By Callum Bennett
The U.S. Centre for Disease Control and Prevention estimates that three out of every four emerging infectious diseases in humans first come from animals. Of all animals, it is the unlikely candidate, bats, that contain the highest proportion of mammalian viruses that are likely to infect people. Bats act as natural reservoirs for a suite of viral diseases, including rabies, Ebola and SARS, all diseases with high fatality rates and the potential for epidemic outbreaks.
COVID-19 is potentially another disease to add to this list of viruses originating in bats, with a recent genomic study finding high genetic similarity between COVID-19 and a strain of coronavirus found in horseshoe bats. This suggests, but does not make it certain, that the original host for the COVID-19 virus was bats, although intermediate hosts have been suggested to have also been involved in transmission to humans (pangolins, a small scaly mammal, have been suggested). This raises a few important and interesting questions: is there anything special about bats that leads to the evolution of viruses that are fatal and spread so quickly? How do these viruses spread to humans, and is there anything we can do to prevent this?
New research has aimed to answer some of these questions, with a recent paper published in eLIFE finding that bats have unique immune systems, which work in a way quite different to other mammals. This may lay at the root of what makes bats such a good reservoir species (a species maintaining the presence of a disease in an area that can pass it to other host species). Research has found that bats have hyper-vigilant immune responses to viruses, with very fast and high-level responses to infection and some species permanently switching on immune pathways such as interferon pathways, which in other mammals are only switched on in response to infection.
In other mammals, having such a hyper-vigilant immune response would cause harmful inflammation, but does not appear to do so in bats, due to changes in genes involved in inflammation (with some even being deleted). This is thought to lead to bats supporting viruses as long-term persistent asymptomatic infections, rather than short-term harmful infections. This is due to the immune response in bats preventing viruses from killing cells, therefore allowing bat viruses to instead evolve to spread quickly between cells.
The evolution of viruses which can spread extremely quickly between cells is all well and good for bats, which don’t experience severe symptoms from these diseases (or are sometimes asymptomatic), but when these diseases infect other hosts, such as humans, they can be deadly. This is due to humans not possessing such a hyper-vigilant immune system as bats, allowing the virus to cause cell death as well as still spreading very quickly between cells, which can overwhelm the immune system, leading to fatalities. It is also possible that the quick spreading of these viruses throughout a host allows for quicker spreading of these viruses between hosts, which may explain fast spreading epidemics in diseases such as Ebola and COVID-19.
It therefore seems that the unique immune system of bats makes it more likely that viruses evolving in bats will be virulent and fatal when they transfer to new hosts. It also seems likely that due to the tendency for bats to carry diseases as long-term persistent asymptomatic infections rather than short-term infections, they act as a perfect reservoir to spread and maintain disease since they seem to be less likely to actually die from diseases, but instead maintain their presence in an area.
As always in science, the question now arises of how this information can be practically applied. As has been raised in some recent papers, the nature of bats as good reservoir species for diseases may mean that as humans increasingly encroach on forest environments and human-wildlife contact increases, the transfer of diseases from animals such as bats may be an increasingly important public health problem. It can also be applied to highlighting the importance of good hygiene and food safety practice, an issue that has recently received a lot of attention in the context of Chinese wet markets, since the potential for the generation of new and potentially deadly diseases from wildlife, especially bats, is high, and is paramount to responsible public health.
Image by Heidi Januszewski