How do new diseases arise
Veterinarian on the corona outbreak: "We have been looking at the live animal markets with concern for a long time"
With Covid-19, another disease has developed because a pathogen has jumped from animals to humans. This is happening more and more often because the contacts between the species are increasing, explains veterinarian and epidemiologist Gertraud Schüpbach.
Ms. Schüpbach, the Sars-CoV-2 virus, which is responsible for the current pandemic, probably circulated in bats before it was transmitted to humans via another intermediate host. Are Wildlife a Threat to Our Health?
First of all, wild animals are a great source of biodiversity: There are very many different types of animals, and within these types there are also very many different viruses - and of course bacteria and other pathogens. But I wouldn't go so far as to say that wildlife is a threat. Pathogens are simply part of nature and our world.
There have always been diseases that spread from animals to humans, so-called zoonoses. Nevertheless, the outbreaks seem to be increasing - think of Sars, Mers, Ebola or currently Covid-19. Is that impression correct?
Yes, the impression is correct. Especially among the newly emerging infectious diseases there is a noticeable number of zoonoses. Because the human population is growing and people are also penetrating into remote areas, the number of contacts between people and the various animal species is increasing. This is the most likely reason why there are more and more diseases that are transmitted from animals to humans.
Humans have always lived near wild animals or have fed on them. What is different today?
Indeed, zoonoses have always existed. What is new is that the contacts between humans and animals have become much more intensive and frequent. You have to consider: the vast majority of viruses are harmless to humans because they are adapted to a specific animal. Only if a virus happens to be such that it can spread to humans, or if it changes accordingly, can it also infect humans and make them sick. That actually happens very rarely. But the more frequent the contact between humans and animals, the greater the likelihood that it will happen.
Gertraud Schüpbach, veterinarian and epidemiologist
The first infection with Sars-CoV-2 probably took place at an animal market. It was similar with Sars or the bird flu. Why are markets always the source of new disease outbreaks?
We have been looking at these live animal markets with great concern for a long time, as are common in Asia, for example. Because there is not only close contact between humans and animals, but also between the many different animal species that are traded. For example, the avian flu pathogen that actually affects birds can also spread to pigs or humans. The fact that these species are so close together promotes the exchange of viruses and the exchange of genes between viruses, so that new types of influenza can emerge. Such markets are therefore rather problematic for human health. But they also have other negative effects.
And what would they be?
Because the animals offered for sale are transported under dubious conditions, kept in confined spaces and slaughtered on site, live animal markets are firstly a problem for animal welfare. Second, they are also a huge problem for species protection, because many of the wild animals traded there are critically endangered.
But with the extermination of certain animal species, potential pathogens also disappear, don't they?
Yes, at first glance. But I think it is a very bad idea to solve the zoonosis problem in this way. Firstly, the diversity of the animal world has a very high value in itself and is also valuable for humans. Second, a reduced biodiversity even more favors the transmission of pathogens.
Can you explain why that is?
In an intact ecosystem there is a well-established balance between the species and their specific pathogens. Each species occupies its own ecological niche and is thus differentiated from other species. But if a species recedes because it is hunted intensively or its habitat is destroyed, the structure gets mixed up. Suddenly animals meet that would never have met before. This favors the transmission of pathogens to new hosts.
Does that also contribute to the emergence of new aggressive diseases?
Yes, and that happens when a pathogen crosses over to a new host to which it is not adapted. For this it is usually more deadly than for its original host.
Can you give us an example of this?
The African swine fever pathogen has been circulating in warthogs in Africa for a long time. Because the pathogen is already well adapted to its host, the warthogs rarely get sick. Because a pathogen that would kill its host immediately could no longer be spread through it. But if the swine fever pathogen spreads to domestic pigs, the disease is fatal because it has not yet adapted to the new host.
We've talked a lot about wildlife now. What role do farm animals play in the development and spread of new zoonoses?
As far as we know, farm animals played no role in Sars-CoV-2. In the case of other zoonoses, however, they are an important factor. The example of avian flu shows it well: although wild birds are the reservoir for the pathogen, humans only become infected with it through poultry. Because he has closer contact with chickens and ducks. A virus that circulates in a population of farm animals is therefore more dangerous to humans than a virus that is unique to a wild animal population.
So one way to reduce the risk of zoonoses would be to improve the health of farm animals?
Yes, at least that's what you try. It is important that the health programs not only take animal diseases into account, but also zoonotic pathogens that can spread to humans. In Switzerland, for example, there is a program to combat salmonella in chickens, the main aim of which is not to keep poultry healthy, but to protect people.
Meat consumption is increasing worldwide, which is why more and more livestock are kept. Does this aggravate the problem of zoonoses?
If for this purpose animal husbandry is expanded to areas where no farm animals were previously kept, then yes. For example, when areas in the rainforest are cleared for this purpose. Because then the farm animals move closer to the wild animals and contacts are made. However, this aspect hardly played a role in the case of Sars-CoV-2. For other zoonoses, however, it can be an important factor.
What is the significance of increasing mobility for the spread of zoonoses?
Mobility and globalization, both in relation to the movement of animals and goods, are important aspects. Travel and flight activity are also increasing more and more. In this way, pathogens can reach us in Europe from even the remotest corner of the world. Another new feature is the short time in which a pathogen can be spread across the world from the country of origin. That was much less the case a hundred years ago.
In your opinion, what measures should be taken to prevent future pandemics such as Covid-19?
If you want to address the origin, then the ecosystems should be preserved as well as possible so that livestock and humans come into less contact with wild animals. Then the live animal markets, where wild animals are offered, would have to be abolished: They are mainly found in countries where the cold chain cannot be adhered to. The situation would immediately improve if only meat were traded. The risk of infection is much lower than with the transport and trade of live animals.
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