The idea that common cold coronaviruses were far more deadly when they first appeared in humans is supported by animal studies. In 2016, for example, scientists caught a coronavirus in the act of jumping species into pigs. “The genetic sequence was closely related to coronaviruses in bats, so it looked like the virus had spilled over directly from bats,” says Linda Saif at Ohio State University.
That virus killed 25,000 piglets in China in just a few months. Such events are common, says Saif, who has been investigating new coronavirus outbreaks in animals for decades. In the 1990s, for instance, a respiratory coronavirus devastated cattle herds with “shipping fever”. And in 1977, a diarrhoeal disease caused by a coronavirus emerged in pigs in Europe, later spreading to China and then the US, where it killed an estimated 8 million pigs.
“It is quite possible that when these [common cold] coronaviruses first jumped over to humans, they would have caused episodes of severe disease,” says Saif. What is surprising, however, is how infrequently such leaps seem to have occurred. “When SARS happened,” says Esper, “people like myself started looking for other coronaviruses that might cause respiratory infections.” Only one new one turned up. In 2005, the fourth common cold coronavirus was
discovered in a 71-year-old pneumonia patient in a Hong Kong hospital. HCoV-HKU1 causes respiratory illnesses and has been recorded worldwide. Its closest relative appears to be a rodent coronavirus. We don’t know when it began infecting humans. Esper points out, though, that people are less likely to be hospitalised with HKU1 and NL63 than with 229E and OC43, possibly indicating that the former pair have more ancient roots in human populations.
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What four coronaviruses from history can tell us about covid-19
