The Search for the SARS-CoV-2 Host Species

When SARS-CoV-2, the virus that causes COVID19, emerged in Wuhan, China in December 2019, it didn’t take long for scientists to recognize the disease as a zoonotic virus: a disease that originates in animals before jumping to humans. Like the coronaviruses epidemics SARS and MERS before it, scientists quickly surmised that SARS-CoV-2 first originated in bats,  and most likely horseshoe bats native to China. But how exactly the disease moved from bats to humans has raised only more questions in the scientific community, even as the virus’s status as an international pandemic has made discovering how zoonotic diseases spread all the more important.

According to the Center on Disease Control, six out of ten diseases originated first in animals before spreading to humans. These include well-known animal-to-human transmitted diseases, such as rabies and Lyme disease, as well as other deadly viruses such as the West Nile Virus. For what is known as “emerging” or “novel” coronaviruses – SARS, MERS, and COVID-19 – bats are usually the viruses’ original host. Scientists estimate that possibly hundreds of coronavirus strains exist in bats (including six new strains recently found in Myanmar), likely due to their unique biology derived from being the only flying mammal.

Why Bats?

For bats, a fast heartbeat and sky-high body temperature (up to 108 degrees Fahrenheit/42 Celsius) require advanced cellular repair mechanisms to prevent DNA damage and a stellar immune system to protect the individuals from the effects of the virus. Because of this, bats are more resistant to viruses such as SARS or COVID-19, allowing the virus to move through their populations without wiping out entire colonies. It takes only one of these viruses to mutate in a way that allows it to jump between species, but direct bat to human transmission is nevertheless rare. Instead, an “intermediate host” is usually required before transmission to humans. In the case of SARS civets – a cat-like animal considered a delicacy in some Southeast Asian countries – and in the case of MERS, camels, became the intermediate host, respectfully. Humans who then used products from these animals, such as meat or milk, increased odds of the virus mutating and gaining further exposure to humans.

What About Pangolins?

In the case of SARS-CoV-2, pangolins – also known as “scaly anteaters” and one of the most trafficked animals in the world– are for now the most plausible intermediate host. Prized in parts of China as a delicacy and for their alleged traditional medical properties, pangolins could have received the disease from bats in so-called “Wet Markets”, where often exotic live animals are placed in close proximity to be sold as meat. A common theory holds that close proximity to horseshoe bats in these markets caused the virus to jump into pangolins and then mutate. When they were then consumed, the virus spread to human hosts in Wuhan and, from there, to the world.

While the pangolin link is a strong contender for the virus’s spread to humans, not all scientists have subscribed to it. A recent study found that, of the 41 original cases of the virus, 13 had no link to the Wuhan Wet Market or persons associated with it. Because of this, many scientists believe that the virus originated independently as early as November 2019 in China, and could have spread through the Wuhan Market through person-to-person, rather than animal-to-person, transmission. In this situation, pangolins may not have been the intermediate host at all.

What’s Next?

In the case of SARS and MERS, it took years for scientists to find the true intermediate host between bats and humans, and COVID-19 will likely be the same. It’s important, however, to remember that bats, nor pangolins, are the problem: instead, it’s our relationship to them. As humans move closer to and infringe upon bat habitats, the likelihood of bat-to-human transmission increases. And even if the Wuhan Wet Market proves not to be the source of this strain of coronavirus, similar markets where animals that typically don’t share ecosystems are in close proximity does in fact increases the chance of interspecies transmission, and from there human infection.

As COVID-19 forces us to re-evaluate the spread of diseases in our global world and how to better respond to future virus outbreaks, so too should it cause us to relook at our relationship to the natural world. As humans and wild animals come into closer and closer contact, whether through illegal animal trafficking or through habitat infringement and population growth, the likelihood of more diseases such as COVID will increase. Respecting natural habitats, ending markets such as those in Wuhan, and disrupting animal trafficking networks are just some of the ways we can be more proactive as we ensure another global pandemic like COVID-19 doesn’t happen again.

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article by Devin Windelspecht

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last updated: May 8, 2020

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