Why bats do not catch COVID
How do bats stay safe from the viruses they harbour?
Bats have been implicated in several outbreaks of viral diseases that come to humans from animal sources. And the current pandemic of COVID-19, caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), has notched up 96 million persons affected
and 2 million dead.
Aaron T. Irving, Matae Ahn, Geraldine Goh, Danielle E. Anderson and Lin-Fa Wang, from Duke-NUS Medical School and Duke-NUS Global Health Institute, Singapore, and the Zhejiang University, China, in a paper in the journal, Nature, examine the immune system
and defences of the bat, which enable the animal to play host to viruses, but steer clear of being affected by them. “Lessons from the effective host defence of bats would help us to better understand viral evolution and to better predict, prevent and control
future viral spillovers,” the authors say.
A reason for the ‘spillover’ of pathogens from animal hosts, which have adapted to the pathogen, to humans, who are susceptible, is that human disturbance of an ecosystem raises the fraction of animals that are host to pathogens, over those that are not.
Why this happens is that smaller and shorter-lived animals are more likely to survive and stay on when humans encroach an environment. These smaller animals, with lesser body weight, cannot afford the large investment of energy that immunity requires. Hence,
they do not throw out several pathogens, as do the larger animals. The capacity to tolerate foreign bodies in the bloodstream without severe immune reaction being provoked enables them to adapt to changes and remain in the new areas that humans inhabit.
Smaller animals are also short-lived. This has been related to the higher resting-metabolic-rate in small animals. The higher rate of metabolism leaves the animals with less energy to support immunity, and hence makes them, in their short life-spans, host
to more pathogens than larger animals.
The bat, the paper points out –in respect of the life-span, is an exception. For its diminutive size, bats are rarely larger that common rats, it has a life-span associated with a much larger animal. The result is that the bat is both host to many viruses
and stays around for a long time. The paper points out that as the only mammal that is capable of flight, the advantages of this capacity come at a high metabolic cost, a feature associated with smaller animals. The life-span, however, is substantially greater
than non-flying mammals of the same size. “When adjusted for body size, only 19 species of mammals are longer-lived than humans: 18 of these species are bats (the other is the naked mole-rat),” the paper says. “As a mammalian model of anti-ageing, bats may
offer vital clues in human attempts to delay mortality and enhance longevity,” the paper says.
The paper says that apart from possible mechanisms to regulate the body responses to infection, the species richness of bat communities may be a reason for the ability to harbour a variety of viruses. This idea flows out of research that shows that the
number of pathogens an animal order can host increases with the richness of species within that order. In the case of bats, this richness is unprecedented - out of the 6,400 species of mammals, 1,423 species, are species of bats. And they are widely distributed,
being found in all parts of the earth, except the poles, extreme desert climes and a few remote islands.
While bats have long been associated with infectious diseases, the discovery of SARS-related coronaviruses in bats, the paper says, has led to identifying bats as the richest source of genetically diverse coronaviruses. The paper adds that apart from several
coronavirus diseases in humans, bats can affect other animals, like pigs and horses. There is even the case of a reversal of infection – from humans to domestic pets or zoo animals. And where civets or pangolins were suspected as sources of SARS or SARS-CoV-2,
this is seen not to be the case, as these animals take ill when infected, and could not be the reservoirs. Whereas bats show no signs of disease even when infected by most viruses.
How do they do it?
The authors refer to proposed answers based on factors that reduce the viral load in bats. But they discount these suggestions, as bats do carry and tolerate high viral loads. And evidence indicates that what makes bats special might not be their ability
to overcome the virus, but rather their ability to avoid the disease that follows. What is likely, they suggest, is that there is an effective balance between the immune reaction of the body, which would protect against viruses, and moderation of the reaction,
so that it does not harm the body itself. This could also be the reason that bats have long life-spans and low incidence of cancer.
The immune reaction of the body gets activated when a foreign body, like microbes, viruses or certain toxins are detected. When a cell is infected by a virus, the cell releases a protein, called Interferon, as a signal to other cells and to trigger immune
cells to set up defence action. The fever, body pain, that accompanies viral fever in humans is caused by the immune system, which acts on healthy tissue too. And there are many bat-borne viruses, the paper says, which set the innate immune system of humans
on a course of prolonged or strong response, leading to serious disease.
Bats, on the other hand, show no signs of disease even with high levels of virus infection. The paper explains that the recognition of invasion by a virus is by way of stimulation of genes that code for the Interferon protein. This mechanism is called
Stimulation of Interferon genes, or STING, and the paper says that STING-dependent interferon response has been found to be dampened in several bat species.
The paper also cites a recent study, where three of the current authors took part, which found that a protein which senses an indicator of stress, to set off inflammation, is dampened in the case of bats. The result is that whatever the load of viruses,
in bats, there is reduced inflammatory response. Which allows the bat to act as a viral reservoir without showing symptoms of infection.
“Deeper understanding (of the mechanism of robust defence and immune tolerance) will provide insights and strategies not only to aid in the prediction, prevention or control of zoonotic virus spillover from bats to humans, but also to potentially combat
ageing and cancer in humans,” the paper says.
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Note to editor: While the sensible thing is to write COVID, SARS with all caps (like the rest of the world), pls ensure that you do not change STING into lc, as the word, ‘sting’ has meaning and would mislead.