Why is it difficult to trace the origin of the virus?

Why is it difficult to trace the origin of the virus?

Marilyn J. Rosinak, Penn State

Pennsylvania (USA), June 11 (The Conversation) Every time a major disease outbreak occurs, the first question scientists and the public ask: “Where did it come from?”

To prevent future outbreaks and predict epidemics like COVID-19, researchers need to trace the origins of the viruses that cause these diseases. This is no easy task. The origin of HIV is still unclear, 20 years after its worldwide spread. Scientists still don’t know the origins of Ebola, even though it has caused periodic epidemics since the 1970s.

As an expert in viral ecology, I am often asked how scientists trace the origins of viruses. In my work, I have found many new viruses and some well-known pathogens that infect wild plants without causing disease. Plant, animal or human, the methods are largely the same. Tracing the origins of a virus involves a combination of extensive fieldwork, thorough laboratory testing, and a fair amount of luck.

Viruses come to humans through wild animals

Many viruses and other disease factors that infect people originate in animals. These diseases are said to be animal-borne, meaning they are caused by animal viruses that have been transmitted to humans and then adapted to spread through human populations.

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.

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Another problem is that people and the animals they eat are not stable. The place where researchers first find an infected person is not necessarily close to the place where the virus first emerged.

In the case of COVID-19, bats were an obvious first place. They are known to house several coronaviruses and are potential sources of other animal-borne diseases such as SARS and MERS.

From the study of 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 Virology Institute in 2011 and 2012.

After the SARS-CoV-1 pandemic in 2003, virologists were looking for SARS-related coronaviruses in bats. They collected bat stool samples and throat swabs at a site in Yunnan province, about 932 miles (1,500 kilometers) from the institute’s laboratory in Wuhan. They brought the samples back to Buhan’s laboratory for further study.

To test whether bat coronavirus could spread to people, the researchers infected monkey kidney cells from Yunnan samples and cells derived from human tumors.

They found that many of the viruses in the samples collected could increase their numbers in human cells, meaning they could potentially be transmitted directly from bats to humans without any intermediate hosts. However, bats and people do not often come into direct contact, so there is still great potential for an intermediate host.

Finding Nearest Relatives

The next step is to determine how closely a suspected wildlife virus is to a virus infecting 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.

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Genetic sequencing of the bat coronavirus RATG13 showed it to be more than 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 may have originated in bats, but is still very difficult to prove. that it is its direct ancestor. Another host is likely to have caught the virus from bats and transmitted it to humans.

Similarly, when a related coronavirus was identified in pangolins confiscated in an anti-smuggling operation in southern China, many concluded that SARS-CoV-2 was transmitted from bats to pangolins and then to humans. However, only 91% similarity was found between pangolin virus and SARS-CoV-2, which eliminates the possibility of it being a direct ancestor of the human virus.

To solve the puzzle of viral origins and its human reach, scientists have to find not only the missing pieces, but also how they all fit together. This requires collecting viral samples from human infections and comparing those genetic sequences to each other and to other animal-derived viruses.

Apart from this, there is also a debate about whether the virus reached humans from an animal or leaked from the lab to humans. Recently, 18 leading virologists have suggested testing the spread of this virus from the lab. However, once it is matched with RATG13, the possibility of its creation in the lab becomes very less.

The Conversation Unity Ekta


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