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The SARS-CoV-2 pathogen developed in bats and then probably jumped into humans through another animal.
Whether it’s Covid-19, the Spanish flu, or HIV: The greatest pandemics in human history were mostly triggered by pathogens that leapt from animals to humans. In the case of infectious diseases caused by viruses, the proportion of these so-called zoonoses is even more than 70 percent. Most influenza viruses originate in birds, HIV is due to a common virus in monkeys, and Ebola and the new SARS-CoV-2 corona virus have developed in bats.
The corona virus outbreak in Wuhan, China has caught the attention of wildlife markets like this one.
The first step: contact
But how do these animals get into humans? And why can they make us sick even if the animal host shows no symptoms of disease? A prerequisite for jumping over the species barrier is obvious: the virus or its carrier must come into contact with humans. Therefore, it is no coincidence that outbreaks of new virus infections increase, especially when people and animals join. This can occur in rural areas with many farm animals, but also in markets where live and dead wild animals are sold, such as the market in the Chinese city of Wuhan.
It also becomes dangerous where we humans invade previously intact areas or destroy wild animal habitat. Because suddenly this creates new contacts between humans and wild animals, which also offer their parasites and pathogens the opportunity to jump. But also in our near surroundings and in our cities, there are many animals that have adapted to our present and can, for example, infect us with viruses through their urine or faeces. An example of this is the red vole, which can also transmit the Hanta virus in Germany.
Cute red chickens can transmit the dangerous Hantavirus to humans. The infection is triggered by direct or indirect contact with infected animals and their excretions: urine, feces or saliva.
Cute red chickens can transmit the dangerous Hantavirus to humans. The infection is triggered by direct or indirect contact with infected animals and their excretions: urine, feces or saliva.
The second step: the docking point
The second prerequisite for a successful art jump is the human cell approach. Viruses cannot multiply, but they depend on the cellular machinery of their hosts. To do this, they first have to dock with the cell and penetrate it. To do this, the virus needs a surface protein that fits like a key in the lock of certain coupling points in the human cell. The virus can only attack cells if the configuration of this binding site is adjusted exactly.
In the case of the coronavirus, a part of the corona proteins that protrudes in this way performs this task. The binding site on this spike protein is so mutated that it can now perfectly couple to a receptor on the cells of our airways and lungs. That is why this virus triggers cough and pneumonia. With other viruses, such as the tick-borne TBE virus, surface proteins bind to coupling sites in cells of the nervous system; that is why this virus causes meningitis.
The more easily the surface proteins of a virus mutate, the greater the probability that this pathogen can adapt to a new host. Because sometimes small changes in the structure of the binding sites are enough to gain access to cells of a new type. Particularly talented “shape changers” in this regard are enveloped viruses, viruses such as coronavirus, which have an additional coat in addition to the protein-rich capsule around their genetic material. This virus envelope consists of a membrane that is covered with many particularly variable surface proteins. Therefore, it is no coincidence that most zoonoses are caused by enveloped viruses.
In the corona virus, some of the corona proteins that protrude from the corona take over the coupling with human cells.
In the corona virus, some of the corona proteins that protrude from the corona take over the coupling with human cells.
The third step: human-to-human transmission
But there is another step that turns an animal virus into a possible trigger for a pandemic: it must be able to multiply in human cells and then be able to jump from person to person. Certainly, there are viral pathogens that are widespread and make many people sick, but they have not taken this step. This applies, for example, to the Hanta virus, but also to the TBE virus or the causative agent of dengue fever.
This is different for today’s SARS-CoV-2 coronavirus, Ebola, or many influenza variants – these viruses no longer need animal carriers, but can be transmitted directly from person to person. This is possible because pathogens reach the outside world with the air we breathe, through blood, saliva, or other bodily fluids and can then be absorbed by other people.
The Covid-19 SARS-CoV-2 pathogen is unfortunately particularly “versatile” in transmission from person to person.
iStock.com, spondylolisthesis
The Covid-19 SARS-CoV-2 pathogen is particularly “versatile” in this regard: It is transmitted by droplet infection by coughing, sneezing, or even breathing and talking, but it can also be present in the feces of infected patients. Furthermore, this virus persists on smooth surfaces for several hours or days, increasing the risk of infection.
To make matters worse, unlike its predecessor, the SARS virus, which became widespread in 2003, those infected release this virus into the environment even before the first symptoms appear. This is why coronavirus infections are often contagious before fever and cough develop, which makes this pathogen so successful and containment of the current pandemic so difficult.
