On the trail of the origin of Covid-19

While the Covid-19 virus (coronavirus SARS-CoV-2) continues to circulate and make victims around the world, its origin remains unknown. Every scientific community puts forward its hypothesis. Some suggest the possibility of an escape of the virus from a lab.

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

A missing link

While certain bat species have now been proven to naturally harbor these coronaviruses, the identity of the domestic or wild animal that would have acted as a relay between them and humans — missing links — remains a mystery. The Pangolin, initially suspected, now appears more as a “collateral victim” than one of these famous missing links. Indeed, a sequence of the coronavirus genome detected in Pangolins was indeed related to that of SARS-CoV-2, but the rest of the genome was genetically too far removed from it.

On the other hand, the pangolins on which viruses genetically close to SARS-CoV-2 were isolated were usually seized from live animal markets, at the end of the trade chain, and therefore had prolonged contact with other animals. animal species. It is very likely that they were infected along this path and not in their natural environment. Mink farms are also suspected in China.
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Finally, pangolins and horseshoe bats do not share the same habitats, making contact between the two species highly unlikely, where the virus would have passed from a bat to a pangolin. Civet cats and/or raccoon dogs could be an intermediate reservoir for SARS-CoV-1). Rodents or primates may also carry pathogens with zoonotic potential, such as hantaviruses, which can especially cause hemorrhagic fever with severe renal syndrome, or filoviruses, including Ebola virus. The latter is transmitted to humans by wild animals, especially fruit bats, porcupines and primates such as chimpanzees or gorillas, and subsequently spreads in the human population primarily through direct contact with blood, secretions and other bodily fluids of infected humans. The average death rate is around 50%.

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

Bushmeat consumption: a risky practice

The risks of animal-to-human transmission, a phenomenon known as spillover, whether from hunting, handling animals or consuming 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 nearly 2 years and since the start of the pandemic, if so, and how pathogens such as coronaviruses are transmitted from wild animals, which are hunted and eaten, on humans.

In Southeast Asia, wildlife trade and bushmeat consumption are indeed common practice. Often opportunistically, this consumption occurs in certain communities as an adjunct to a low-protein diet. It can also be regular and focused. In Cambodia, 77% of the 107 families interviewed at ZooCov said they had eaten bushmeat in the past month.

Medicinal use is also widespread. In Vietnam, the analysis of reports of seizures of pangolins and derivatives carried out by the Vietnamese authorities between 2016 and 2020 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 wealthy classes, and often in large cities, this consumption may be motivated by a need for social recognition, the belief that the consumer of this meat appropriates the physical or physiological virtues of the animal consumed, or by a desire to consumption of industrial meat that is harmful to health. Breeding wildlife 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 fringes of these forests were interviewed in an attempt to analyze the structures and functioning of commercial, illegal bush meat. . Statistical analyzes are underway to identify the people most at risk of coming into contact with such pathogens. We already know that the exposed people are mostly middle-class young men. Some communities are also more exposed than others. Sociological studies 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 their motivations, linked to the wildlife trade and their consumption, and the evolution of these context on the different health crises (avian flu, Ebola, SARS-CoV-1, etc.).

Which populations may 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 families interviewed still reported hunting or catching, and 11% said they sold bushmeat and/or wild animals. In addition, and at the same research sites, more than 2000 samples of wild animals subject to trade 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 zoonotic potential. Finally, more than 900 people surveyed in the same area had blood samples taken to find out if they had been in contact with one or more coronaviruses. The analyzes are still in progress, but we already know that these people had not been exposed to SARS-CoV-2 at the time of the study.

The Covid crisis has clearly demonstrated this: it is essential to detect this emergence early in order to take measures as soon as possible to prevent the spread of pathogens. And if many questions remain about the mechanisms of attendance, logically the same applies to the surveillance systems that are put in place to control them. The results of the ZooCov project will be used to develop a system for the early detection of zoonotic virus overflow events, in particular by strengthening the wildlife health monitoring system that already exists and is in place in Cambodia. set up by the Wildlife Conservation Society WCS. Other important research and development projects will contribute to the understanding of these emergence phenomena, their prevention and early detection.

The authors thank the Ministries of Health, Agriculture and Animal Husbandry and Environment of Cambodia, 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.

The original version of this article was published on The Conversation on May 2, 2022.

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