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Health

Why did swine flu appear in a person in the UK – and what’s next?


A pig stands in its pen on a farm in south-east England.

In 2009, a swine flu pandemic killed hundreds of thousands of people.Credit: Daniel Leal/AFP via Getty

Scientists are working to better understand a type of swine flu first detected in a person in the UK. Swine flu usually circulates in pigs, but caused a human pandemic in 2009 that killed about 284,000 people.

The virus detected in the British individual was the H1N2 influenza A virus, which is believed to have infected 50 people worldwide since 2005. None of these previous cases are genetically linked to the British strain, according to information released on November 27 by the British Health Safety Agency. (UKHSA).

The UK case was detected after doctors carried out a genetic test on a patient in North Yorkshire who reported flu-like symptoms. The patient is now fully recovered. UKSHA says it is still investigating how the person became infected.

Scientists and doctors are now working to better understand the genetics of this human infection and monitor for evidence of human-to-human transmission.

“It’s really important that these cases are monitored, because if something goes further, we really want to know,” says Ed Hutchinson, a virologist at the University of Glasgow.

Common spillover effects

Infections with respiratory viruses, including influenza, across species are quite common. “The reason it appeared now is because it was spreading in pigs,” says Paul Hunter, an epidemiologist at the University of East Anglia.

Respiratory viruses are spread through close contact, so people working with pigs are at particular risk – although any close contact with an infected animal can transmit the virus. In the majority of cases, the fallout is thought to go undetected because the person never feels sick and the virus does not spread further.

The UK’s spillover was detected in part because the infected person felt unwell enough to seek medical attention. The UK has a respiratory virus surveillance system, and if a patient visits a doctor with flu-like symptoms, the doctor is encouraged to collect it and send that sample for analysis, Hutchinson says.

Once a virus has spread among humans, scientists look for evidence that it is spreading among them. “Until you have evidence that it’s starting to spread more widely and increase in numbers, you don’t know it’s going to pose a threat,” Hunter says. There is no evidence yet that this has happened in the case of H1N2 detected in the UK.

Hutchinson says such spread is rare. “The question is whether the virus can replicate enough in that person to not only infect them but also transmit it to another human being. It’s actually very difficult for viruses to do that if they don’t not found in the right host,” he says. “So usually in an overflow case, that doesn’t happen.”

When viruses multiply

However, influenza viruses are unique in that they can replicate, meaning that different viruses can combine their genes as they replicate. This helps the virus adapt from one species to another.

“If two different viruses infect the same cell at the same time, the virus can emerge and have some genes from one parent virus and some genes from the other parent virus,” Hutchinson says. This is called reassortment.

The process is of particular concern when it leads to a virus that has many human-adapted genes and is therefore fit to replicate in humans, and which has proteins on its exterior – which are what antibodies humans use to recognize and destroy viruses – which come from a new non-human source.

The virus that caused the 2009 swine flu pandemic was a mixture of different viruses: a swine flu, a human flu and a bird flu all exchanged genes, Hutchinson explains. “And then another swine flu mixed with that, to produce a virus capable of transmitting to humans, and so it was an unusually complex version of mixing and matching,” Hutchinson says.

Over the coming days, scientists will work to better understand the genetics of the strain that infected the person in the UK, using known data from large data sets on the virus to see if it is linked to other viruses.



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