2023-04-15 16:10:34
Why is it that bats are so unaffected by the many viruses they carry? The answer lies partly in genetic protection and partly in a highly tuned immune system.
In recent years, scientists have examined the viruses of nearly two thousand wild animals that are traditionally on the menu as a delicacy in China, such as porcupines. There were species such as raccoon dog and palm civet that are sometimes also farmed, including for the fur trade. The results appeared in the journal Cell and were sobering: 102 virus species were discovered, of which 21 could pose risks to humans. More than half of the viruses were new to science.
Although it is occasionally suggested that the coronavirus that caused the latest pandemic escaped from a Chinese research lab, the top journal published Science two studies last summer that may not prove conclusively that the virus made the leap to humans at a wildlife market in the Chinese city of Wuhan, but nevertheless found very strong evidence for it. Extensive research showed that the virus occurred in wild animals that were offered for sale. It would have even jumped to humans twice in a short time, independently of each other. Wildlife markets are ideal as a hub for zoonoses: diseases that jump from other animals to humans. Heavily stressed and therefore unhealthy animals in cages come into close contact with each other and with a crowd of people, which increases the risk of transmission.
66,000 infections
Research into bats and their viruses is still in its infancy. A study in Frontiers in Ecology and Evolution concluded that perhaps half of the bat species believed to occur in Asia have not yet been formally described by science. There are already about 1400 known bat species worldwide, making the family the second largest in the mammal world, after the rodents.
An analysis Nature Communications estimated that around 66,000 people in Asia would be infected each year with mostly unknown coronaviruses originating from the animal world – the total number of ‘jumps’ of viruses from other animals to humans around the world is estimated at 400,000 per year . Most viruses die out immediately after infecting a human body, but, as one scientist aptly put it, “If viruses constantly rain down on people, you’re going to have a pandemic one day.” Pandemics have increased in frequency in recent centuries. A study in Science Advances even concluded that all the pandemics in the human world since the early 1900s were due to a viral jump from other animals to humans.
A recent article in Nature nicely illustrated how human interference with natural habitats can increase the risk of bat virus transmission. The study from Australia involved the largest bats in the world (flying dogs), which are carriers of the Hendra virus, among other things. This mainly affects horses, but can end up in humanity through horse lovers. The virus was probably already circulating in bats before humans arrived in Australia, but it wasn’t until 1994 that the first jump to the human world was detected. From 2006, the number of reports of infections increased.
The study made a link between the increase in infections and changes in the landscape. Due to human expansion and global warming, more and more wild fruit and other blossoming trees are disappearing. The bats originally exhibited a wandering behavior in large groups over hundreds of miles in search of fruit and blossoms for their nourishment. But they were forced to adapt to the deterioration of their biotope and thus ended up in scattered small groups in an agricultural landscape that was less interesting to them, where they came into contact with horses and people. In this way they became a ‘problem’ for humans – a problem that we created ourselves.
Insensitive to cancer
Scientists have long puzzled over how bats can carry so many viruses, seemingly without being affected. For critters with such relatively small dimensions, they also live relatively long, partly because they seem to be insensitive to cancer. An exploratory study reported in 2021 Nature Reviews Immunology that the immune system of bats contains cell types that have not been found in any other animal species. There were also indications that the genetic material of bats contains quite a few pieces of virus genes. The bats appear to include them in their own genetic heritage.
Last year, scientists reported in Science Advances that copies of a gene have been made in the bat genome, which contains information about an enzyme with a key role in organizing the immune system. Most mammals have one copy of that gene, but some bats have several. Because the individual genes can mutate slightly independently of each other and thus expand their attack repertoire together, they offer a broader defense against viruses.
Last month appeared in Nature an overview of what has been gathered in recent years about the relationship between bats and their viruses. For example, it has been found that bats have a much higher permanent circulation of interferons in their blood than we do. These are molecules that sound the alarm when they detect a threat and thus mobilize the immune system. Bats also produce much more proteins that paralyze the multiplication of virus particles in their bodies and clean up the waste products that are released in the process. So their defenses seem to be more efficient than ours.
But perhaps the most important difference from what happens in a human body is that bats’ defenses rarely overreact to a viral infection, causing less severe inflammatory responses. The animals have found a natural way to restrict the responsible molecules (so-called inflammasomes). A large part of the symptoms that people show after an infection with the corona virus is a result of an overreaction of our immune system to the intruder.
It is also interesting that bats generally react differently to the presence of a virus than we do. Instead of pumping a lot of energy into trying to eradicate it completely, over time they seem to find a modus vivendi with it: they accept a modest presence of the virus. “They seem to be making some kind of peace treaty with their viruses,” one scientist argued Nature. Thus, they can become a reservoir for a wide variety of viruses. Man can learn a lot about his not always optimal functioning by studying how other species have fared in the course of their evolution.
Self vaccination
It cannot be ruled out that viruses and bats enter into a kind of symbiosis with each other: a relationship that is beneficial to both parties. A study published recently in Cell appeared. The advantage of bats for viruses is clear: they provide a safe and reliable biotope in which the viruses are not permanently attacked. But the reverse is less clear. How can viruses benefit their carriers? An impetus for an answer comes from the observation that quite a few virus genes end up in the bat genome, which can remain active and produce proteins. The proteins can function as a form of self-vaccination against new infections, with or without similar viruses.
Over time, bats would thus partially incorporate viral invaders into their defenses against new attackers. It is a stimulating thought, which may provide new medical insights for our own defense. Viruses can also work against each other in our body. An analysis Science offered evidence for it, based on observations of how corona, flu and other viruses of our respiratory tract interfere with each other’s functioning. ‘They really don’t match well’, one researcher summed it up concisely. “They can even show bullying behavior towards each other.” In this sense, some form of cohabitation with a number of viruses could provide additional protection against new attackers.
For the foreseeable future, the risk of new pandemics is bleak unless we take drastic measures to limit the risk of zoonoses. For the nature-unfriendly naive among us: it is pointless to try to exterminate bats and other virus carriers in order to protect humanity against disease. Not only is it technically impossible, because the animals are ubiquitous, but it would also entail natural catastrophes, partly because in much of the world bats play an important role in the fertilization of fruit and other crops and the liquidation of insects. So we have to look for a safer way of living together with other animals.
Loopholes
This implies, above all, more attention for a more nature- and environmentally friendly human existence. A study in Proceedings of the National Academy of Sciences predicted that the probability of a pandemic will multiply in the coming decades if we do not curb our destructive expansion in other biotopes. A study in Nature decided that as a result of global warming, the biotopes and behavior of other animals could change so much that the chance of virus transmission to humans will significantly increase.
However, the urgency does not penetrate equally quickly everywhere. In an 86-page report on the coronavirus pandemic from the World Health Organization (WHO) in May 2020, the term “wild animals” appeared barely twice and the term “deforestation” once, as if they were irrelevant. recently reported Science it is that the Chinese authorities are beginning to realize that there is a problem with their game markets. On May 1, China will ban the capture and consumption of wild animals, such as raccoon dogs that may have served as intermediate hosts for the coronavirus. That should be the first step towards a solution.
Unfortunately, here too the terrible specter of regulatory loopholes has surfaced. Because the Chinese policymakers did not dare to go so far as to also prohibit the cultivation of their fur, for example from raccoon dogs. Which implies that intense contact between humans and the unfortunate animals will continue. It also offers poachers opportunities to ‘launder’ their wild-caught animals through a fake passage in a fur farm. It is a pity that the seriousness of the situation is so difficult to grasp for the leaders who have to organize our society. It is no different here with us, with our lamentable coexistence with the meager remnants of our nature.•
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