A study published in 2015 in the Nature journal had warned that severe acute respiratory syndrome (SARS) like viruses circulating in Chinese bat populations may pose a serious future threat.
The study was co-authored by Shi Zhengli, virologist at the Wuhan Institute of Virology who last month published a study identifying Covid-19 to be originating from bats. The February 3 study published in Nature journal concluded that 2019-Ncov which is causing widespread pneumonia-like illness in China is 96% identical at the genome level to a bat coronavirus.
The 2015 study had focused on SHC014-CoV, another coronavirus strain which was found to be circulating among Chinese horseshoe bat populations but the study highlighted that recent metagenomic (study of genetic material) studies have identified sequences closely related to the SARS like viruses in bats which could spill over leading to outbreaks.
Many scientists and virologists working on animal to human transmission of viruses saw the Covid 19 pandemic coming.
Peter Daszak for example, a disease ecologist and President of EcoHealth Alliance, a US based research organisation wrote in The New York Times that in a World Health Organisation (WHO) meeting in 2018, experts had coined the term “disease X” a virus emerging in animals which would be transmitted to humans and would spread as easily as the flu but with a higher mortality—very similar to the current Covid 19 outbreak.
Daszak and other scientists like David M Morens from the National Institutes of Health wrote in an analysis in the New England Journal of Medicine that with emerging zoonoses we are living dangerously close to pandemics. This is mainly because “in our crowded world of 7.8 billion people, a combination of altered human behaviors, environmental changes, and inadequate global public health mechanisms now easily turn obscure animal viruses into existential human threats.”
But out of the several zoonoses, bats are one of the most important reservoirs of deadly viruses this is because of a combination of factors.
A 2017 study on diversity of coronaviruses in bats from China found a total of 73 coronaviruses in the sample of 1067 bats from 21 species in China. Another assessment of why bat viruses are so deadly conducted by University of California-Berkeley in February after the Covid 19 started spreading suggested that bat cells have a very strong immune response, constantly primed to respond to viruses. High virulence and infectivity wreak havoc when these viruses infect animals with tamer immune systems, like humans, the study published in Science Daily concluded. Bats ability to fly also allows them to spread and pick up pathogens over a vast area.
Several other scientific studies have suggested that land-use change or wild habitat loss are leading to more contact between bats and recipient hosts. “Disrupting bat habitat appears to stress the animals and makes them shed even more virus in their saliva, urine and feces that can infect other animals,” says the Science Daily study. Researchers working on the ground say the interplay between these factors make virus spillover and an out-break situation even more likely.
“The relationship between habitat loss and virus spillover from bats is not that simple. There are many areas where we live in close proximity with bats, but there is no spillover. The exceptional thing about bats is that they have the ability to tolerate high viral load without getting sick themselves. In general though, interfaces between bats and people provide opportunities for spillover. We need to study and better understand which particular viruses spillover and cause outbreaks,” said Uma Ramakrishnan, Associate Professor, Senior Fellow, Wellcome Trust, National Centre for Biological Sciences.
Rohit Chakravarty, a wildlife biologist and Indian Bat Conservation Unit says certain human behaviours make us more vulnerable to Covid-19 like spillovers. “Habitat loss and bushmeat consumption bring humans in closer contact with wildlife. Large commercial wildlife markets that exist in China and Southeast Asia bring humans in contact with live animals that are kept in unsanitary conditions thereby compounding the risk of disease transmission,” he said adding that “habitat loss has an indirect, yet significant role in disease outbreaks.
Habitat loss and increasing human disturbance have been linked to increased stress in forest-dependent wild animals. High stress results in low immunity in those animals which augments their risk of getting infected.”
Chakravarty explained that in India myths cause ill-informed people to kill bats which adds to the stress. “It is true that bats are reservoirs for numerous viruses due to their extraordinary immune system and physiology, but the chances of contracting a disease in natural conditions are extremely low…of course unless one heckles bats needlessly.”
In case of the Nipah outbreak in southeast Asia at various times including the 2018 outbreak in Kerala which killed 17 persons has been linked with loss of tropical forests which stresses Nipah carrying flying foxes–Pteropus bats. Due to loss of tropical forests, they have moved to other habitats according to some analyses.
Despite being reservoirs of dangerous viruses, bats have irreplaceable ecological value because they are pollinators and control pest and mosquito populations in different parts of the world. So, scientists say the strategy to prevent such pandemics is better surveillance in wildlife, reduce interaction and keeping a watch on communities with wildlife interaction for potential spillover events.
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