Contents
What dangerous infections do bats transmit to humans and is it possible to prevent viral expansion from the wild?
In popular culture, bats are traditionally associated with the supernatural forces of evil and are feared by many. But 2020 seems to have dealt a truly devastating blow to their reputation. Not so long ago, it turned out that bats are a natural reservoir for a huge number of dangerous viruses. Among them is SARS-CoV-2, which caused the COVID-19 pandemic. All this was the reason for the complete demonization of bats. Because of the fear of the spread of deadly infections, people in different parts of the world even began to exterminate these animals, endangering some species.
What is the uniqueness of bats
Today there are more than 1,2 thousand bats in the world. This is approximately 20-25% of the total diversity of mammals. Bats are found on all continents of the Earth except Antarctica. However, most of them live in tropical regions. Despite the fact that the population of bats is so huge, they remain one of the most mysterious and little-studied animals. But even the little that scientists know about them today is surprising.
Bats live very long lives. On average, a seven-gram individual can live up to 43 years, which is approximately 1 thousand years of human life. No animal in the world of similar size and metabolic rate lives that long.
Chiropterans are the only winged ones capable of very long and high-speed horizontal flights. Female bats weighing 11-12 grams can reach speeds of up to 160 km / h and more. For example, the maximum horizontal flight speed of a black swift is about 120 km/h.
Despite their longevity and high numbers, bats remain a very difficult object to study. And not only because most species live in hard-to-reach areas. First of all, they are very difficult to keep in captivity. Unlike the countless rodent vivariums around the world, there are very few such establishments for bats. And if fruit-eating species are still relatively easy to feed, then insect-eating species are much more demanding. Accustomed to hunting, they do not perceive dead insects in a plate as prey. Therefore, researchers have to arrange whole performances in order to feed them.
Stephen Goodman, Field Biologist, Department of Zoology, Field Museum of Natural History, Chicago, USA:
“We often use long tongs in which we place small worms and other insects. In the process of feeding, we wave them in front of bats, simulating flight. That’s the only way they agree to eat. Another difficulty is that some bats have distorted spatial perception. Being in cages, they continue to think that they can fly without restrictions – therefore they often crash into the cage and get seriously injured. It is often necessary to use smaller cells, but this greatly limits the activity of animals, and, as a result, makes it difficult to study their metabolism and physiology in general. But that’s not all. The study of each new colony of bats is a completely unique challenge. Because all bats are very different: both genetically and immunologically. For this reason, experience with one species can provide a very limited understanding of another species.”
But perhaps their most striking feature is their unique immune system. Bats are a natural reservoir for a huge number of viruses. Some researchers suggest that the virome of these mammals – that is, the total number of all viruses – is the largest in the animal world and may number in the thousands. However, any modern ideas about the total number of bat viruses are only models and extrapolations; science still does not have exact data.
What viruses are carried and transmitted by bats
The first attempt to count and systematize all bat viruses was made by scientists in 2006. This was done by an international team of five researchers led by virologist Charles Kalisher. To collect all the data on bat viruses known at that time, a group of scientists worked for several months without days off. Of course, today this list has been significantly expanded. But 12 years ago, data on the number of infections carried by bats impressed many scientists. At the same time, bats themselves only occasionally get sick and die from rabies: their immune system surprisingly copes with other infections. Exactly how, until the end, scientists do not know. All assumptions about what features of the immune system allow them to avoid infection with many dangerous viruses lie today only in the field of hypotheses.
Charles Kalisher, Virologist, Professor Emeritus, Colorado State University, USA:
“So far, we know that bats maintain relatively high levels of interferons, signaling molecules that elicit initial immune defenses against infection and are usually only generated after host immune cells detect the virus.
Such features lead to the fact that some viruses that manage to survive in the body of bats begin a new path of evolution: they produce interferon-blocking proteins and become more dangerous. When transmitted to a person, such viruses can significantly overwhelm the immune system before it has time to react.
One of the most common hypotheses explaining the unique features of their immunity is related to the adaptation of the body of bats to long horizontal flights. At such moments, their body temperature can approach 38-42 ° C (depending on the species). And the heart rate can reach up to 1 thousand beats per minute. For example, in humans, such prolonged activity could cause serious stress, however, as in any other organisms. This is probably why bats have a much more efficient and more finely tuned immune system.”
Of all the viruses that bats carry, zoonotic (that is, transmitted to humans) are:
- rabies virus;
- Marburg virus;
- Ebola virus;
- genipavirus (namely Nipah and Hendra virus);
- coronaviruses (SARS-CoV and SARS-CoV-2).
If you can save yourself from rabies by getting vaccinated, then, for example, there is no cure or vaccine for the Hendra and Nipah viruses. They are transmitted from person to person through intermediate hosts: pigs, horses, goats, sheep, dogs – that is, domestic animals. In humans, henipaviruses cause acute respiratory infections, as well as meningitis and severe febrile encephalitis. Mortality is very high: from 30 to 85% of cases.
Charles Calisher:
“The first reported outbreak of the Nipah virus that affected Malaysia occurred in 1999. 105 out of 265 infected people died. The spread of the infection began on pig farms, which were located close to the wooded area where the fruit bats lived. It is not known exactly how the infection occurred, but there are two versions. Either the bats dropped the remains of their food down from the trees, where the pigs walked around and picked it up, or the bats and pigs somehow competed for food. But one way or another, the virus was transmitted orally, that is, through food or water, to all farm pigs. To stop the spread of the deadly infection, hundreds of thousands of these animals had to be slaughtered, because they not only died from the Nipah virus themselves, but also transmitted it to people who cared for the pigs on the farm. Today, Nipah virus outbreaks are not uncommon in Bangladesh. About 70% of infected people usually die – that’s a lot.”
