Denmark has urged its farmers to cull their minks - an estimated 15 million to 17 million animals - to contain a mutated strain of coronavirus that was detected at mink farms and has spread to people.

The Straits Times speaks to Professor Wang Linfa, director of the emerging infectious diseases programme at Duke-NUS Medical School, to find out how diseases spread between people and animals, and if this latest development is cause for alarm.

Q How did the minks get infected in the first place?

A The World Health Organisation (WHO) said the minks were infected from being exposed to humans with Covid-19. To date, several countries, including Denmark, the Netherlands, Spain, Sweden, Italy, Greece and the United States, have reported Sars-CoV-2 in their farmed minks.

The WHO said animals such as minks could act as a "reservoir" for the virus, passing it between them and posing a risk of uninfected humans catching it from them.

Q What enables a disease to jump between different species, such as animals and humans, and how commonly does it occur?

A A disease that can spread between humans and animals is known as a zoonotic disease.

Prof Wang said a virus needs to clear at least three hurdles to jump from species to species.

First, both species must have contact with each other.

Second, the cells of the receiving species must have the right cell surface receptors, allowing the virus to enter and start replicating.

Third, the receiving species must have an immune system that allows the virus to replicate and transmit itself.

"As one can appreciate, it is not easy to overcome all three hurdles and hence, cross-species jump remains a rare event," he said.

Q What are some well-known zoonotic diseases?

A Rabies is one of the most deadly zoonotic viruses jumping from animal to human, said Prof Wang.

Another is the human immunodeficiency virus (HIV), which not only spread from animals to humans, but has also successfully maintained itself in the human population, he said.

Other examples include the Hendra virus, Nipah, Sars-CoV, Mers-CoV and Sars-CoV-2.

Prof Wang added that Sars-CoV-2, which causes Covid-19, poses the highest risk of a human-to-animal jump, known as "reverse zoonotic transmission", as demonstrated by the large-scale infection of minks in many countries.

Q Is the coronavirus that causes Covid-19 likely to be zoonotic, and are humans at risk of catching Covid-19 from animals? What about minks specifically?

A Prof Wang said scientists are almost 100 per cent certain Sars-CoV-2 was an animal virus which spread to the human population.

"What we don't know is when and where that initial jump happened," he said.

The risk of catching the coronavirus from animals like minks exists, but is lower than that of catching it from another human.

"We have over 50 million people infected and roaming around the globe. The minks are farmed animals and the risk of spreading the virus to the general public is very low," he added.

Q Is culling minks a necessary step to prevent the spread of the coronavirus?

A Countries have mass-culled animals to stop the spread of diseases, said Prof Wang.

For instance, Malaysia culled more than a million pigs during the Nipah virus outbreak in the 90s. During the Sars crisis in the 2000s, about 10,000 civet cats were killed.

But it is very difficult to judge, from a scientific standpoint, if the culling of minks is necessary, he added. "In dealing with a pandemic like Covid-19, overreaction may be better than underreaction."

Q Should the infection of minks be a cause for alarm? What can we learn from it?

A While health officials and people must remain alert, there is no need to be alarmed, said Prof Wang.

The reason is that it was already known "very early on" during the pandemic that animals such as dogs and cats could be infected.

"Not all zoonotic transmission leads to severe disease or pandemic in humans," he said.

"If we are serious about the prevention of future pandemics, we need to do a much better job of monitoring all zoonotic transmission events, not only those causing severe diseases. To achieve that, we need much more international collaboration and funding."