Scientifically Speaking | Decoding the evolution of coronaviruses
Coronaviruses infect many animals. Bats are often singled out as culprits in spillover events, but they are not unique. In fact, humans have transmitted Sars-CoV-2, the coronavirus that causes Covid-19, to animals like cats, minks, and gorillas.
Viruses don’t always respect species boundaries. They recognise receptors to attach to and cells to infect. There are currently around 220 viruses known to infect humans, of which seven are coronaviruses.
Coronaviruses have been responsible for two epidemics and one pandemic in the last two decades. In recent months, scientists have discovered two more novel coronaviruses that infect humans. There are certainly many more. The simple fact is the more we search in the “virosphere”, the more viruses we will find.
Before Sars, there was limited interested in coronaviruses. A perspective in Science in 2003 called coronavirology one of the “backwaters” of virology. After Sars, when scientists searched intensively for new coronaviruses, they found two that cause common cold-like infections.
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There are many reasons we will find more viruses.
First, the molecular tools of virology are relatively new, and sampling of animals has traditionally been limited to species and classes of viruses with greatest pandemic potential. Second, some spillover events from animals may be relatively recent. Third, some of viruses may be infecting people but escaping detection because they are not causing disease. Others may cause disease, but may be self-limited to non-specific symptoms of pneumonia or gastrointestinal problems that get better within a few days.
In COVID-19: Separating Fact from Fiction, I mention the various interactions between viruses and humans. There are millions of viruses that don’t infect human cells at all. There are a few viruses that infect people in spillover events, but don’t spread further. There are others can infect people but do not cause recognisable disease. Yet others that result in disease may not have outward transmission to other people. Viruses that transmit from one person to another might stop spreading if they don’t transmit well.
Of course, of greatest concern are viruses like Sars-CoV-2 that transmit well and cause disease. These viruses cause significant disease and death and they threaten to remain in human populations.
A preprint on MedRxiv describes a pig coronavirus found in blood samples of children in Haiti who had acute fever of unknown cause a few years ago. By sampling the genome and comparing with other known viruses, researchers found that these infections were the result of at least two past spillover events. The causative coronavirus belongs to a family that was not thought to typically infect humans. It is currently unknown whether the virus caused the fever in the children or whether it was simply coincidentally present. We also don’t know how prevalent this coronavirus is in broader populations.
Another description of a novel coronavirus is in an article in Clinical Infectious Diseases. Eight samples collected from children hospitalised in rural Malaysia with pneumonia many years ago turned up a canine coronavirus. This virus seems to have sequences that are similar to dog coronaviruses but also to coronaviruses found in cats and pigs.
Sars-CoV-2 is also a virus that seem to have different genetic sequences that are similar to coronaviruses from different animals. What the discovery of the hybrid canine coronavirus in Malaysia reinforces is that recombination events, which result in new viruses that are made up of parts of other viruses, are common in nature.
The newly identified virus is the first canine coronavirus found to infect people, a trait that may have taken years of evolution to acquire. Like the other novel coronavirus which spilled over from pigs, it hasn’t yet been shown to cause disease; it might even result in dead-end infections.
Both studies underscore a fact. We do not know how many coronaviruses infect humans. Better surveillance for coronaviruses is needed especially when humans and animals are in close proximity and there are outbreaks of pneumonia of unknown causes. The virus identified in Malaysia was found because of a highly sensitive RT-PCR test that checked for many different coronavirus types.
Neither of these two new coronaviruses are capable of causing a pandemic right now. They were found because scientists were looking for them. But there is a need to search for viruses that infect people, because a virus might lurk undetected and be just a few years away from causing the next pandemic.
We can speculate what might happen when viruses remain undetected and cause infections in people. Over years, in the test tube of human cells, they might acquire additional mutations that allow them to become more capable of replicating and transmitting to others. In the process of using up the cell’s resources, they might cause severe disease. These scenarios are catastrophic for us, but there is nothing nefarious here, only evolution at work.
Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction
The views expressed are personal