Like other RNA viruses, henipaviruses are characterized by a high level of mutations. If any of the strains of the same Nipah virus finally adapts to humans, this could be the beginning of a new pandemic. With consequences much worse than from COVID-19.
An additional factor that increases the likelihood of such an outbreak, scientists believe continuous urbanization and the expansion of agricultural land. Such activity only encourages humans and bats to closer contacts. And, therefore, to the transmission of dangerous zoonotic infections. Actually, thanks to similar events in 2019, the spread of the new coronavirus SARS-CoV-2 began.
How the SARS-COV-2 virus jumped from bats to humans
Coronaviruses are a huge family of viruses. Some of them have a name, others do not. Some of them are found only in bats, others are found in bats and other mammals. For example, camels. However, different coronaviruses are found in whales, dolphins and even lizards.
So far, scientists know about the three most infectious and deadly coronaviruses for humans. These are SARS, MERS and SARS-CoV-2, which provoked the COVID-19 pandemic. And if the one-humped dromedary camel is considered the host of the MERS virus, then bats are the permanent natural reservoir for both SARS and SARS-CoV-2. Presumably from the horseshoe family.
The group of SARS viruses is extremely diverse. A huge number of genetic variants of these viruses circulate in the population of different bat species, in particular the same horseshoe bats. They constantly mutate within a large population of bats.
Charles Calisher:
“When two different coronaviruses of the virus enter the same cell, they exchange parts of the genome. Thus, new and surprisingly diverse genetic types arise. This giant population of coronaviruses circulating in bats has probably been around for many millions of years. They continuously recycle, lead their lives and their evolutionary history.”
One of the popular hypotheses that explains the unexpectedly high infectiousness of SARS-CoV-2 is this: due to the transition to an intermediate host, the virus began a new round of evolution. Another recombination of genes occurred in its RNA, as a result of which SARS-CoV-2 received a unique ability to penetrate human cells extremely efficiently. According to scientists, a similar chain of events was behind the spread of the SARS-1 virus, which caused an outbreak of SARS in 2002. Then the intermediate host between bats and humans became predatory mammals – Himalayan civets.
In the case of the new coronavirus SARS-CoV-2, which spread in the Chinese city of Wuhan, most likely (scientists are still arguing) pangolin lizards were the intermediate host.
Pangolins live in Africa and Southeast Asia. These are the only mammals on the planet that are completely covered with scales. Pangolins are widely used in oriental medicine: it is believed that the skin scales of these pangolins have a healing effect against many diseases, including cancer.
Pangolin meat is considered a rare delicacy. And this is despite the fact that representatives of all genera of pangolins are listed in the international Red Book. But it is the black market of these animals that is the most active and massive in Asian countries. According to many scientists, the wild market in Wuhan is just illegal trade in pangolins.
In such markets, birds, small mammals, and rodents are sold in one place. Nowhere in the world is there such an intense population interaction between different species in one place. All animals are traditionally sold alive. It is likely that pangolins may have been infected by bats, which are also sold in such markets. As a result of eating infected pangolin meat, people became infected.
But not all scientists support this hypothesis. After all, bats themselves are considered no less a delicacy. Their meat, like pangolin meat, is also cooked and served in restaurants at wild markets. Therefore, this method of transmission of the SARS-CoV-2 virus to humans could well have taken place.
Stephen Goodman:
“People take great pleasure in eating bats in some countries in Southeast Asia. To stop the spread of dangerous infections, you need to stop these animals from entering wild markets, and of course, stop eating them. Today, zoologists observe how people all over the world have become hardened against bats and even began to ruin their colonies. This is a monstrous mistake. Bats are not the kind of animals that can destroy humanity. Bats are extremely useful. They eat a huge number of insects, which, in turn, are carriers of dangerous diseases. As paradoxical as it may sound, bats play a critical role in limiting the spread of disease. In addition, fruit bats eat and disperse the seeds of fruits and other plants, which contributes to the restoration of forests and trees. In a broad sense, bats are an essential component of an ecosystem.”
The problem is human activity: wildlife markets and deforestation
The problem is that wild animal markets are all over Asia, not just China. Today, scientists suspect that absolutely all of them can be giant breeding grounds for viruses, including coronaviruses. By the way, this has already been proved by a group of zoologists who worked in 2013 and 2014 in four provinces of Vietnam. Scientists collected samples of biological material from animals that were sold in wild markets or raised for sale on farms. A total of 2164 samples were collected from some species of field rats, bats and porcupines. As a result, different strains of coronaviruses have been found in all markets studied. Scientists suggest that the spread of viruses occurs through the mixing of animal waste products, or through direct transmission from one species to another.
The COVID-19 pandemic has brought the wildlife trade to public attention, but shutting down all of Asia’s wild markets at once is unlikely to be possible just yet. According to scientists and conservationists, this industry annually brings in about $20 billion. To reduce the risk of future pandemics, Asian governments should decide at the state level to limit the trade and consumption of wild animals that can carry dangerous highly pathogenic viruses.
And yet, according to many scientists, this industry is only part of a big problem. An equally important factor contributing to the spread of zoonotic viruses in the human population is deforestation. As humans reduce biodiversity by expanding agricultural land and building additional infrastructure, they increase the risk of new pandemics.
Many ecologists believe that it is those species that tend to survive and thrive in any conditions (primarily rodents and bats) that are more likely to remain in human-developed territories. This means that the dangerous pathogens that these animals carry will inevitably be transmitted to humans.
To date, scientists cannot predict where, when and due to which zoonotic virus a new outbreak of the disease will occur. And it is impossible to predict in advance how dangerous this disease will be for a person.
More information and news about how business, law and society are “greener” in our Telegram channel. Subscribe.