Could bats hold the secret to viral immunity?
Written By Luca Miler
Bats have been the facilitators of great suffering in our species for many years due to being the second most significant reservoir hosts, surpassed only by rats. The fact that bats can handle a much greater viral load for many diseases means that viruses can much more freely mutate in bat colonies and so are more likely to become transmissible to other species. In fact, bats have rarely been recorded dying from any virus other than from lyssaviruses, according to the Bat Conservation Trust, showing how effective their immune systems are. Many recent pandemics have been traced back to bat species, including Ebola epidemics and the COVID-19 pandemic. The importance of researching interspecies transmission was highlighted in a 2017 study by Ben Hu, which identified virus strains in bats within the Yunnan province of China that bore the same genetic building blocks as the strain which caused the 2002 outbreak of SARS-CoV. Thus, it was suggested that bat populations like this one could cause another pandemic, an unfortunate reality which did come to fruition. This scenario could well happen again as increased urbanisation causes bats to nest in closer proximity to humans, increasing the likelihood of transmission.
Conversely, the same attributes that lead to bats being such dangerous reservoir species could illuminate methods for handling great quantities of viruses in humans and revolutionise treatment for viral diseases. The trouble is that the mechanism which allows bats to be so unaffected by most viruses is still not well understood. There are a variety of hypotheses, the majority relating to the most prevalent difference between bats and every other mammal – flight.
Dr Thomas J O’Shea suggests that it may be the act of flying itself which allows tolerance to viruses as a bat’s basal metabolic rate (BMR) rises by fifteen times during flight, whereas humans only raise their BMR by two times and rodents by seven times respectively when sprinting. This results in bat body temperatures being raised to fever-like temperatures, 38–41 degrees or higher on a regular basis, which could increase immune system function while inhibiting viruses. However, bats only fly for short periods of time and so it is unknown whether this fever effect would be sustained enough to have a significant impact. Additionally, many bat species hibernate and so during these periods their BMR is constant and low, but there is not a significant increase in disease or morbidity during these periods.
The bat immune system has not been fully researched but Sonu Subhudi summarises the hitherto collected literature to conclude that bats have a far greater basal interferon expression, suggesting that bats are capable of constantly suppressing viral replication. Interferon-stimulated genes are also expressed at higher levels and are more sensitively activated than in humans. The bat immune system may also have adapted better to viruses as a form of co-existence rather than eradication. Bats have been shown to be able to transmit viruses to other bats months after being infected and after the viral load is no longer detectable. This suggests that viruses live in the organs of bats without causing pathology in an extended latent phase, although this does mean that viral shedding can happen once a bat experiences significant external stress. While these factors certainly contribute to the impressive immune response of bats, they do not appear sufficient to fully explain the near complete lack of fatal infections.
Dr Cara Brook suggests that mammalian flight is such a great stress on bats that it has effects equivalent to ageing. In order to handle regular flight therefore, a selection pressure to have anti-ageing properties evolved which could have had a side effect of offering anti-viral capabilities. This could explain why, in proportion to their body weight, bats have the longest lifespan of any mammal. Bats exhibit a constant inflammatory suppressant response, particularly against tumour necrosis factor alpha, with inflammation being heavily linked to ageing and the prevalence of many chronic diseases. This may explain why bats can handle a persistent viral load without it impacting their lifespans. An evolutionary pressure which may have caused this is an increased prevalence of oxygen free radicals during flight (due to the massively increased BMR) which can cause DNA damage. This damage in humans would result in chronic inflammation but does not occur in bats. However, a potential flaw in this theory is that flight is an extremely slow evolving trait. Immune system evolution typically occurs much faster and could have been instigated by the massive size and therefore rate of transmission of bat colonies.
Why bat conservation is vital:
You might conclude, given the zoonotic risk, that bat populations should be controlled or exterminated; however, that does not appear to be a sensible course of action for multiple reasons. Firstly and most practically, it is not feasible as bats can reproduce in many environments, being found on every continent other than Antarctica, and can reproduce in urban environments as well. One bat population in Uganda was targeted for extermination but this only served to cause a Marburgvirus resurgence in the local human population, showing the risk of employing such a tactic. Bats are also major predators of many insects and serve to protect crops, with some species also eating mosquitoes, which controls the risk of transmission of malaria in endemic areas. Bats should be conserved and allowed to thrive in healthy environments as shedding occurs at a much higher rate when a bat population is impaired(for example, from a lack of suitable habitat or raising temperatures), so keeping bats healthy benefits humans. Finally, in terms of doing further research on bat virology or anti-ageing properties, having a healthy, abundant and accessible population to study is vital.
So, although bats have the potential to facilitate a global pandemic, their unique and fascinating biology could be the key to understanding how to tolerate viruses or even develop our understanding of anti-ageing compounds. The safest way to combat this threat is through co-existence and understanding, rather than radical and risky measures.
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