While the Covid-19 virus (coronavirus SARS-CoV-2) continues to circulate and claim victims worldwide, its origin remains unknown. Every scientific community makes its own hypothesis. Some suspect the possibility of an outbreak of the virus from a lab.
Another hypothesis, based on recent studies on the Chinese market by Wuhan and others in Cambodia, Laos, Japan, China and Thailand, is that of evolution from an ancestral virus of the family horseshoe bat present in bats in domestic or wild animals in particular, then the transmission of the virus from these animals to humans. In fact, during these various studies, several viruses with genetic sequences very similar to SARS-CoV-2 have been isolated in these bats.
A missing link
While certain species of bats are now proven to harbor these coronaviruses naturally, the identity of the domestic or wild animals that would have served as intermediaries between them and humans — missing links — remains a mystery. The initially suspected pangolin now appears as more of a “collateral victim” than one of those famous missing links. Indeed, while a sequence of the coronavirus genome detected in pangolins was related to that of SARS-CoV-2, the rest of the genome was genetically too distant.
On the other hand, the pangolins on which viruses genetically similar to SARS-CoV-2 were isolated were most of the time confiscated from live animal markets at the bottom of the supply chain and were therefore in prolonged contact with other animal species. It is very likely that they were contaminated in this way and not in their natural environment. Mink farms have also been suspected in China.
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Finally, pangolins and horseshoe bats do not share the same habitats, making potential contact between the two species, which would have involved transmission of the virus from a bat to a pangolin, very unlikely. Civet cats and/or raccoon dogs could represent 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 in severe renal syndrome, or filoviruses, including the Ebola 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 the human population mainly through direct contact with blood, secretions and other bodily fluids of infected individuals. The average mortality rate is around 50%.
In 2013, the first cases of Ebola virus disease (EVD) were detected in West Africa. This emergence 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 game meat, are therefore real and potentially devastating.
It is the characterization and quantification of this risk in Cambodia that the ZooCov project has been investigating, 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 hunted by wild animals and eaten, transmitted to humans.
In fact, wildlife trading and bushmeat consumption are common practices in Southeast Asia. This use is often opportunistic and occurs in certain communities to supplement a low-protein diet. It can also be regular and targeted. In Cambodia, of 107 families surveyed during ZooCov, 77% said they had consumed bushmeat in the previous month.
Medical use is also widespread. In Vietnam, analysis of reports of seizures of pangolins and derived products conducted 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, a belief that the consumer of that meat is appropriated the physical or physiological merits of the animal being consumed, or a desire to challenge industrial consumption unhealthy meat. Wild animal husbandry to meet this demand and/or fur production are also common.
In Cambodia, in Stung Treng and Mondolkiri provinces, where forest reserves still exist, more than 900 people living on the edge of these forests were interviewed to analyze the structures and workings of commercial, illegal bushmeat. Statistical analyzes are underway to identify those individuals who are most at risk of exposure to such pathogens. We already know that the exposed persons are mainly young, middle-class men. Some communities are also more exposed than others. Sociological surveys have also made it possible to better understand the current context – the legal framework, the profiles of the actors involved in this trade, their obstacles and their motivations related to the trade in wild animals and their consumption, as well as the evolution of this context over the various health crises (Bird flu, Ebola, SARS-CoV-1 etc.).
Which population groups may be at risk?
These successive crises appear to have had little impact on the practices of these communities. Beyond normal consumption, a quarter of families surveyed still reported hunting or trapping and 11% reported selling bushmeat and/or wildlife. In addition, more than 2000 samples of wild animals traded or consumed for personal consumption – bats, rodents, turtles, monkeys, birds, wild boar, etc. were analyzed at the same study sites. Some of the samples, in particular, tested positive for coronavirus and will be 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 interviewed in the same area to find out if they had been exposed to one or more coronaviruses. Analyzes are still ongoing, but we already know that these individuals were not exposed to SARS-CoV-2 at the time of the investigation.
The Covid crisis has clearly shown this: it is essential to identify these phenomena early on in order to take measures as quickly as possible to prevent the spread of pathogens. And if many questions about the mechanisms of formation remain unanswered, this logically also applies to the monitoring systems to be set up to monitor them. The results of the ZooCov project will be used to develop a system for the early detection of zoonotic virus spillover events, in particular by strengthening the wildlife health surveillance system already in place in Cambodia and set up by the Wildlife Conservation Society WCS. Further important research and development projects will contribute to the understanding of these phenomena, their prevention and their early detection.
The authors thank the Ministries of Health, Agriculture and Livestock and Environment of Cambodia and all project partners: Institut Pasteur du Cambodge (IPC), Wildlife Conservation Society (WCS), Flora and Fauna International (FFI), Research Institute for Development (IRD), Hongkong 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.