How humans and animals can be linked in disease

Current scientific research agrees that the COVID-19 coronavirus was transmitted to us by the encounter of two animal species. However, this encounter was probably forced by the human being himself. How can human health be linked to that of other animals? A subject specialist explains it to us.

Veterinarians and virologists regularly discover new unknown viruses in bat species in Asia (Credits: Smithsonian’s National Zoo / YouTube).

In the context of a pandemic, despite the self-centered vision that often governs it, humans are not just alone against a virus. Whatever the “fake news” about its origin (whether caused by the new 5G wave frequencies or coming from a laboratory accident), the COVID-19 coronavirus is of natural origin. The SARS-CoV-2 coronavirus would not exist without having encountered several animal species on its way. This fact should remind us that humans are just another animal species among others that this virus is capable of infecting.

However, the fault is not to be blamed on our wild fellows who passed it on to us. It is not enough to eradicate them to eradicate them, on the contrary. A greater diversity of species dilutes the chances that a virus will preferentially attack us. In addition, humans have a non-negligible share of responsibilities in the emergence of this pandemic through its disruptive action on natural ecosystems. Coralie Martin, researcher in parasitology for Inserm (National Institute of Health and Medical Research) and at the National Museum of Natural History in Paris, explained everything (or almost) to us: in reality, everything is connected.

What about zoonosis?

To understand how the health of the human species can be linked very concretely to that of other animal species, we must first focus on the concept of “zoonosis”. This phenomenon is observed when a disease crosses the biological barrier between species and is transmitted from one specific species to another and therefore by extension, possibly, to humans (Homo sapiens). Virologists call an infectious agent “zoonotic” “when it infects, for example, humans when normally it is not expected to be on its host spectrum,” said Coralie Martin. In fact, an infectious agent manages to live by exploiting, in one way or another, the biological resources of one or more hosts belonging to as many different species. It can be in the form of a parasitic worm like the Tapeworm, a bacterium like Escherichia coli or a virus like the coronavirus of COVID-19. Some are more or less pathogenic than others and make some of their hosts sicker than others. As the parasitologist points out, “there is no animal that does not carry a pathogen. We all host one or more infectious agents, including us. ” According to some researchers, there are still 1.6 million unknown viral species in mammals and birds, in wildlife – not counting on the other microorganisms that they are able to transmit to us before we passed them on to each other ourselves.


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This phenomenon of zoonosis can seem worrying when it produces a chimeric virus (that is to say, resulting from genetic recombinations during its adaptation in different species) unknown and deadly like the coronavirus SARS-CoV-2. However, it is not uncommon and is at the origin of the vast majority of epidemics, even pandemics, known to date. The three so-called permanent pandemics, still at work today, all originate from zoonotic transmissions: tuberculosis, related to bovine tuberculosis caused by the bacterium Mycobacterium bovis, malaria, induced by the protozoan Plasmodium whose mosquito is a vector and HIV, or AIDS virus, which would come from several species of great apes of Cameroon. In these three cases, as in that of COVID-19, the emergence of these pandemics comes only from one thing: “closer contacts between wildlife and the human population which create stronger possibilities of transmission. ”

Human, serving their own pandemics?

These connections between humans and wildlife are not the result of a simple meeting between two individuals of different species. Most often, they are the consequence of a territorial enlargement of human activities and the degradation of natural habitats that follows. As the human population increases, so does its need for resources and housing. To satisfy them, certain populations are forced to go and extract resources in nature or “make room” by opting for the deforestation of wild areas. These situations lead them to come into contact for the first time with local wildlife and the pathogens it carries. This unprecedented promiscuity considerably increases the risks of zoonosis but above all disturbs the established ecosystems by inserting a species which normally does not belong there. “If we touch on a natural phenomenon or a parameter on which biodiversity depends, we unbalance it, notes Coralie Martin. This human intervention causes cascading effects on the populations responsible for it and subsequently leads to unexpected and often dangerous consequences. ”

