How do bats carry diseases?

Dangerous pathogens : Prince of Viruses

In November 2002, a new virus suddenly appeared in southern China. Patients suffered from high fever, cough, and aching limbs. One in ten died. Within a few months, the pathogen spread to more than 30 countries. The World Health Organization named the disease Sars (Severe acute respiratory syndrome). In order to curb their spread, travelers at airports were examined. Hospitals where Sars patients were lying were cordoned off. Parents could not visit their terminally ill children, women could not visit their dying husbands.

Then the virus disappeared again.

What remained were the relatives of almost 800 people who had been killed by the pathogen, many of them doctors and nurses - and the question of where this virus had come from, which had struck the world as if it wanted to show mankind their impotence.

On Thursday, around ten years after the Sars epidemic, researchers reported in the journal "Nature" that they had found closely related viruses - in bats in a cave in southern China. The animals have long been suspected of being a reservoir for Sars. As early as 2005, researchers found similar viruses in horseshoe bats, but the pathogens lacked an important gene segment without which they could not infect human cells. This time, the researchers succeeded in isolating a live virus and using it to infect human cells in the laboratory. "This shows that there are now bats in China that carry a virus that can directly infect humans and trigger a new Sars epidemic," says Peter Daszak, one of the researchers and president of the non-governmental organization EcoHealth Alliance.

Bats is the generic term for bats and the larger fruit bats. They are fascinating creatures, the only mammals to have mastered flying (see box). In public it is mainly the leathery wings, the night activity and the fact that a few (three) species feed exclusively on blood that have sparked the imagination and spawned characters like Batman or Dracula. But for many researchers, bats are more like princes of viruses than princes of darkness.

Scientists have known for over 60 years that bats can carry the rabies virus. But it stayed that way for a long time. Then in 1994 in Hendra, on the east coast of Australia, 13 horses and one of their trainers died within a few days. The causative virus has been traced to fruit bats. When examining the animals, the researchers discovered another deadly pathogen, the Lyssavirus. Another virus jumped from bats to pigs and then to humans in Malaysia in 1998, killing more than 100 people. Researchers now assume that Ebola and the Marburg virus also come from bats, as do numerous lesser-known but dangerous diseases with names such as Kasokero, Duvenhage or Menangle.

So do bats have special properties that make them fly virus throwers? This is hotly debated at international conferences. Researchers like Daszak point out that about one in five of the 6000 known mammal species belongs to the order of the bats. Given this number, it is not surprising that many emerging diseases come from animals. In addition, bats live on every continent except Antarctica and some species can travel hundreds of kilometers. It is therefore possible that they come into contact with more different viruses than other animals.

In some species, thousands or millions of animals join together to form huge colonies. “This is usually only done by humans among mammals and they have been doing it for a much shorter time,” says Christian Drosten, virologist at the University of Bonn.

A particularly dangerous type of virus may have developed in the big flying cities. Many viruses such as herpes or HIV lie dormant in their host for decades and often cause only mild symptoms. Other viruses, such as measles or flu, cause serious illness immediately. If the immune system defeats them after a few weeks or months, the host is protected from the pathogen for the rest of its life. Such viruses have to jump quickly from one host to the next, what researchers call “hit and run”. However, the strategy only makes sense if there is a constant supply of new individuals who do not yet have immune protection, that is, enough newborns. This has only been the case with humans since they first built large cities a few thousand years ago. Bats, on the other hand, have lived in huge groups with many offspring for millions of years. Such viruses could therefore have developed in them. "And these are precisely the pathogens that we are afraid of jumping to humans and then sweeping through humanity," says Drosten.

But Linfa Wang, who heads a team of bat researchers at the Australian Animal Health Laboratory in Geelong, is not enough to explain this. “We believe something very unusual is going on in bats,” he says. Wang suggests that the answer may lie in the animals' immune system. His hypothesis: During the flight the mammals consume a lot of energy and produce waste materials that can damage the genetic make-up. To compensate for this, they developed a particularly efficient system of DNA repair. In return, evolution neglected the immune system, which uses some of the same molecules as the DNA repair system. Wang believes that this is why the bats are particularly susceptible to viruses.

He found the first evidence of this in the animal genome. At the beginning of the year, Wang and other researchers published the first genetic sequences of a flying fox and a bat in the journal “Science”. Both animals lacked an entire segment of genes that help the mammalian immune system recognize pathogens and act against them.

Even if bats harbor more dangerous viruses than other animals, that does not mean that they also infect humans. Many bats in the United States carry the rabies virus, says Michael Osterholm, an infectious disease expert at the University of Minnesota. "If that were enough for the transmission, we would all have to die of rabies in the US." In fact, there are only one or two cases a year, says Osterholm. In order to become infected, a person has to be bitten by a bat and that rarely happens.

Sars also probably did not reach humans directly from bats. There are many indications that crawling cats were initially infected and that the virus was then transmitted from them to humans.

In some cases, blood-sucking insects that feast on humans and bats could act as a kind of virus bridge between humans and bats, says parasite researcher Sven Klimpel from the University of Frankfurt am Main. So far, however, this has hardly been investigated. Another possible way of transmission: Many bats feed on fruit, they chew on the fruit to draw out nutrients and then spit them out half-digested. Pigs and other animals that eat the fruit from the ground could become infected and then pass the disease on to humans.

But there could also be a direct route. In China in particular, many bats are eaten, says Daszak. “People should stop hunting and eating bats,” he warns.

It is unclear whether the Sars-like coronavirus that Daszak and his colleagues discovered in China poses an imminent threat. Just because the virus can infect human cells in the laboratory does not have to be the same in nature, says Drosten. Further experiments are necessary for this.

Nevertheless, the scientists agree that it makes sense to search for new viruses in bats and fruit bats. If researchers knew all the dangerous viruses that the animals harbor, they could pinpoint the origin of an epidemic more quickly and prepare vaccines or drugs against particularly dangerous viruses, says Daszak. "That would be the beginning of the end of global epidemics."

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