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What is H10N3, the bird flu reported by China in a human?

A 41-year-old resident of Zhenjiang has been hospitalized with the first recorded case of transmission of a rare strain of bird flu, known as H10N3.

Workers vaccinate chicks with the H9 bird flu vaccine at a farm in Changfeng county, Anhui province, April 14, 2013.

A 41-year-old man in China's eastern province of Jiangsu has been confirmed as the first human case of infection with a rare strain of bird flu known as H10N3, Beijing's National Health Commission (NHC) has said.

The man, a resident of the city of Zhenjiang, was hospitalized on April 28 and diagnosed with H10N3 on May 28, the health commission said on Tuesday, adding that his condition is stable.

It did not give details on how the man was infected but said investigation of his close contacts found no other cases and the risk of spread was very low.

What do we know about H10N3?

Little is known about the virus, which appears to be rare in birds, according to the Food and Agriculture Organisation (FAO), and does not cause severe disease.

The World Health Organization (WHO) said while the source of the patient's exposure to the H10N3 virus was not known and no other cases were found among the local population, there was no indication of human-to-human transmission yet.

Yet avian influenza viruses that have little impact on birds can be much more serious in people, such as the H7N9 strain that killed almost 300 people in China during the winter of 2016-2017. The WHO has said there had been only rare instances of person-to-person spread of the H7N9 virus.

What are the risks to humans?

The risk of further infection with H10N3 is currently believed to be very low, with experts describing the case as "sporadic".

Such cases occur occasionally in China which has huge populations of both farmed and wild birds of many species.

And with growing surveillance of avian influenza in the human population, more infections with bird flu viruses are being picked up.

In February, Russia reported the first human infection with the H5N8 virus that caused huge damage on poultry farms across Europe, Russia and East Asia last winter. Seven people infected with the virus were asymptomatic, authorities said.

Experts will be on alert for any clusters of H10N3 cases, but for now, a single case is not much of a concern.

"As long as avian influenza viruses circulate in poultry, sporadic infection of avian influenza in humans is not surprising, which is a vivid reminder that the threat of an influenza pandemic is persistent," the WHO told Reuters in a statement.

The strain is "not a very common virus," and only around 160 isolates of the virus were reported in the 40 years to 2018, according to Filip Claes, regional laboratory coordinator of the FAO's Emergency Centre for Transboundary Animal Diseases at the regional office for Asia and the Pacific.

Still, flu viruses can mutate rapidly and mix with other strains circulating on farms or among migratory birds, known as "reassortment," meaning they could make genetic changes that pose a transmission threat to humans.

H10N3: what happens now?

The genetic sequence of the virus that infected the patient has not yet been published, and will be needed to fully assess its risk.

Scientists will want to know how easily H10N3 can infect human cells to determine if it could become a greater risk. For example, the H5N1 variant that first infected people in 1997 has been the most deadly, killing 455 people globally so far.

It would only take a few mutations before the H5N1 variant gains the ability to spread easily from person-to-person, said Ben Cowling, professor at the School of Public Health at the University of Hong Kong, making it a high priority for surveillance.

Having the genetic information for the H10N3 variant would help assess if it was "close to being the type of virus we should be worried about", he said.