Since SARS-related coronaviruses are found in bats across Asia, including Thailand and Japan, it’s a very big haystack to search for a very small needle. This is a difficult task – sampling bats is time-consuming and requires strict precautions against accidental infection. To pinpoint the origin of SARS-CoV-2, a lot more wild samples need to be collected. The pangolin virus was found to be only 91% identical to SARS-CoV-2, though, making it unlikely to be a direct ancestor of the human virus. Likewise, when a related coronavirus was identified in pangolins confiscated in an anti-smuggling operation in southern China, many leaped to the conclusion that SARS-CoV-2 had jumped from bats to pangolins to humans. However, researchers never found the coronavirus in animals from the market. AP Photo/Silvia Izquierdoīecause some of the earliest cases of COVID-19 were found in people associated with the wildlife market in Wuhan, there was speculation that a wild animal from this market was the intermediate host between bats and humans. In order to find the intermediate host between bats and humans, researchers have to cast a large net and sample many different animals. There likely was another host that caught the virus from bats and passed it on to humans. This level of similarity means that RaTG13 is a pretty close relative to SARS-CoV-2, confirming that SARS-CoV-2 probably originated in bats, but is still too distant to be a direct ancestor. Genetic sequencing of bat coronavirus RaTG13 showed it to be over 96% identical to SARS-CoV-2. The more nucleotides two genetic sequences share, the more closely related they are. Scientists do this by figuring out the genetic sequence of the virus, which involves determining the order of the basic building blocks, or nucleotides, that make up the genome.
The next step is to determine how closely related a suspected wildlife virus is to the one infecting humans. Bats and people don’t come into direct contact very often, however, so an intermediate host is still quite likely.
They found that a number of the viruses from this collection could replicate in the human cells, meaning they could potentially be transmitted directly from bats to humans without an intermediate host. To test whether the bat coronaviruses could spread into people, researchers infected monkey kidney cells and human tumor-derived cells with the Yunnan samples. They collected fecal samples and throat swabs from bats at a site in Yunnan Province about 932 miles (1,500 kilometers) from the institute’s lab in Wuhan, where they brought samples back for further study. The virologists were looking for SARS-related coronaviruses in bats after the SARS-CoV-1 pandemic in 2003. This virus is part of a collection of bat coronaviruses discovered in 20 by virologists from the Wuhan Virology Institute. They’re known hosts for many coronaviruses and are the probable source of other zoonotic diseases like SARS and MERS.įor SARS-CoV-2, the virus that causes COVID-19, the nearest relative scientists have found so far is BatCoV RaTG13. In the case of COVID-19, bats were an obvious first place to look. LLuis Alvarez/DigitalVision via Getty Images These diseases are zoonotic, meaning they are caused by animal viruses that jumped to people and adapted to spread through the human population.Ī challenge in viral origin tracing is the wide range of human and animal samples that need to be collected and tested. Many viruses and other disease agents that infect people originate in animals.
Viruses jump from wild animal hosts to humans Tracking down the origins of a virus involves a combination of extensive fieldwork, thorough lab testing and quite a bit of luck. Plant, animal or human, the methods are largely the same. In my work, I have found many new viruses and some well-known pathogens that infect wild plants without causing any disease. Scientists still don’t know the origin of Ebola, even though it has caused periodic epidemics since the 1970s.Īs an expert in viral ecology, I am often asked how scientists trace the origins of a virus. The origin of HIV was not clear until 20 years after it spread around the world. In order to predict and prevent future pandemics like COVID-19, researchers need to find the origin of the viruses that cause them. Every time there is a major disease outbreak, one of the first questions scientists and the public ask is: “Where did this come from?”