This intrusion into an ecosystem previously untouched by interactions with humans can be formulated in various ways. In the case of COVID-19, the scientific community currently agrees that the sale of live wild animals at the market in Wuhan, China, is very likely to be the cause of the adaptation and transmission of ” a coronavirus specific to the Bat Horseshoe Bat (Rhinolophus affinis) in humans. The promiscuity of animal species foreign to each other has allowed the “fortuitous passage” of the bat coronavirus RaTG13 to the Malaysian pangolin (Manis javanica) where the virus has recombined to be, by the effect of the chance of meetings , sufficiently adapted to infect a third species, the human being. Without the human desire to poach and then sell different species in one place, this zoonotic transmission may never have occurred. The same goes for the pandemic that followed. The drastic increase in freight transport and movement of people from one corner of the world to the other, which has become the norm in modern society, has not helped either. Ditto in the case of intensive or semi-intensive farming which gives foreign viruses the real culture broth they need to mutate quickly and, sometimes, contaminate humans. This is what gave birth to the H5N1 avian flu virus in the 2000s.

Despite the pandemic, poaching and pangolin trafficking continue: 9.5 tonnes of pangolin scales were reportedly seized in January in Nigeria (Credits: Adam Tusk / Flickr).
The increased risk of zoonotic transmissions and pandemics can also be induced more indirectly by human activity. The role of global warming on our health also has its share of responsibility. As the parasitologist points out, the increase in global temperatures “can have an effect on the distribution of animals carrying pathogens.” Some areas free of disease-carrying species are becoming more welcoming to them. Their distribution area increases with this rise in temperature and regional climate change. Thus hematophagous arthropods (which suck the blood of other animals to live), such as the tiger mosquito or the hard tick (or Ixode) which are respectively vector of malaria and Lyme disease, now bask well beyond their natural habitat and comes into contact with vulnerable human populations. “At the time of the construction of the Palace of Versailles, the temperature reached record levels and workers died of malaria in France, recalls the researcher. Until its eradication thanks to massive DDT mosquito control campaigns (which we would no longer use today and rightly so) in the 1960s, it was impossible to go out in the afternoon in certain regions such as Languedoc-Roussillon during the hottest summers. ” Proof of this is that the health of mankind is dependent on the health of biodiversity and the health of the planet. The more these two are damaged or disturbed, the more our own health suffers.

The “One Health” concept

In his song “Side Effects” addressed to humanity in response to the COVID-19 pandemic, the slammer Grand Corps Malade is not afraid to proclaim: “Nature rules its law by taking back its rights, taking revenge of our arrogance and contempt. ” In his own way, he reminds us that this health fight that our civilization is facing does not in reality have humans against the virus as a poster. Despite the perhaps excessively self-centered vision of human society, the SARS-CoV-2 coronavirus which is the cause of this health crisis is not just a simple adversary against which it must fight. If she treats him that way, as soon as he is eradicated, another will come later to take his place. Like, moreover, he himself succeeded SARS-CoV, the first of its name, in 2003 and MERS-CoV in 2012.

According to the scientific and medical community, to learn to control and limit the devastating effects of such zoonotic viruses, humanity must consider them in a broader context, regaining its position as an animal species among others and becoming aware of its responsibility as such towards its peers. This idea, that in order to treat humanity, we must first think of caring for the whole of nature, is carried by a group of researchers, doctors and veterinarians behind the concept of “One Health”. Coralie Martin is one of her fervent supporters. “This is a more global way of thinking about the spread of infectious diseases,” says the parasitologist. It allows you to have a sufficiently broad vision, which transcends borders. ” For about four years, this method of thinking has united the World Health Organization (WHO), the World Organization for Animal Health (OIE) and the Food and Agriculture Organization of the United Nations (FAO) to discuss environmental solutions for human and animal health. This alliance around the same vision does not only accelerate medical and epidemiological research in times of pandemic, by unlocking emergency funding funds as at present. It can also provide an opportunity to take a closer look at human-animal relationships around the world and find a way to prevent them from causing harmful consequences for each other.

The “One Health” concept has great potential that researchers are longing to realize. “It could bring together more professionals in order to decipher the complexity of the cultures and the environmental context on the ground”, underlines Coralie Martin. Despite the relevance of this idea, it is up to government authorities to put it into effect. As the researcher notes, “without a strong political decision, it cannot work. COVID-19 helped highlight the role of the human-animal relationship in the emergence of the pandemic and produced a breath of fresh air that pushed China to limit the trafficking of wild animals on its territory in order to reduce the risks of zoonotic transmission. ” This necessary drop of water on the geopolitical and ecological chessboard could perhaps encourage the rest of the world to change things … before the next pandemic emerges.

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