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For the longest time, bats as an animal have been getting a lot of attention in terms of harboring some of the deadliest viruses on the planet. Even the recent COVID-19 pandemic has seen evidence of this despite many reports indicating that might not be the complete picture. So, why is this so? Are they actually the only animal responsible for the many human infections that we see due to spillover? What do the stats say and how do they compare to other species of animals? Let’s find out!

Let’s begin with the biology of bats

These ‘flying foxes’ that we know as bats belong to the Chiroptera order and there are over 6400 species that can be found all over the world. What makes them unique is that barring polar regions, they can be found anywhere and in those regions, they are keystone species. What are keystone species, you ask? Well, they’re species that are dependable species that are involved in activities such as pollination, control of insect population, fertilization, and seed dispersal. They’re one of the most biologically flexible mammals out there because they are used to varying diets, have the ability to cope with different seasons, and can even prolong their pregnancy if they’d like to.

From the perspective of bats harboring viruses within their bodies, it has been found that evolutionary it’s been a LONG ride. There is evidence dating back to over 50 million years ago where multiple bat species were evolving as the global temperatures rose to give them a better-suited climate. What is interesting is that with them, even multiple viral species were co-evolving with them making this is a long-standing association! Looking back at this, it is not surprising that bats can harbor so many different viral species because it’s given them ideal conditions and intricate cellular and molecular pathways to do so.

What are the drivers impacting bats as a viral host? (Adapted from Streicker & Gilbert, 2020)

Considering all these factors plus there are additional factors that contribute as well. For example, their comparatively long life spans give longer times for the harboring, collection, and evolution of viral strains. Their ability to live in packed enclosures eventually allows the viruses to pass between different bat species. Also, their ability to fly long distances means that they can contract different viral infections over time and also pass them over. Immunologically, it seems that they have something else going on.

Understudied immune system

Since bats are one of the oldest mammal species, it means that they have some of the most primitive immunological features which are much different from other mammals. This also means that they are able to mount certain responses that are extremely different from the way our bodies do. There is speculation about the fact that they are able to fight viral infections well but their immune systems are not able to clear these infections from the body making them a long-term host. They can mount similar responses to these infections even after two times of being infected, unlike what we see in humans, especially in the case of COVID-19. But as bats are not an extensively studied mammal, it becomes very difficult to actually understand the mechanisms behind them.

That being said, recent studies that have focused on bats entirely have found a very interesting mechanism in place to understand how viral infections just persist for so long. Let’s look at the immune dampening mechanism below.

How does immune dampening work in bats?

In a study done by researchers at Wuhan Institute of Virology during the 2003 SARS outbreak, they found out that there the secret for high tolerance lies in their genes. The study found that there were certain genes that were positively selected over time and are conserved in multiple species and it dampens an interferon pathway which basically prevents the system from going into overdrive. So, what happens is that they are able to fight the infection and tolerate the presence of viruses in their bodies. Normally, in our bodies when we’re infected with a virus, our bodies go into an immune overdrive, and as a result, we feel feverish, worn down and there can be severe consequences as well. But this just doesn’t happen in bats, they can just tolerate it, or rather we could think of it as the virus choosing a perfect host without killing them.

Comparison of the immune activation process in humans/ mouse & bats (Source: Irving, Ahn, Goh, Anderson & Wang, 2021)

The entire immune dampening mechanism is quite complex and intricate with multiple components that work in conjunction to maintain the balance of survival in them. Some of these include the reduction of proteins such as caspase-1 that can induce key inflammation cascades in the body. Another involves the suppression of the maturation of cytokines that are inflammatory in nature. The idea is that the body needs to fight the invaders but not to an extent that it kills itself and that is exactly what is happening to bats.

But are they the best viral hosts out there?

Short answer, no. It’s mainly a numbers game because while bats do have the necessary mechanisms to propagate these viruses, we’ve just not studied other possibilities enough. For example, in a study done at the University of Glasgow, it was found that it simply depends on which species a study focuses on. When they studied birds and other mammals such as rodents, it was found that they also harbored a comparable number of viruses that eventually passed onto humans. So, can we really say that bats are the only culprits we need to look out for? And also, are we only looking at pandemic-related scenarios or much smaller but highly impactful scenarios such as epidemics?

Another aspect that they looked into was viral factors and they found that more than the host, it depends on how the virus replicates and whether there are intermediary hosts as well. When we’re looking at a spillover event, it’s more useful to see the virus’s compatibility with humans rather than the animal it spilled over from as that would help us fight it better. Not all viruses that infect bats, tend to infect us. It is much easier to monitor and surveil interfaces where humans and animals meet such as in the case of live markets and in those scenarios, more often than not, you will find other contributing animal species as well!

References

Duke-NUS Medical School. (2020, October 26). Why bats excel as viral reservoirs without getting sick. ScienceDaily. Retrieved May 8, 2021 from https://www.sciencedaily.com/releases/2020/10/201026154002.htm

Streicker, D. G., & Gilbert, A. T. (2020). Contextualizing bats as viral reservoirs. Science370(6513), 172-173.

Calisher, C. H., Childs, J. E., Field, H. E., Holmes, K. V., & Schountz, T. (2006). Bats: important reservoir hosts of emerging viruses. Clinical microbiology reviews19(3), 531-545.

Irving, A. T., Ahn, M., Goh, G., Anderson, D. E., & Wang, L. F. (2021). Lessons from the host defences of bats, a unique viral reservoir. Nature589(7842), 363-370.

Zimmer, K. (2018, June 1). Why Bats Make Such Good Viral Hosts. Retrieved May 8, 2021 from https://www.the-scientist.com/notebook/why-bats-make-such-good-viral-hosts-64251

Watson, C. (2020, April 12). Bats are a key source of human viruses — but they’re not special. Retrieved May 8, 2021 from https://www.nature.com/articles/d41586-020-01096-z

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