Disease spillovers: "a common occurrence"

If the coronavirus did first get into humans somewhere far south of Wuhan, why did no one notice? One possible answer is that these spillovers from animals to humans are more common than you might think, and often difficult to detect. Maureen Miller from Columbia University worked with Zhengli Shi, from the Wuhan Institute of Virology, to test people in rural China for an entirely different coronavirus - as Phil Sansom heard…

Maureen - We went to a place in Yunnan where she had found a bat that could cause potential harm to humans. And what we found was that 3% of the population that lived near the bats that were infected with these coronaviruses, had already been infected. So spillover is quite a common occurrence.

Phil - This isn't the coronavirus that's caused the pandemic; this is just a random other one?

Maureen - It is a separate coronavirus, it is not close at all. There are many, many coronaviruses that bats carry. Only 1% of them are estimated to cause any kind of disease in humans. And what had been believed prior to this was that it always had to go through a secondary animal and then get transmitted to humans. Zhengli Shi was the first person to discover that bat coronaviruses could be potentially transmitted directly to humans; and we were able to prove that yes, indeed, that was so.

Phil - Why on earth did you suspect it might infect humans then?

Maureen - That is the work that Zhengli Shi does. She has collected genetic sequences for various coronaviruses that bats carry. This particular one had a spike that is able to directly infect humans.

Phil - And who did you go out and test?

Maureen - This was Yunnan province. And we just tested all kinds of community members: farmers, foresters, hunters. And when we found 3% positivity, there was no particular demographic profile that had higher risk. Because we know in general, hunters have a higher risk - they kill the animals, the animals can scratch them - but they were no higher risk than anybody else in that community.

Phil - How is that possible?

Maureen - Exactly! So bats are really everywhere. They often live in the roofs of houses. People go into caves where bats live to collect bat guano, because it is a very valued fertiliser for crops. There's all kinds of mechanisms of exposure, and bats are fairly revered in China, so everybody knows where they are and respects them and thinks nothing of them.

Phil - Is that not kind of scary? I mean, in Yunnan province, why wasn't there a pandemic that began?

Maureen - With this particular coronavirus, it appears to not have caused noteworthy disease. In COVID-19, many of the cases were asymptomatic; so we wouldn't notice that anyone was infected, because everyone feels fine. When we start to notice it is when there is an increase in deaths, particularly among the elderly, from pneumonia; but pneumonia is a leading cause of death among the elderly, particularly in rural areas. So people may not have noticed that there were excess deaths.

Phil - Is the implication that the new coronavirus started like this?

Maureen - Absolutely. Spillover is much more prevalent than we think it is. What's different about COVID-19 is that it spreads human to human. A lot of zoonotic disease spillover spreads from the animal directly to the human, and then the human does not transmit it to another human. When it becomes dangerous is when it mutates and then can become transmissible from human to human.