The 17th of April marks International Bat Appreciation Day. This is because bats begin to emerge from hibernation at this time of year. Despite all the bad press bats get, especially in light of the coronavirus outbreak, they actually play an important roll in nature.

Bats are insectivorous creatures, and reduce the number of many annoying insects. Did you know a bat can eat up to 1000 mosquitoes in an hour?!

Bats are fascinating mammals. Unique in several ways, including their ability to fly, they also have a harmonious relationship with many viruses that can be devastating to humans. 

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Viruses such as Ebola virus, Coronaviruses (e.g. SARS), and Hendra virus are examples of Zoonotic viruses that are found in bats. Zoonotic is the term for viruses that jump species barriers or “spill over”, i.e. from bats to humans. These viruses often cause no physical symptoms in bats, and due to their lifestyle of roosting in large colonies, can spread easily through large populations. 

Wynee and Wang (2013) from CSIRO Australian Animal Health Laboratory, Geelong, Australia, released an open-access article in PNAS asking whether bats are friends or foe. They remind us that bats are just as diverse as the viruses that infect them. The picture below, also found in their article shows different species of bats, and electron microscope images of the viruses that can infect them (1).

“Bats are diverse, as are the viruses that infect them.

The Chinese horseshoe bat (ARhinolophus sinicus) is one of many Rhinolophus sp. that are a natural host of SARS-like coronaviruses (B; scale bar 100 nm). The spectacled flying fox (CPteropus conspicillatus) along with other Pteropus sp. are reservoirs for the Australian Bat lyssavirus (D; scale bar 100 nm). A number of African fruit bats including Hypsignathus monstrosus (E) have been found to host Ebola virus (F; Ebola Reston, scale bar 200 nm). The Malayan flying fox (GPteropus vampyrus) is the natural host of Nipah virus (H; scale bar 200 nm). All four pteropid Australian bat species including Pteropus alecto (I) have been found to carry Hendra virus (J; scale bar 200 nm).”

https://doi.org/10.1371/journal.ppat.1003651.g001

The reasons for why bats can tolerate these viruses are not fully understood, but there are some characterised differences in their immune system response that are thought to account for this (2)

One relatively well supported hypothesis for the underlying reason is due to their flight, which puts a large amount of stress on the body. Evidence suggests that they have managed to deal with this stress by the evolution of an altered immune response, which then allows the bats to control viral replication whilst minimising any counter-productive over response (3)

All of this is great for bats, but becomes a problem for us when these viruses jump into the human population. Our immune systems respond differently to bats, and this results in the diseases we see. So, what can we do?  

To some, the obvious response may be to exterminate wild bat populations. Even without considering the obvious moral objections to this, it would also be counter-productive for many reasons. Bats are incredibly important to ecosystems, playing crucial roles in pollination, insect control, and seed dispersal (4)

In order to reduce the chances of spill over events, it is important to look at the ways that human activity brings us into more frequent contact with bats, such as deforestation and the possibility of bats passing diseases to us via animals in our food supply chains. We can then find ways to minimise these.  

In the meantime, researchers can learn a lot from the differences between the immune system response of humans and bats, in order to identify ways in which ours is not effective in tackling viruses. (2)

We should appreciate these wonderful mammals, understand the ways they can be dangerous, and learn to live alongside them whilst minimising contact. It will suit both us and them!  

https://i.pinimg.com/originals/b2/e3/98/b2e3989557c230febf0b46c69c0969ab.jpg

References: 

1: Bats and Viruses: Friend or Foe? – Wynne & Wang, 2013

2: Innate Immune Responses of Bat and Human Cells to Filoviruses: Commonalities and Distinctions – Kuzmin et al, 2017 

3: Novel Insights Into Immune Systems of Bats – Banerjee et al, 2020 

4: Education to Action: Improving Public Perception of Bats – Hoffmaster et al, 2016 

Written by Ben Jones