In the wake of the COVID-19 global pandemic, which started when the virus SARS-CoV-2 jumped from animals into humans, we ask an important question why are infections acquired from animals so dangerous to human health?
Humans may have contracted the new coronavirus from pangolins, which are often trafficked illegally in China. While it is not yet clear which animals were the source of the new coronavirus ó was it bats? Was it pangolins? Was it both? scientists are sure that SARS-CoV-2, the virus that causes COVID-19, originated from animals.
The numbers of confirmed COVID-19 cases across the world are staggering. According to Johns Hopkins University, more than 730,000 people have contracted the virus and 34,000 people have died.
But zoonotic diseases that are, diseases acquired from animals were affecting vast numbers of people across the world before COVID-19 took center stage. Stay informed with live updates on the current COVID-19 outbreak and visit our coronavirus hub for more advice on prevention and treatment.
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An international report from 2012, for example, informed that a total of 56 such diseases were responsible for 2.5 billion cases of illness and 2.7 million deaths across the globe each year. These illnesses included rabies, toxoplasmosis, Q fever, Dengue fever, avian influenza, Ebola, and anthrax.
Animal virus vs. the human immune system
One reason viruses from animals are so dangerous to humans is that people have no means to deal with them. Our immune system was never ëintroducedí to these novel viruses, so it doesnít know how to respond to the uninvited guest.
Researchers explain that most of the viruses that enter the human body are successfully destroyed by the immune system or pass through our gastrointestinal system. However, now and then, an animal virus manages to replicate within a human host.
An evolutionary arms race
In other words, the animal virus and the human immune system have entered an arms race and like with any arms race, one of the two competitors could win, or both competitors could reach a stalemate.
Medical News Today spoke to Christopher Coleman, assistant professor of Infection Immunology at the University of Nottingham in the United Kingdom, about animal viruses, human hosts, and the role of evolution and natural selection.
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[T]he general assumption, he explained, is that as viruses evolve to a host, they become less dangerous to that host (they want to ensure their own transmission so donít want to rapidly kill the host before they get a chance [to replicate]).
This is by no means [always] true, but a virus that adapts to humans might be less dangerous in the long term because the ëevolutionary arms race between virus and host has reached a sort of stalemate where neither is perfectly happy, but neither is killed off.
The scientist whose main research focuses on ëhighly pathogenic human coronaviruses gave examples of aggressive animal viruses within the coronavirus family. These include the ìëinfectious bronchitis virus of chickens, feline infectious peritonitis virus in cats, or ëtransmissible gastroenteritis virus, which is near 100% fatal in piglets.
None of these [viruses] are known to infect or cause any disease in humans, Coleman said.
Bats and their ëhyper-vigilantí immunity
For instance, the fact that very harmful viruses such as SARS, MERS, and Ebola have all originated in bats begs the question of what do bats have that we don’t?
How can bats fly around carrying viruses that, in some cases, are extremely deadly to humans (such as Ebola), but that do not seem to harm these creatures in the slightest?
"You can only play viral Russian roulette for so long, which is why public health experts concerned about zoonotic diseases have for years been ringing the alarm about the industrial farming of animals."
We can't afford to keep playing this game.https://t.co/oEHhgmMA7T
— Liz Specht (@LizSpecht) March 28, 2020
A new study, led by Cara Brook, a postdoctoral Miller Fellow at the University of California Berkeley, asked this very question. The research shows how batsí unique immune capabilities enable them to carry and maintain a high viral load without getting sick themselves.
[S]ome bats, explain Brook and colleagues in their paper, have an antiviral immune response called the interferon pathway perpetually switched on.
From strength to strength
This is all great news for bats, but what does it do for other mammals? Sadly, not much. The fact that bats have such good defenses means that the virus has all the encouragement it needs to replicate more quickly.
The batsí unique immune capabilities eventually make the viruses stronger. It is like training with an outstanding competitor and getting stronger as a result.
Brook and her team carried out experiments using cell lines from two species of bats. The results showed that in ìboth bat species, the strongest antiviral responses were countered by the virus spreading more quickly from cell to cell.
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ìThis suggests that bat immune defenses may drive the evolution of faster transmitting viruses, and while bats are well protected from the harmful effects of their own prolific viruses, other creatures, including humans, are not.
COVID-19: bats, pangolins, or snakes?
In the case of the new coronavirus, multiple theories are circulating about the specific animal that passed on SARS-CoV-2 to humans. Scientists have implicated pangolins or even snakes as possible carriers.
Pinning down specific mammals is vital because the animal can offer insights into the genetic structure of the virus and ways to tackle it. However, it is essential not to discount the possibility that the new coronavirus might have several animal sources.
Two alternative scenarios for COVID-19
In a recent study, researchers led by Kristian Andersen, Ph.D., an associate professor of immunology and microbiology at the Scripps Research Institute in LaJolla, CA, used the available genomic data to determine whether the origin of the new coronavirus was natural or made by humans.
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In other words, if the virus evolved to its current state in animals, then animals would continue to pass it amongst each other, and the virus could jump back into humans at any point.