If bats transmit so many viruses, why don't we kill them?

IGNACIO LOPEZ-GOÑI Professor of Microbiology, University of Navarra

It has just been news that a group of virologists have been able to isolate and cultivate the Lloviu virus in the laboratory for the first time from blood samples of live bats obtained in caves in Hungary. Although at the moment we do not know what role this virus could play in human health, its finding highlights the importance of virological surveillance in wild animals, especially bats.

But who is the Lloviu virus? In 2011, a new virus of the Filovirus family was identified in samples of dead bats found in the Asturian cave of Lloviu (hence its name). The dangerous Ebola and Marburg viruses belong to that same group of viruses, among the most lethal in primates, including humans. The finding was significant, because both viruses are endemic to Africa and the Philippines, and this was the first time a Filovirus had been detected in natural samples in Europe.

However, at that time, the virus could not be cultured or isolated in the laboratory. Its RNA genome was only detected by molecular methods. There was no data to suggest that it could be pathogenic for humans. In fact, not all Filoviruses are.

A virus that circulates in bats

Since 2016, Lloviu virus genomes had been detected in other bats in different European caves, suggesting that the virus could be circulating in bat populations. Specifically, in the insectivorous bat Miniopterus schreibersii.

In the work published now, it has been possible to isolate the virus, it has been cultivated in cell lines and it has been verified that it is capable of infecting and replicating in human and monkey cells in the laboratory. Therefore, it could have the ability to spread to other species.

In addition, the researchers have verified that there is no cross reaction of antibodies between the Lloviu virus and the Ebola virus. This implies that, probably, the vaccines that have been developed against Ebola would not be useful to prevent a potential infection by Lloviu.

Within the Filoviridae family, there are several genera: Ebola (Zaire, Sudan, Reston, Tai Forest species, Bundibugyo ebolavirus), Marburg (Marburg marburgvirus species), Cuevavirus (Lloviu Cuevavirus species), among others. CDC/ Dr. Erskine Palmer, Russell Regnery, Ph.D., CC BY

Bats, a repository of dangerous viruses

Bats, whose scientific name is bats (Chiroptera), are a type of mammal whose upper extremities developed as wings. They are the only flying mammals, and few know that there are more than 1,400 different species of bats and that they represent approximately 20% of all mammal species. In fact, within mammals they are, after rodents, the largest group. They are present on all continents except the polar regions and the largest deserts.

Some species of bats are a natural reservoir or storehouse for large numbers of pathogenic microbes. Furthermore, as the covid-19 pandemic has shown, they can play an essential role in the transmission of many infectious diseases.

A few years ago, the virome (the set of virus genomes) of the giant bat Pteropus giganteus (the Indian flying fox) was analyzed and 55 different viruses were found. Fifty of them were new, from various families of viruses such as Coronavirus, Paramyxovirus, Hantavirus, Astrovirus, Bocavirus, Adenovirus, Herpesvirus and Polyomavirus.

Other studies have shown that bats are the natural host of many zoonotic viruses that cause infections, some of which are very serious in humans. From the already mentioned Ebola and Marburg filoviruses, to the rabies virus. Also the coronaviruses that cause acute respiratory syndromes such as SARS-CoV1 and CoV2 or MERS. Without forgetting Paramyxoviruses, such as Nipah and Hendra viruses or different types of Influenza A.

Why do bats harbor so many viruses without getting sick?

Despite being a virus storehouse, bats appear to be immune to infection. Why don't bats, carriers of so many different viruses, some so dangerous, infect themselves and die from the action of a massive attack? What is so special about bats that they are immune?

These questions have always intrigued researchers. Some think that there is nothing special about them, that it is just a matter of numbers: there are so many different species of bats and so many individuals that it is not surprising that they have so many viruses. Some bat colonies, for example, can be made up of millions of individuals!

However, there are other researchers who think that bats do have something peculiar. For example, the genome of a couple of bat species has been sequenced and it has been found that, unlike other mammals, the genes of the DNA damage detection and repair system are constitutively active. It is speculated that this could be related to the bats' type of flight, which consumes a lot of energy. This requires a very active metabolism, which generates a lot of stress, which in turn causes damage to the cells' DNA, which is quickly detected and repaired. These systems are also often the target used by many viruses, so having them so active has been able to make bats immune and capable of carrying viruses without suffering the consequences.

Another hypothesis suggests that the flight of bats generates such an active metabolism that it can also produce an increase in temperature similar to fever. The body temperature of bats during flight can reach 40ºC. In most mammals, fever is related to the stimulation and activation of the immune system and helps fight infection. Therefore, by increasing their body temperature, bats could be able to control their viruses better than anyone else.

What do we do with the bats

Now that we know that bats were the most likely origin of the ancestor of the coronavirus that has caused covid-19, and given the potential for new infections that these small mammals pose, one might think that it would be best to kill them all. But beware, because bats play a fundamental ecological role and are also beneficial to humans.

There are more than 100 different diseases in humans caused by microorganisms transmitted by arthropods (mosquitoes, flies and other insects). This is the case of malaria, yellow fever, dengue, Zika, chikungunya, West Nile, encephalitis and other hemorrhagic fevers.

Insectivorous bats help keep their populations at bay. Some have estimated that these types of bats can eat up to 1,200 mosquitoes per hour, the equivalent of their body weight each night. Therefore, bats act as biological control agents, reducing or limiting the growth of populations of insects or other arthropods that transmit diseases.

Moreover, they play a vital ecological role in seed dispersal and as pollinators. Many bats are also frugivorous (they feed on fruit), which they later excrete elsewhere, and contribute to the dispersal of their seeds and the regeneration of forests. In addition, by also feeding on nectar, they act as very active pollinators. The biological cycle of many tropical plants depends entirely on bats.

For all these reasons, there are international agreements for the conservation of bats and, in addition, in many European countries they are protected by law.

In short, the solution is not to kill all the bats. If we want to prevent new threats, we must know what viruses or other pathogens are around us, and how the alteration of ecosystems could affect their biology. The solution lies in virological surveillance and by insisting on the One Health or Global Health strategy: our well-being depends directly on animal health and the environment.

This article has been published in The Conversation

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