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How viruses spread from animals to humans

Audrey Tan on 02 Feb 2020

The Straits Times


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Viruses are everywhere, and in almost every living thing on the planet.


But there are several barriers that normally prevent their jump from animals to humans.


One of these species barriers, as scientists call them, is the mismatch between the "shape" of the virus and that of the binding receptors on the host cell. These receptors are located on the surface of the cell.


When other molecules bind to a cell's receptors, they kick-start other processes within the cell.


Just like pieces from a different puzzle, virus surface proteins from an animal host are usually unable to latch on to the receptors on human cells.


However, genetic mutations in a virus can cause its structure to change.


Dr Stathis Giotis, a virologist at the Department of Infectious Disease at Imperial College London, explains: "Often, viruses accumulate mutations over time in their genomes, which can eventually enable them to bind to new host receptors."


These changes allow them to cross the so-called species barrier, he adds.


The coronavirus that has so far killed more than 200 people in China is believed to be most closely associated with bats.


While earlier reports had linked the emergence of the disease with pneumonia-like symptoms to a market in Wuhan, China, where wildlife was sold, the conditions that led to the emergence of the virus are still being studied, Dr Giotis says.


He tells Insight: "The transmission of viruses can often be simple, with a direct transmission from an initial host to humans, or more complicated, where transmission occurs through one or multiple intermediate hosts."


Professor Peter Piot, director of the London School of Hygiene and Tropical Medicine, says greater clarity on the origins of the coronavirus and how it was transmitted to humans could help in further prevention strategies.


When it comes to avian influenza (H5N1), for example, birds have proven to be, literally, the canary in the coal mine.


Birds, including poultry consumed by humans, are especially susceptible to it.


"Now, there is not only epidemiologic (disease study) surveillance of the infections among humans, but also among birds - detecting changes in mortality rates among the animals, which can serve as a warning," Prof Piot says.


In the wake of the escalating number of people infected by the coronavirus, the Chinese government last Sunday implemented a temporary ban on trading wild animals "until the national epidemic situation is over".


But there are growing calls among the scientific and nature communities to make the ban permanent.


Banning the trade in wild animals would reduce the exposure that humans have to them, lessening the likelihood of mutated viruses crossing the species barrier, says Prof Piot.


A spokesman for nature group World Wide Fund for Nature (WWF) Singapore says the current situation highlights the lesser-known link between human health risk and wildlife markets.


A virus has greater potential to mutate and infect humans by jumping the species barrier in places where people come into close contact with infected animals.


"Wildlife markets provide a potentially fertile environment for this type of viral mutation and infection of humans," says a WWF Singapore spokesman.


"Weak to no enforcement of laws in these markets, coupled with the absence of any veterinary controls, makes them a threat to the health of both people and domestic animals."


WWF says it will work closely with governments in the Asia-Pacific region to further strengthen national and international legal systems and engage public health sectors to eliminate the illegal wildlife trade, including closing unregulated wildlife markets.


Source: The Straits Times © Singapore Press Holdings Limited. Reproduced with permission.



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