Pangolins, once suspected as the missing link from bats to humans in the origin of the coronavirus pandemic, may not have played that role, some scientists say, although the animals do host viruses that are similar to the new human coronavirus.
Peter Daszak, the president of EcoHealth Alliance, an organization that works on animal-to-human spillover diseases, said that accumulating evidence on pangolins made it “doubtful that this species played a role in the outbreak.”
“We need to keep looking for the original reservoir” — likely a bat,” he said, adding that the potential intermediate host would likely be another mammal species that’s more widely traded in the Yunnan-to-Wuhan corridor of China.
While the pangolin trade is vast, Dr. Daszak said most of it “is in their scales, dried, in which viruses would almost certainly not be able to persist.” He opposes trade in the highly endangered animal.
Kristian G. Andersen of Scripps, who wrote a recent paper with other viral disease experts deciphering some aspects of the pangolin virus link, was more neutral. “In my opinion, none of the data I have seen so far is suggesting that pangolins did serve as an intermediate host, however, that doesn’t mean they didn’t serve as an intermediate host,” he said.
Pangolins do play host to viruses that are similar to the new virus, known as SARS CoV-2, that causes the disease sweeping the globe.
But those viruses are not as close to the human virus as one called RaTG13, found in Chinese horseshoe bats. None of those viruses are similar enough to the human virus to indicate a direct spillover to humans. And there has been no clear path established from bats to pangolins. It is still possible there could have been a direct leap from bats to humans, although that was not the case with earlier coronavirus outbreaks, like SARS and MERS.
In several reports from research conducted since the pandemic began, including a paper published Thursday in Nature that had previously been posted as a preprint, scientists have painted a picture with a missing middle. Both bats and pangolins have viruses similar to the new coronavirus. Somehow they form the raw material for the new virus, but exactly how that raw material was transformed — in what animal, or in humans — remains unknown.
The initial public interest in pangolins arose from reports out of China that a virus found in sick Malayan pangolins was almost identical to the virus sickening people. Since pangolins are considered the most trafficked animal in the world, the possibility attracted a great deal of attention.
When the research was posted on BioRxiv, it became apparent that there had been some confusion. It was not the entire pangolin virus, only something called the spike protein, that was nearly identical to the same part of the new coronavirus that infects humans.
Several research papers by Chinese scientists have questioned the likelihood of the virus spilling over from pangolins directly to humans, noting differences in the pangolin virus and the human virus.
The paper published Thursday in Nature examined a second set of Malayan pangolins seized from a smuggling operation and also found coronaviruses much like the ones previously studied. The authors concluded that pangolins should not be traded, because although there is no evidence these viruses jumped directly to humans, they have the potential to sicken people.
There are many, many coronaviruses in animals, most of which do not infect people, and although knowing how they get to humans won’t affect the course of the current pandemic, such knowledge will help for the future.
Dr. Andersen said there are several paths the new virus could have taken. Assuming that it began with a bat virus, it could have jumped directly to humans, although that didn’t happen in the other coronavirus outbreaks of SARS and MERS.
Or it could have passed from a bat to another animal, one of the many that humans hunt, raise for food and sell in markets. Dr. Andersen said that the virus didn’t necessarily first spill over to humans at the Wuhan market initially marked as the likely source of the outbreak. It could have happened elsewhere.
Dr. Daszak said that South China “has an abundance of mixed wildlife-livestock farms that house chickens, ducks, civets, porcupines, pigs, bamboo rats, altogether all in conditions that would be conducive to viral spillover and spread.”
“These farms are often open, rustic, with access to bats which are abundant in the region,” he said. A spillover could have occurred at one of the farms and an infected human could have infected others at the Wuhan market.
There is also the question of where the virus evolved its ability to easily pass from human to human. That could have happened in animals, Dr. Andersen said, meaning it would have sprung forth ready to cause a pandemic. Or it could have circulated in humans in a less efficient form, from the virus’s point of view. Then it evolved its pandemic equipment in humans.
In any case, Dr. Andersen said, although the current pandemic is the war we are now fighting, there is no question that another one could come. This one was no surprise, he pointed out. He said, “We hear that nobody could have predicted a pandemic,” he paused, “except for everyone who works in infectious diseases.”
He admitted, however, that before the current pandemic, his main concern was not the coronavirus family. “If I had to make a guess before,” he said, “I would have said flu.”
The best defenses for the future, Dr. Andersen said, would be broadly effective vaccines, like a universal flu vaccine and a vaccine against SARS-like coronaviruses.
Surveillance of animal sources and reaching an understanding of how viruses evolve is essential, but, he said, would not prevent the next pandemic.
As to finding the animal that was the intermediate host, it may happen. But without viral genetic material from the very first human cases or from animals with highly similar viruses, including animals that might have been present at the Wuhan seafood market, that search is very difficult.
“Chances are that we just won’t know,” Dr. Andersen said.