Why discovering the origin of the coronavirus is so important and so complicated?

The bat is the ideal culprit, but it needs an intermediate host to infect humans. And if the snake’s trail is very fragile, the research has only just begun.

As China tightens increasingly stringent restrictions to limit the spread of the new coronavirus that has already killed 17 people, researchers around the world are trying to understand exactly what we are dealing with.

We know that the epidemic began in a market in Wuhan where live animals are traded. The virus genome, collected from patients, has also been fully sequenced. It is clearly a new form of coronavirus, dubbed 2019-nCov. And it is also clear that it has been transmitted to humans by an animal, still unidentified, which is commonly called a “reservoir”.

For the rest, unfortunately, “we’re still in the dark,” Sandrine Belouzard, a researcher at the Lille Center for Infection and Immunity, told HuffPost. And if the snake or the bat are mentioned for the moment, these tracks are to be taken with tweezers.

The stake is however of importance because “the confirmation of the presence of an animal reservoir is essential for the intervention and the prevention of the epidemic”, explain two American researchers in an update published this Wednesday January 22 in the Journal of medical virology. To identify the path of the virus and this animal reservoir, it will therefore be necessary to compare the viruses with each other.

The bat, original source?

A very simple example: as Wired reports, the information available to us shows that the virus has hardly changed since it infected humans. Out of 15 viral genomes analyzed, almost no difference was found. This means that either the virus has spread quickly between animals in Wuhan and has passed from animal to man on multiple occasions, or that there were few humans infected at the start and that it is then very virulent between humans.

This second hypothesis is not the one retained for the moment in view of what we observe. But to be sure, the current data is not enough, the magazine said. Except that if we knew the animal that transmitted this coronavirus to humans, we could then test its virality in the laboratory. This is why, among other things, it is important to know which animal has infected humans.

Unfortunately, things are not simple. “Once we have the genome, we compare it to databases in which we can check the closest known viruses,” explains Sandrine Belouzard. We have already done this. Result: it is indeed a coronavirus close to SARS, whose epidemic came from China in 2003 had killed more than 700. Above all, it is close to “SARS virus found in bats,” says the researcher.

In fact, bats are all kinds of virus nests, healthy carriers who have the gift of intriguing science. They represent, for example, 20% of the diversity of mammals when they are the only ones to fly. And for some mysterious reason, they seem to carry many viruses … without being negatively affected. A 2007 study explained that bats, including the Great Horseshoe species, are “a natural reservoir of a group of coronaviruses related to SARS”.

what could be the intermediate host?

But related does not mean identical. The genome of the coronaviruses found on bats is close, but not enough to be able to pass to humans. In the case of the 2019-nCov, the resemblance is around 85%, says Sandrine Belouzard.

For a coronavirus to infect humans, it has to mutate several times. First to infect another mammal. Then, in this other animal, called “intermediate host”, again to infect humans. In 2003, it was via the palmist civet that SARS was born. For the Mers virus, the intermediaries are dromedaries. “What we are looking for today is the intermediate host which must be infected with a virus very similar to, or even identical to, that found in humans,” explains the researcher.

Wednesday, January 22, still in the Journal of medical virology, Chinese researchers hypothesized an unprecedented intermediate host: the snake. But Sandrine Belouzard calls for caution. “These results are surprising and very indirect,” she explains. To put it simply, the authors have analyzed in detail part of the virus, which is “preferentially used by the snake when it makes proteins”.

research and analysis continue

This is far from being direct proof, especially “that we know of no coronavirus in snakes, which are cold-blooded creatures”, recalls the researcher, who says she is “a little skeptical”. Especially since the market in Wuhan was filled with many other mammals and birds, known to be affected by various coronaviruses. Samples were taken from several animals present, according to the Chinese authorities.

In the update, the two American researchers also mention the trail of the snake, while also recalling that checks must be carried out and that many other suspects were present in Wuhan. The simplest: “determine if 2019-nCov can be isolated and infect snakes”.

If positive results are found on animal samples, it will also be necessary to verify that the animal virus is capable of infecting humans. In this race against the clock, the quest for origin is far from over.