On the trail of the origin of Covid-19

While the Covid-19 virus (SARS-CoV-2 coronavirus) continues to spread and cause casualties around the world, its origin remains unknown. Each scientific community presents its hypothesis. Some suggest that the virus can escape from the laboratory.

Another hypothesis, based on recent studies related to the Chinese market of Wuhan and others conducted in Cambodia, Laos, Japan, China and Thailand, is an evolution from an ancestral virus found in bats, to the Horseshoe Bat family in particular, in domestic or wild animals, then transmission of the virus from these animals to humans. Indeed, during these various studies, several viruses with a genetic sequence very close to SARS-CoV-2 were isolated in these bats.

missing link

While it has now been established that certain species of bats naturally harbor these coronaviruses, the identity of the domestic or wild animal(s) that would have served as a relay between them and humans – the missing links – remains a mystery. The pangolin, suspected at first, now appears to be more of a “side victim” than one of these famous missing links. In fact, the sequence of the coronavirus genome detected in pangolins was already related to the sequence of SARS-CoV-2, but the rest of the genome was very far from the genome.

On the other hand, pangolins from which viruses genetically related to SARS-CoV-2 were isolated most of the time were confiscated from live animal markets, at the end of the commercial chain, and thus were in long-term contact. animal species. It is very likely that they were contaminated along this path rather than in their natural environment. Mink farms in China have also been suspected.

It is unlikely that the pangolin was the intermediate host of the virus.
Arif Bodhi Kusuma/Shutterstock

Finally, pangolins and horseshoe bats do not share the same habitat, which makes possible contact between the two species highly unlikely, as the virus is transmitted from a bat to an anteater. Civet and/or raccoon dogs can constitute an intermediate reservoir for SARS-CoV-1). Rodents or primates can also carry pathogens with zoonotic potential, such as hantaviruses that can particularly cause hemorrhagic fever with acute kidney syndrome, or filamentous viruses, including Ebola disease virus. The latter is transmitted to humans by wild animals, particularly fruit bats, porcupines, and primates such as chimpanzees or gorillas, and then spreads to humans mainly by direct contact with the blood, secretions and other bodily fluids of infected people. The average case fatality rate is about 50%.

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

Bushmeat consumption: a risky practice

The risk of transmission from animals to humans, a phenomenon known as Extension and diffusionWhether hunting, handling animals, or eating wild meat, they are real and potentially devastating.

The characterization and estimation of this risk, in Cambodia, is what the ZooCov project has explored through a “One Health” approach, for nearly two years and since the beginning of the epidemic, if so, how pathogens such as coronaviruses are transmitted from wild animals, which are hunted And eat it, to humans.

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

Medicinal use is also widespread. In Vietnam, an analysis of reports of confiscation of pangolins and derivative products conducted by the Vietnamese authorities between 2016 and 2020 showed that 1,342 live pangolins (6,330 kg), 759 dead or carrion pangolins (3305 kg), and 43,902 kg of scales.

But this consumption also has a cultural and social aspect that is still poorly understood. For the affluent classes, often in large cities, this consumption could be motivated by the need for social recognition, the belief that the consumer of this meat appropriates the physical or physiological virtues of the animal consumed, or the desire to challenge the consumption of industrial products. Unhealthy meat. Wildlife farming is also spreading to meet this demand and/or produce fur.

In Cambodia, in Stung Treng and Mondulkiri provinces where protected forest areas still exist, more than 900 people living on the outskirts of these forests were interviewed in an attempt to analyze the structures and performance of illegal commercial bush meat. Statistical analyzes are underway to identify those most at risk of contact with pathogens. We already know that the people revealed are mostly middle-class guys. Some societies are more vulnerable than others. Sociological surveys have also made it possible to better understand the current context – the legal framework, the profiles of the players in this trade, their obstacles and motivations, related to the trade and consumption of wild animals, and the evolution of this context around the various health crises (avian influenza, Ebola, SARS-CoV-1, etc.) .

What population groups may be at risk?

These successive crises seem to have little effect on the practices of these societies. In addition to regular consumption, a quarter of the families interviewed reported that they still hunted or hunted, and 11% said they sold bushmeat and/or wild animals. In addition, at the same study sites, more than 2,000 samples of wild animals subjected to trafficking or subsistence consumption – bats, rodents, turtles, monkeys, birds, wild boars, etc. have been analyzed. Some of the samples tested positive for coronaviruses in particular, and are being analyzed at the Institut Pasteur du Cambodge (IPC) to sequence the genome and learn more about its origin, evolution and animal potential. Finally, blood samples were taken from more than 900 people surveyed in the same area to see if they had been in contact with one or more coronaviruses. Analyzes are still ongoing, but we already know that these people were not exposed, at the time of the investigation, to SARS-CoV-2.

The Covid crisis has made this clear: it is essential to detect these outbreaks early in order to take measures as quickly as possible to prevent the spread of pathogens. And if many questions remain about the mechanisms of emergence, the same logically applies to the monitoring systems that are put in place to monitor them. The results of the ZooCov project will be used to develop a system for early detection of disease events. Extension and diffusion zoonotic viruses, in particular through strengthening the current Cambodian wildlife health monitoring system established by the WCS Wildlife Conservation Society. Other important research and development projects will contribute to the understanding, prevention and early detection of these emerging phenomena.

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

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