The evolution of COVID-19 and why bat flight is important

  • With COVID-19 spreading at unprecedented rates taking the world by surprise, investigations into its origin began. Before crossing the species barrier to humans, seemingly for the first time in Wuhan, China in 2019, the SARS-CoV-2 coronavirus originated in bats.

    The likes of the global spread of COVID-19 have not been seen since the Spanish Flu pandemic in 1918. In recent years, however, there have been epidemics including Zika virus, Ebola virus, Middle Eastern Respiratory Syndrome (MERS), and Severe Acute Respiratory Syndrome (SARS) which affected millions of people across the world.  

    What do they - and many other tropical diseases - have in common? Bats: all of these diseases spread to humans from the primary host bats. In most cases, intermediate hosts amplify the viral transmission, such as civets in the SARS outbreak and camels in the MERS outbreak. Furthermore, snakes have been rumoured as intermediate hosts for COVID-19, despite the fact that coronaviruses only infect mammals and birds. However, this is refuted by closer genetic analysis which suggests that snakes have not carried COVID-19. Pangolins have also come under fire as COVID-19 spreaders, and very similar amino acid sequences of SARS-CoV-2 and the SARS-CoV-2 spike protein suggest they are most likely intermediate hosts.

    What makes bats so good at transmitting viral diseases in the first place? Bats have very strong immune defences, whilst some species have immune systems constantly primed for mounting anti-viral responses. In these bats, immune defences are quickly deployed to prevent viral entry into cells and protect the bat from infection. This process, however, causes rapid evolution of the virus for quicker reproduction to defeat the immune response of the host, subsequently becoming more efficient pathogens.

    Such intense immune responses can lead to systemic inflammation to allow the immune cells to clear damage to tissues or cells. Yet inflammation can also worsen situations and enhance disease. Bats can survive mounting such an intense immune response as they counteract it by releasing anti-inflammatory molecules, such as interferon-α, to balance systemic effects of immune defence.

    Using the same anti-viral strategy, human immune systems would encounter widespread inflammation and be unable to deal with the systemic stress placed on the body, hence why we succumb to infections that bats seem to avoid.

    With such powerful immune responses, bats begin to act as reservoirs of highly pathogenic and transmissible viruses which they can tolerate without any negative effects. Yet once these viruses spread to other organisms whose immune systems cannot cope, organisms quickly show increased pathology and sometimes fatality. This can be seen with the recently emerged COVID-19 which remains asymptomatic in bats but has a potential fatality rate of around 1% in humans.

    It turns out that bats have even more up their sleeve as they have a uniquely long lifespan for a mammal of up to 40 years. Meanwhile the same sized mammal, such as a rodent, lives for around 2 years. Researchers have discovered that this fact could be associated with bats being the only mammal that can fly.

    Bats have developed physiological mechanisms to deal with the stress induced by flight and elevated metabolic rates. During flight, systemic inflammation occurs due to reactive oxygen species produced in response to the physical activity. Since they developed strong anti-inflammatory responses, including release of interferon-α, bats can dampen the inflammation which comes in useful when managing viral infections.

    With such successful immune responses shutting the virus out of cells, bats maintain infection for slightly longer as the virus remains in their system, thereby allowing the virus time to evolve and improve its method of attack in the host.

    In flight, bats are destroying viruses as their temperature reaches 40°C at which many viruses cannot survive. Evolving to survive at higher temperatures means viruses can survive human fever conditions increasing pathology in a human host. Viruses that can survive in bats have thus evolved mechanisms to tolerate harsh conditions, consequently giving them power to cause much more damage in subsequent hosts.   

    Despite the wide spread of COVID-19, the mortality rate remains lower than SARS at 15%, which also had a much-reduced transmission. This is due to the lower fitness of the SARS coronavirus which had lower capacity to transmit successfully through the human population, hence resulting in its eventual decline. COVID-19 appears to be much better at transmitting through human populations, however it also seems to induce less detrimental symptoms compared to SARS.

    Global research into COVID-19 is working towards vaccines to control the coronavirus, whilst further work is looking into the molecular biology and transmission through hosts to advance vaccine research.

     

    Image by NIAID via Flickr: Novel coronavirus SARS-CoV-2 particles.