Are we responsible for the SARS-CoV2 pandemic?

Source of inspiration: “Les futures épidémies que nous vivrons” (Future epidemics we have yet to live through) from DirtyBiology

These days our planet is threatened by society’s way of life. Some people try to protect the Earth by changing their habits so as to not impact the environment as much: eat less meat, avoid flying, etc… At the beginning of 2020, our behavior changed drastically. However, it wasn’t caused by climate change: it’s the result of the SARS-Cov2 pandemic. Taking into account the development of these events, it’s important we change our behaviour to save the climate, but also to prevent future pandemics that could threaten humanity.

Three main factors

SARS-CoV2 is a pandemic, which means a disease that affects a large part of the population on the global scale. There are three main factors that can come into play when it comes to the appearance of new diseases of this type, and all three will be detailed in this post.

It has been observed that some equatorial regions, notably in Asia and in the Pacific Ocean, face more serious epidemics, and more often when compared with other parts of the world [1]. This can be explained by especially high biodiversity (how many different kinds of animals, plants and bacteria are present), which is the first factor.

The second one is mobility that has developed thanks to human activity. Since the very beginning, Homo sapiens has been travelling more and more, and nowadays all corners of the world are connected.

The third and last factor is our way of life. Humans started out as hunter-gatherers, but thanks to agriculture, they established permanent settlements and started growing food.

Biodiversity – a lever to future epidemics?

As mentioned above, biodiversity is correlated with the abundance of epidemics. In fact, the more there are different species, the more opportunities occur for pathogens to develop and diversify. Such diversity could indirectly allow a virus to pass from one species to another. This phenomenon is called a zoonosis [2] and is defined as an infectious disease that has passed from an animal to a human. This type of transition isn’t new and has been happening for ages – it has been shown that 58% of human diseases came from animals. In contrast, only 26% of human pathogens are able to infect wild and domestic animals [1].

A higher biodiversity therefore leads to a higher number of pathogens. As a result, we could think that a lower biodiversity prevents the development of new viruses. Partially, it’s true. However, low biodiversity is a symptom of future epidemics [3]. In fact, destruction of natural habitats of numerous species makes them approach human infrastructures, where food and water are easily accessible. This new proximity favours interactions between humans and animals, and so also favours the potential risks of transmitting a pathogen from one to another. Abundance of potentially infectious microbes (like virus or bacteria) and the risk of zoonotic infection are two similar, but not identical notions. Indeed, if we look at repercussions of the loss of biodiversity [4] in the equatorial regions, we see that they result in particular diseases.

The following example is presented to illustrate this concept, which is not specific neither to bats nor to deforestation: there are other species that can lead to a zoonose and there are other ecological or climate disturbances forcing animals’ migration. We can easily imagine a situation like this: deforestation makes the bats look for a place to live in our gardens, where they eat fruit and contaminate them with their saliva (which contains viruses and bacteria). From this point, contact between a virus and a human is just a question of time.

These two factors, which are the abundance of microbes and a small distance between animals’ habitats, will probably increase the risk of a new epidemic.

Global level transport

Human activity is also connected to propagation of viruses coming from equatorial regions. First of all, it’s important to differentiate epidemics and pandemics. Epidemics are diseases that touch a big part of the population, but they are limited to one region, country, or another, well defined zone. A pandemic refers to a disease that spreads on a population of a continent, or even globally. This way, since humans travel among different places, the risk of transporting a disease is higher, which increases the risk of the development of a pandemic.

A known example is the Black Plague, an illness of bacterial origin. The Black Plague was initially transmitted to humans by rodents because of the great drought in China in the XIV century [5]. Heat and lack of water made the wild rodents migrate to the cities – and with them came the fleas, vectors of the plague. After that, the Plague spread across the world by the silk road, which used to connect East Asia and Europe, as shown on the picture below. Thanks to the popularity of this trading route, the Plague quickly developed into one of the most infamous pandemics.

Settled down way of life and livestock

These new outbreaks can also be explained by the fact that we are no longer “hunters-gatherers”, but settled down as farmers, growing all we need [6]. Such a system has its advantages and inconveniences, which we will approach in the light of epidemics.

In the actual animal husbandry system (the agricultural system put into place that provides us with meat and other animal-derived products), it’s possible to control the conditions in which the animals are living. By such means as vaccination, diagnostics and prophylactic treatments (treatments that prevent an illness), we protect the livestock from diseases. However, we select the animals that are most profitable for us, so they are genetically most alike. This genetic similarity acts as a brake, slowing down natural selection and resulting in all individual animals suffering exactly the same mortality and symptoms of a pathogen.

Adding to the lack of genetic diversity, we must take into account the lack of space – all animals are in close contact. This can then result in new epidemics: if one animal gets sick, it will easily contaminate all the others. This raises the risk of a spontaneous mutation in the disease. A mutation means a change in the virus’ or bacteria’s genetic material, and possibly a change of its characteristics. In this manner it could potentially adapt and infect other species, humans included [6].

It’s equally possible that a virus adapts at first to an animal species, called intermediate, before infecting humans – in this case, the animal species becomes known as a “reservoir” where the disease can stay until it can complete its life cycle in humans (the “final host”). This was probably the case of SARS-CoV2, which could have passed from a bat, to a pangolin, to a man [7].

Hypothetically, in a couple of years, human activity will take a different turn, one that leads to durable consumption while preserving biodiversity, lowering the level of greenhouse gases, preventing climate change and deforestation. Maybe humanity will take this turn to avoid the next new global crisis that followed the SARS-CoV2 pandemic. Or maybe our way of life will stay as it is and there will be more and more new pandemics, but we won’t be there to witness.

Sources:

Jakub Korzeniowski

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Sara Ezzat

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