Why has swine flu emerged in a person in the UK — and what’s next?

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

The virus detected in the UK individual was influenza A H1N2, which is known to have infected 50 people worldwide since 2005. None of those previous cases are related genetically to the UK strain, according to information released on 27 November by the UK’s Health Security 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 has now made a full recovery. UKSHA says it is is still investigating how the person got infected.

Scientists and medics are now working to understand more about the genetics of this human infection as well as monitoring for evidence of human-to-human transmission.

“It’s really important that these cases are monitored because if anything is going any further we really want to know about it,” says Ed Hutchinson a virologist at the University of Glasgow.

Common spillovers

Spillover infections of respiratory viruses, including influenza, from one species to another are quite common. “The reason it’s popped up now is that it was spreading in pigs,” says Paul Hunter an epidemiologist at the University of East Anglia.

Respiratory viruses spread through close contact, so people working with pigs are at particular risk — although any close contact with an infected animal could pass on the virus. In the majority of cases, spillovers are thought to go undetected because the person never feels ill and the virus travels no further.

The UK spillover was detected in part because the infected person felt unwell enough to see a doctor. The UK has a surveillance system for respiratory viruses, and if a patient comes to a doctor with flu-like symptoms, the physician is encouraged to swab them and send that sample off for analysis, says Hutchinson.

Once a virus has spilled over into people, scientists look for evidence that it is spreading between them. “Until you get evidence that it’s starting to spread more widely and increasing in numbers you don’t know it’s going to be a threat,” says Hunter. There is no evidence that this has happened yet in the case of H1N2 detected in the UK.

Hutchinson says that such onward spread is rare. “The question is can the virus replicate enough in that person in order to not just infect them but to pass on to another human and that’s actually really hard for viruses to do if they’re not in the right host,” he says. “So usually for a spillover case, that doesn’t happen.”

When viruses breed

However, influenza viruses are unusual in that they are able to breed, meaning that different viruses can combine their genes during replication. This helps the virus to adapt from one species to another.

“If you get two different viruses infecting the same cell at the same time, the virus can come out which has some genes from one parent virus, and some genes from the other parent virus,” Hutchinson says. This is called reassortment.

The process is especially concerning when it leads to a virus that has a lot of human-adapted genes and so is adept at replicating in humans, and has proteins on its outside — which are what human antibodies use to recognize and destroy viruses — that are from a new non-human source.

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

Over the coming days scientists will be working to understand more about the genetics of the strain that infected the person in the UK, using known data from large sets of virus information to see if it is related to other viruses.