Whenever a major epidemic strikes, the first question scientists and the public ask themselves is, “Where did it come from?” To prevent future outbreaks and predict outbreaks like COVID-19, researchers need to trace the origins of the viruses that cause these diseases. It is not an easy task. The origin of HIV is still uncertain, 20 years after its global spread. Scientists still do not know the origins of Ebola, even though it has been the source of periodic epidemics since the 1970s.
As an expert in viral ecology, I am often asked how scientists trace the origin of viruses. In my work I have found many new viruses and some well-known pathogens that infect wild plants without causing disease. Plants, animals or humans, the methods are largely the same. Tracing the origins of a virus involves a combination of extensive fieldwork, extensive lab testing, and a good dose of luck.
Viruses reach humans from wild animals. Many viruses and other disease factors that infect humans originate from animals. These diseases are said to be of animal origin, which means that they are caused by animal viruses that have been transmitted to humans and then adapted to spread in human populations.
Symptoms not seen in wild animals
It may seem easy to begin the search for viral origins by testing sick animals at the site of the first known human infection, but wild animals often show no symptoms. Viruses and their hosts adapt to each other over time, so viruses often do not cause obvious disease symptoms until they migrate to a new host species. Researchers can’t just look for sick animals.
Another problem is that the people and the animals they eat are not stable. The place where researchers first find an infected person is not necessarily near where the virus first appeared. In the case of COVID-19, bats were an obvious first place. They are known to harbor several coronaviruses and are potential sources of other animal diseases such as SARS and MERS.
From studying SARS-CoV-2, the virus that causes Kovid-19, scientists have so far found that Batcov RATG13 is its closest variant. The virus is part of a collection of bat coronaviruses discovered by virologists at the Wuhan Institute of Virology in 2011 and 2012.
samples found in yunnan
After the SARS-CoV-1 pandemic in 2003, virologists were looking for coronaviruses linked to SARS in bats. They collected bat stool samples and throat swabs at a site in Yunnan province, about 1,500 kilometers from the institute’s laboratory in Wuhan. They took the samples back to Buhan’s lab for further study.
To test whether the bat coronavirus could spread to humans, the researchers infected monkey kidney cells from samples from Yunnan and cells derived from human tumors.
They found that many viruses in the samples collected could increase their numbers in human cells, meaning that they could potentially be transmitted directly from bats to humans without any intermediate host. However, bats and humans do not often come into direct contact, so there is still great potential for an intermediate host.
find closest relatives
The next step is to determine how close a suspected wildlife virus is to a virus that infects humans. Scientists do this by tracing the genetic sequence of the virus, which involves determining the sequence of the basic building blocks, or nucleotides, that make up the genome. The more nucleotides two genetic sequences share, the more closely related they are.
Genetic sequencing of the bat coronavirus RATG13 has shown that it is over 96% similar to SARS-CoV-2. This level of similarity means that RATG13 is a very close relative of SARS-CoV-2, which confirms that SARS-CoV-2 can originate from bats, but is still very difficult to prove that it is its direct ancestor. Another host is likely to have caught the bat virus and passed it on to humans.
Likewise, when a related coronavirus was identified in pangolins seized during a smuggling operation in southern China, many concluded that SARS-CoV-2 had passed from bats to pangolins. , then to humans. However, only 91% similarity was found between the pangolin virus and SARS-CoV-2, which rules out the possibility that it is a direct ancestor of the human virus.
To solve the puzzle of viral origins and its human reach, scientists must find not only the missing pieces, but also how they all fit together. This requires collecting viral samples from human infections and comparing these genetic sequences with each other and with other viruses of animal origin.
Apart from that, there is also a debate as to whether the virus has reached humans from an animal or if it has leaked from the laboratory to humans. Recently, 18 leading virologists suggested testing the spread of this virus from the lab. However, once it is combined with RATG13, the possibility of its creation in the laboratory becomes very reduced.
Corona: Important data received by the WHO team from the Wuhan Lab of China