by wild animals, in particular fruit bats, porcupines, and primates such as chimpanzees or gorillas, and then spreads in the human population primarily through direct contact with blood, secretions, and other bodily fluids from infected people. The average fatality rate is around 50%.

In 2013, the first cases of Ebola virus disease (EVD) were detected in West Africa. This emergency will cause more than 10,000 deaths mainly in Guinea, Liberia and Sierra Leone.

Hunting meat consumption: a risky practice

The risks of transmission from animals to humans, a phenomenon known as overflowwhether hunting, handling animals or eating wild meat are therefore real and potentially devastating.

It is the characterization and quantification of this risk, in Cambodia, that the ZooCov project has explored through a “One Health” approach, for almost 2 years and since the beginning of the pandemic, if so, and how, pathogens such as coronaviruses could be transmitted from wild animals, hunted and eaten, for humans.

In fact, in Southeast Asia, the trade in wild animals and the consumption of bushmeat is a common practice. Often opportunistic, this consumption comes in certain communities to supplement a low-protein diet. It can also be regular and directed. In Cambodia, of the 107 families interviewed during ZooCov, 77% said they had consumed bushmeat in the previous month.

Medicinal use is also widespread. In Vietnam, analysis of reports of confiscation of pangolins and derived products carried out between 2016 and 2020 by the Vietnamese authorities shows 1,342 live pangolins (6,330 kg), 759 dead pangolins or carcasses (3,305 kg), and 43,902 kg of scales.

But this consumption also has a cultural and social aspect that is still poorly understood. For the wealthy classes, and often in large cities, this consumption can be motivated by a need for social recognition, beliefs that the consumer of this meat appropriates the physical or physiological virtues of the animal consumed, or by a desire to contest consumption. of industrial products. unhealthy meat. Wild animal husbandry to meet this demand and/or fur production is also widespread.

In Cambodia, in the provinces of Stung Treng and Mondolkiri where protected forest areas still exist, more than 900 people living on the periphery of these forests were interviewed, in an attempt to analyze the structures and functioning of illegal, commercial bushmeat. Statistical analyzes are underway to identify people most at risk of being in contact with these pathogens. We already know that the people exposed are mostly young, middle-class men. Some communities are also more exposed than others. The sociological surveys also allowed a better understanding of the current context – the legal framework, the profiles of those involved in this trade, their obstacles and their motivations, linked to the trade in wild animals and their consumption, and the evolution of this context . about the various health crises (avian flu, Ebola, SARS-CoV-1, etc.).

Which populations might be at risk?

These successive crises seem to have little impact on the practices of these communities. In addition to regular consumption, a quarter of the interviewed families also reported hunting or trapping and 11% said they sell game meat and/or wild animals. In addition, and at the same study sites, more than 2000 samples of wild animals subject to trafficking or subsistence consumption – bats, rodents, turtles, monkeys, birds, wild pigs, etc. were analyzed. Some of the samples tested positive for coronavirus in particular and are being analyzed at the Pasteur Institute of Cambodia (IPC) to sequence the genome and learn more about its origin, evolution and zoonotic potential. Finally, blood samples were taken from more than 900 people surveyed in the same area to find out if they had been in contact with one or more coronaviruses. Analyzes are still ongoing, but we already know that these people had not, at the time of the investigation, been exposed to SARS-CoV-2.

The Covid crisis has clearly demonstrated this: it is essential to detect these outbreaks early in order to implement measures as soon as possible to prevent the spread of pathogens. And if there are still many doubts about emergency mechanisms, the same logically applies to the surveillance systems to be implemented to monitor them. Results from the ZooCov project will be used to develop a system for the early detection of disease events. overflow zoonotic viruses, in particular through the strengthening of Cambodia’s existing wildlife health surveillance system created by the Wildlife Conservation Society WCS. Other important research and development projects will contribute to the understanding of these emergency phenomena, to their prevention and early detection.

The authors thank the Cambodian Ministries of Health, Agriculture and Livestock and Environment, as well as all project partners: Institut Pasteur du Cambodge (IPC), Wildlife Conservation Society (WCS) Flora and Fauna International (FFI), Research Institute for Development (IRD), Hong Kong University (HKU), GREASE Network, International Development Enterprise (iDE), World Wildlife Fund (WWF), Elephant Livelihood Initiative Environment (ELIE), BirdLife International, Jahoo, World Hope International.

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