People are claiming that snakes are the source of the Wuhan coronavirus (2019-nCov), but that’s based on a flawed understanding of early research by Chinese scientists.
We will explain what those Chinese scientists discovered, and why snakes are not the source or origin of the Wuhan coronavirus.
Wuhan Coronavirus Originated From Snakes?
Every news outlet and pundit who claimed that the Wuhan coronavirus originated from snakes based their reports and opinions on a single paper by Wei Ji et. al. published in the Journal of Medical Virology on 22 January 2020.
Viruses hijacks the host’s cells to manufacture copies of itself. To replicate more efficiently, they can evolve over time to use the codons (RNA / DNA instructions for amino acids) preferred by their host’s cells.
When Wei Ji’s team analysed the Wuhan coronavirus, they discovered that it favoured codons that are most similar to those used by two snakes :
- Bungarus multicinctus (the many-banded krait), and
- Naja antra (the Chinese cobra)
They therefore concluded that “snakes could be the most likely wildlife animal reservoir for the 2019-nCoV“.
Everyone assumed that because it was published in a journal, it was therefore conclusive. However, scientists publish papers like this all the time, so that their findings can be debated, criticised, or verified by other scientists.
The fact of the matter is – their paper is merely suggestive. It is not conclusive. And it was wrong for any publication or person to state unequivocally that the Wuhan coronavirus originated or evolved in snakes.
And here’s why…
Why Snakes Are Unlikely To Be Source For Wuhan Coronavirus
2019-nCov Is A Betacoronavirus
The Wuhan coronavirus (2019-nCoV) has been confirmed to be a Betacoronavirus, a genera of coronaviruses that mainly infects bats, and to a lesser extent – humans, camels and rabbits.
Incidentally, MERS and SARS are also Betacoronaviruses. SARS has been conclusively traced to bats, and scientists believe MERS also originated from bats.
2019-nCoV Is Most Closely Related To SARS
Analysis of the Wuhan coronavirus genome showed that it was most closely related to two SARS-like coronaviruses of bat origin.
And guess who was the culprit for the human SARS virus? Horseshoe bats living in a remote Yunnan cave.
Hence, bats are likely to be the origin of the Wuhan coronavirus, not snakes.
Coronaviruses Infect Only Mammals + Birds
Coronaviruses are a very large family of viruses, but they have so far been able to only infect mammals and birds. Basically, they are “warm-blooded” viruses.
It would be very unlikely for 2019-nCoV to evolve to infect snakes, which are cold-blooded reptiles.
Intermediate Hosts Have So Far Been Mammals
Let’s look at the last two novel coronavirus outbreaks – SARS and MERS, both from the same Betacoronavirus genera as the Wuhan coronavirus.
They spread to humans through intermediate hosts – civet cats for SARS, and camels for MERS. Not from the act of eating them, but from close contact.
This is further evidence that the intermediate host for 2019-nCov is much more likely to be a mammal, rather than a reptile.
Codon Adaptation Does Not Always Happen
While scientists use codon adaptation as a way to identify hosts and trace the evolutionary history of a virus, it does not always happen.
Codon adaptation happen by genetic mutation – by happenstance – and multiple codons code for the same amino acids.
So viruses can evolve codon preferences that are different from the host, and yet efficiently replicate in its cells.
Some viruses also “choose” not to adopt the host cell codon preferences, favouring a slower replication rate.
This phenomenon, known as survival-of-the-flattest, is an adaptation strategy that presumably helps to avoid triggering the immune system or killing the host too quickly.
So it is possible that the Wuhan coronavirus did not evolve to favour the codon preferences of its intermediate host.
Codon Adaptation Takes Time
Even if codon adaption did happen, it takes a long time for viruses to evolve to adapt to the host’s codon preferences.
It’s very unlikely for the Wuhan coronavirus to adapt so quickly to a snake’s codon preferences in the Wuhan market, because an infected snake would most likely be sold and killed before that happened.
Yet the Wuhan coronavirus has not been found outside of the Wuhan Huanan seafood market, which is currently the sole known origin of the virus.
For codon adaptation to occur, the Wuhan coronavirus should exist (and therefore be detectable) in the wild snake population for quite some time.
How long does codon adaptation take? We have no idea, but we can reasonably assume that codon adaptation in snakes would take as long as it would in humans.
Consider the fact that viral samples taken recently are genetically similar to samples taken from patients in December 2019.
That means there has been no codon adaptation to the new human host, even after the virus has infected thousands of people.
Two Antigenic Shifts Are Very Unlikely
Viruses mutate rapidly in order to escape detection by the host’s immune system though relatively minor changes called antigenic drift.
But they can also “mate” with other viruses to create a completely new virus that can infect a new host, or become highly virulent – a process called antigenic shift.
For example, an exclusively avian influenza virus can “mate” with a human influenza virus to create a highly virulent new virus that also infects humans.
Fortunately, antigenic shift doesn’t happen very often, because it requires different viruses to co-exist within the same host.
For the Wuhan coronavirus to evolve enough to “jump” from bats (or some other mammal) to a snake, and then evolve again to “jump” to a human host – it would be like getting struck by lightning… twice.
What Do Experts Say About Snakes As Wuhan Coronavirus Source?
Paulo Eduardo Brandão, virologist at the University of São Paulo, said,
They have no evidence snakes can be infected by this new coronavirus and serve as a host for it. There’s no consistent evidence of coronaviruses in hosts other than mammals and Aves (birds).
Professor Guizhen Wu, Chinese Center for Disease Control and Prevention, said,
These data are consistent with a bat reservoir for coronaviruses in general and 2019-nCoV in particular.
Dr. Peter Daszak, President of EcoHealth Alliance, said,
When you look at the genetic sequence of the virus, and you match it up with every known coronavirus, the closest relatives are from bats
Cui Jie, virologist at the Pasteur Institute of Shanghai, said,
Clearly this 2019-nCov is a mammalian virus.
We should note that Cui Jie was part of the team that traced the origin of the human SARS virus to the horseshoe bats in a remote Yunnan cave in 2017.
But Could Snakes Still Be Responsible For The Wuhan Coronavirus?
Now this is important – no one should rule out snakes as an intermediary host for the Wuhan coronavirus just yet.
Yes, there is no known coronavirus that infects a snake or any other reptile, but that doesn’t mean it will never or could not happen.
The key thing is to understand that science isn’t straightforward. It takes many studies – blood, sweat and tears – to investigate, verify and replicate – before we can reasonably draw a conclusion.
So everyone should stop jumping on every study as if it is conclusive evidence that snakes or civet cats or the animal of the week is responsible for the Wuhan coronavirus.
Rome wasn’t built in a single day, and evidence that conclusively identifies the culprit for 2019-nCov won’t be found in a single study.
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