Even as officials work to contain the outbreak of a new form of coronavirus that first appeared in Wuhan, China, experts say the emerging public health crisis is shaping up into a crucial battle between two very different evolutionary processes.
On one side is a pathogen that is known for its propensity to swap out genes and then slip across the biological boundaries that divide one host species from another. On the other is the international health research apparatus, a knowledge ecosystem that is vastly more connected and responsive than it was in 2003. That was the year that the coronavirus associated with SARS (severe acute respiratory syndrome) claimed nearly 800 lives and came frighteningly close to triggering a much larger global disaster.
Initial analyses of the current outbreak, published Friday in the medical journal Lancet, reveal that there are some important differences that distinguish it from SARS. For example, one cluster of cases from a family in Shenzhen shows that the new virus produces symptoms within four days of exposure. In comparison, the incubation period for SARS was found to be as long as 10 days. A shorter incubation period means that new cases of infection can be identified and quarantined sooner, reducing the spread of infection.
Whether the current epidemic proves to be as serious as SARS, it marks the third new coronavirus in less than two decades to have made the jump from animal to human. The second, MERS (Middle East respiratory syndrome), has killed more people than the now-vanquished SARS despite fewer cases overall, and it remains a persistent threat because of its presence in camels, which serve as a reservoir for the virus.
“That’s three coronaviruses now,” said Allison McGeer, an infectious-disease physician with the Sinai Health System in Toronto. “This is clearly something we’re going to have to learn to deal with."
One factor behind the trend is almost certainly the ability of coronaviruses to undergo “recombination," a process that allows them to exchange genetic material with one another and, in doing so, help disguise their presence when navigating a host’s immune system. Recombination can also equip a coronavirus to switch species more readily, leading to new outbreaks in humans when the opportunity for exposure arises, such as through the consumption of wild animals.
Influenza viruses do something similar, but coronaviruses differ in one key respect: their genetic sequences are composed of a single strand of RNA – the longest of any RNA virus. Since it’s hard to maintain such a large genome without fatal mutations, coronaviruses also carry additional mechanisms to ensure that their genes are copied correctly. That investment suggests that the virus must benefit from carrying around so much genetic material.
“Bigger viruses can have more tricks up their sleeve,” said Joel Wertheim, an assistant professor at the University of California San Diego who has studied the deep evolution of coronaviruses.
As a family, Dr. Wertheim said coronaviruses are “inestimably old.” They can be subdivided into four main branches, two of which occur in bats and two in birds. These are thought to be the original source of coronaviruses that are found in other animals, including the relatively benign human-adapted coronaviruses that are responsible for 15 per cent to 30 per cent of common colds.
According to Simon Anthony, a virologist at Columbia University in New York, the fact that bats are so diverse – at least 900 species have been identified – has helped to accelerate coronavirus evolution. Like a flying archipelago, each species provides an island where versions of the virus can evolve new survival strategies that are then exchanged in the form of RNA sequences.
Genetic sequencing shows that both the SARS and MERS coronaviruses are related to bat coronaviruses that recently infected humans by first using another species as a bridge. For MERS, the other species is camels. For SARS, it was the palm civet, a relative of the mongoose, which is consumed as wild game in China and Southeast Asia.
Part of this week’s response to the new outbreak was the rapid sequencing and release of a draft genome for the virus. In practical terms, this makes it easier for public health agencies to identify new cases and distinguish them from more common respiratory ailments. It also helps in the search for the source of the new variant.
According to a study posted online Thursday, the genome bears a close resemblance to that of another coronavirus first identified in bats in China’s Yunan province. This hints at the ultimate origin of the outbreak, but it’s not clear how a variant of the bat virus found its way into people – a key detail that could prevent additional outbreaks.
One theory suggests snake meat as the link. This is viewed with skepticism by many experts, because reptiles are not known to play host to coronaviruses. More surprising is the speed at which the debate is playing out, as scientists race to post their work on an open-access website called bioRxiv. Launched in 2013, the site did not exist during the SARS emergency, and the difference is telling.
“For me, the most striking thing about this is just how quickly we have been able to respond,” Dr. Anthony said.
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