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Scientists confirm transmission of influenza A H7N9 from birds to people

Virus showing signs of mammal adaptation, raising possibility of person to person infection

Caroline White

Friday, 26 April 2013

Scientists in China have confirmed for the first time that the influenza A H7N9 virus has been passed on from birds—specifically, chicken at a wet poultry market—to people, in research published in The Lancet

As yet there is no person to person transmission, but the virus is showing signs that it has adapted to mammals, raising the possibility that it will eventually be passed on from person to person, say the researchers.

Since its emergence in February 2013, 108 people are thought to have been infected with the H7N9 virus, which has now appeared in several different regions of China, and is thought to have killed 22 people. 

While early reports centred on cases in the Shanghai region, the new research details four confirmed cases of human H7N9 infection in Zhejiang, an eastern coastal province south of Shanghai. 

After it was discovered that a 39 year old admitted to hospital in the region was infected with the H7N9 virus, Chinese researchers conducted additional and retrospective testing of 486 other patients in three hospitals who had been admitted with respiratory infections.

This testing identified three further patients in the region who had been infected with H7N9, and the scientists proceeded to perform a comprehensive series of tests to characterise the infection in more detail.

All four of the patients had been exposed to poultry, either through their jobs or through visiting wet poultry markets. Swabs from 20 chickens, four quails, five pigeons, and 57 ducks, all from six wet poultry markets likely to have been visited by the patients, showed that two out of five pigeons (40%), and four out of 20 chickens (20%) tested positive for H7N9, but the virus was not found in any of the duck or quail tested.

After analysing the genetic make-up of H7N9 virus isolated from one of the patients, and from one of the wet market chickens, the researchers conclude that similarities between the virus isolates suggest that it is being transmitted sporadically from poultry to human beings.

This is the first time that definite bird to human transmission has been shown for the H7N9 virus. Unlike other types of avian influenza, no increase in poultry deaths appears to have been noticed before the onset of human infections.

Medical surveillance of 303 of the patients’ household and workplace contacts, as well as 82 health care workers who had dealt with the patients, revealed that no one else had developed symptoms within 14 days of the start surveillance, suggesting that the virus is not currently being passed on between people

But genetic analysis has shown that the virus has acquired some characteristics which adapt it specifically to infection in mammals, and the authors caution that further adaptation of the virus could lead to infections with less severe symptoms and more efficient person-to-person transmission.

The researchers note that the symptoms of H7N9 are broadly similar to those of H5N1 influenza, with patients presenting with high fevers and lower respiratory-tract symptoms (especially difficulty breathing), followed by respiratory failure within 3–14 days. Two of the patients died 1–3 weeks after onset of the symptoms; the other two patients were recovering in hospital at the time of publication. 

In light of these results, the authors recommend more aggressive measures to curb the chances of the virus adapting to more efficient transmission among people, including vaccination programmes to control its spread in poultry.

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