Bengaluru/New Delhi: Bats are one of the biggest reservoirs of viruses, many of which can cause severe human diseases. Because of close contact between bats and humans, a high possibility exists for transmission of bat-borne viruses to people.
Bats are the most likely reservoir of the new coronavirus, SARS-CoV-2, as it had been in the case of other novel viruses that caused diseases such as the severe acute respiratory syndrome (SARS) and the Middle East respiratory syndrome (MERS).
So, why do bats, living close to human settlements, carry so many viruses that can cause diseases in people?
ThePrint takes a look at the effects of habitat loss, the kinds of viruses that bats harbour, and the questions on whether there will be more pandemics caused by coronaviruses in the future.
Furry flying mammals
There are over 1,000 species of bats, which make up over one-fifth of all mammal species on Earth. Bats are critical for the survival of several ecosystems — in dispersing seeds, pollinating rare flowers or eating large quantities of insects.
These mammals are a key species that help promote biodiversity in varied ecosystems, such as deserts and swampland. They also act as pollinators for many commercial crops such as mangoes and bananas.
Bats can eat its own body weight in insects in one night, keeping crops safe. These mammals are found on all continents of the world, except in Antarctica.
Bats have mostly been portrayed negatively in literature and entertainment, associating these mammals with darkness and death. This, in turn, has led to stigma and a fear of bats in people’s consciousness.
Since they are a huge reservoir of viruses, the species is also plagued by its own group of diseases. The white-nose syndrome is one of the diseases that kills bats in large numbers.
Bats also come under attack by cats, chemical residues, emissions and other effects of urbanisation. Their habitats are being threatened due to developmental activities, including construction of roads and turbines.
Habitat loss has also led to large-scale economic impact and structural damages to forest ecosystems. It has forced bats to inhabit cities and other places with dense human population, increasing the risks of transmitting novel viruses.
Along with rats, bats are also considered as having the ability to transmit more deadly viruses to humans in the future.
These viruses are usually transmitted by an intermediate animal: civet cats during the SARS outbreak, dromedary camels in case of MERS, horses passed on the hendra virus, and pangolins are thought to probably have transmitted Covid-19 to humans via a wet market in Wuhan.
Kinds of viruses in bats
While SARS and MERS are coronaviruses, ebola and marburg are filoviruses. Hendra and nipah are henipaviruses. There are other kinds of viruses that bats harbour such as rhinoviruses, lyssaviruses, and adenoviruses.
Emerging zoonotic (transferred from animals) diseases — especially those related to transmission of coronaviruses — in bat reservoirs has also been the plot of many science-fiction movies, such as the 2011 drama Contagion, as research into them is well established.
One of the main reasons for the spread of deadly viruses is the fact that bats are mammals, like humans. Both human and bat bodies are “sufficiently related” for the viruses to thrive.
Bats also live close to humans and share food. The mammals often eat fruits from orchards, and vampire bats, often harmless to humans, can suck small quantities of blood a day from domesticated animals such as cows and goats.
These mammals are active in the densest of cities and the sparsest of desserts, feasting on insects, fruits, and blood of large animals.
They often live in large clusters and one infected mammal can lead to a colony of infected bats, which could then transmit viruses via another animal or directly to humans.
Covid-19 manifests differently in bats, just like many other bat-origin viruses. The mammals infected with the deadly virus don’t seem to display any symptoms associated with the disease.
Scientists have found that bats are able to dampen inflammation caused by viruses. Bats have a physiological mechanism that helps them combat inflammation from flying. This mechanism is thought to also protect the bat against the disease.
Diseases have also been caused due to transmission of viruses from ducks (influenza) and pigs (swine flu). Small pox, measles, tuberculosis, etc. were caused due to inter-species transmission from animals to humans.
A 2017 study by researchers at the Tata Institute of Fundamental Research’s National Centre for Biological Sciences showed that viruses may regularly jump from humans to bats and vice versa, without even causing an epidemic.
In this study, conducted on members of a tribe from Nagaland, the team looked at the viral transmission between bats and humans in areas where they come into close contact with people.
Researchers found ebola antibodies in some of the human subjects despite the fact that the disease, which has killed thousands in Africa over the years, has never been reported in the area.
The ebola virus is classified as filoviruses, which like the coronavirus is a zoonotic pathogen. Unlike coronaviruses, which cause respiratory diseases, filoviruses are known to cause haemorrhagic fever in human beings.
The study, published in the PLOS journal, showed that antibodies reactive to two distinct filoviruses (including ebola) were detected in human serum samples, and to three individual filoviruses in bats in Northeast India, where local tribes engaged in annual bat hunting.
“In the Northeast Indian state of Nagaland, local ethnic groups have conducted bat harvests for at least seven generations as a source of food and traditional medicine. These bat hunters are exposed to saliva, blood, and excreta from the bat species Rousettus leschenaultii and Eonycteris spelaea,” the research said.
The study also warned that bats in South Asia act as a reservoir host of filoviruses, which often jump not only from bats to humans, but also vice versa.
Previous studies of coronaviruses in bats have found up to 73 viruses in just over 1,000 bats. Many of these viruses are novel coronaviruses which humans never have seen before, and they are constantly evolving.
Studies have also shown that numerous bat species naturally reside in trees, buildings, and caves that can be in proximity to areas inhabited by humans.
Apart from coronaviruses and filoviruses, jenipavirus — such as nipah — can also be transmitted from bats to humans as a result of repeated spillover events. This puts populations in India and Bangladesh at risk of regular zoonotic infections.
The fact that epidemics caused by coronaviruses are likely to recur has been known for a long time. A 2007 research had predicted that the presence of a large reservoir of SARS-CoV-like viruses in horseshoe bats, together with the culture of eating exotic mammals in southern China, was like “a time bomb“.
The study, which explored the biology of the SARS-CoV that caused an epidemic in 2003, had warned the possibility of the re-emergence of SARS and other novel viruses from animals or laboratories.
Coronaviruses are known to constantly evolve, making it difficult to predict the timings of new outbreaks. But there is a clear scientific consensus about the possible emergence of new diseases from bats through direct or indirect human contact.
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