How did the coronavirus get from bats to us?
It seems extremely likely that bats were the ‘reservoir’ for COVID. But what we don’t know is how it got from them into us, because scientists haven’t yet found an animal that’s actually currently carrying the new coronavirus. Disease ecologist Raina Plowright told Phil Sansom where that animal might be - and how it might have transmitted the virus to humans...
Raina - This virus hasn't actually been sampled in bats. And that's fair, because it would take sampling thousands and thousands of bats to find every single coronavirus that is in existence. But we know that viruses - similar viruses - circulate in horseshoe bats. We know that they circulate in the area of southeast Asia, and southern China, and northern Vietnam, Myanmar, Laos. And so perhaps the virus came from that region. Did it come within a bat to Wuhan, or did it come within a human? We don't know. One of the challenges with coronaviruses is that of the three coronaviruses that have spilled over from animals to people in the last 20 years, each of those potentially has spilled into the human population through a single event. And then for this pathogen, the circumstances are still a mystery, but perhaps it was a single event from a bat, either into a human or into a bridging host. We've talked about pangolins as a potential bridging host; civets, ferrets; we know that felines are actually very susceptible as well.
Phil - Is there any way of telling whether there was a species in the middle, or if it went straight from bats to humans?
Raina - We don't know at this point. But one of the really critical questions is: was the virus supercritical in bats? What I mean by supercritical is that when it infects a human, that human can not only be infected, but that human can then infect others at a rate that's high enough to sustain a chain of transmission. And earlier on we thought that perhaps in bats, the virus wasn't in a form where it could just take off and spread in humans; that it needed a little bit of a genetic mix-up, and that maybe that mix up happened in another species. But more recent evidence is seeming to suggest that, actually, these may be supercritical already in bats; it's just that we haven't sampled the right bat to find the right pathogen. But I do think that we could still try to understand this event by going into the regions where we know the similar viruses occur, the southern area of China and that area of northern Vietnam, Laos, Myanmar; and looking at people, and then trying to do the serological surveys, looking for exposure to other coronaviruses, trying to understand how spillover occurs from animals to people.
Phil - Is that kind of what a spillover means?
Raina - We often think of spillover as this simple jump, and I think because we often don't see it we think of it very simplistically. But when you start to deconstruct all of the things that have to happen for spillover to occur, it's very, very complicated, and it's really quite extraordinary that it occurs at all. And I think it is actually very rare. So if you think about it, every time you leave your house - which isn't that often these days, right - but when we do, we're breathing the air; and that air is full of microbes, but we don't often get sick. So a whole bunch of things have to align: you have to have the reservoir host, that host has to be infected, that virus has to get out of the reservoir host - for example, coronaviruses are shed in the faeces of bats - often the virus has to survive in the environment for some time, and then it gets really complicated because then the virus has to go through a whole bunch of barriers within us to allow it to infect our cells, to replicate in the cells, to be able to exit the cells, disseminate through our body. It has to be able to overcome our innate immune system. And then it's got to be able to exit us, it's got to be able to be transmitted to the next person. Because it's somewhat rare for all of those things to line up at one point in space and time, spillover is a relatively rare phenomenon.