H7N9 flu virus found to transmit from birds to humans but not person-to-person

The LancetApr 25 2013

Scientists in China have confirmed for the first time that the influenza A H7N9 virus has transmitted from birds—specifically, chicken at a wet poultry market—to humans, according to an Article published Online First in The Lancet. 

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 Lancet report details four confirmed cases of human H7N9 infection in Zhejiang, an Eastern coastal province south of Shanghai. 

After it was initially found that a 39 year old patient admitted to hospital in the region was infected with the H7N9 virus, a team of researchers led by Professor Lanjuan Li, of Zhejiang University, Hangzhou, China, and Professor Professor Kwok-Yung Yuen of the University of Hong Kong, China, conducted additional and retrospective testing of 486 further patients in three hospitals who had been admitted with symptoms of respiratory infection. 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 terms of its clinical symptoms, duration, and genetics.

All four of the patients had been exposed to poultry, either through their occupation or through visiting wet poultry markets. To determine whether the virus might be transmitting from poultry to humans, the researchers took cloacal 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. 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 ducks or quails tested.

After analysing the genetic makeup of H7N9 virus isolated from one of the patients, and comparing it to an H7N9 virus isolate taken 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 came into unprotected contact with any of the patients, revealed that nobody else who came into contact with the H7N9-infected patients began to show any symptoms within 14 days from the beginning of surveillance, suggesting that the virus is not currently able to transmit between human beings. However, 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.

Speaking to The Lancet podcast, co-lead author Professor Kwok-Yung Yuen said, “Overall, the evidence, in terms of epidemiology and virology, suggests that it is a pure poultry to human transmission, and that controlling [the epidemic in humans] will therefore depend on controlling the epidemic in poultry.”

Providing a detailed clinical characterisation of the patients’ symptoms and outcomes, 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 outline the best course of action to minimise the chances of the virus adapting to more efficient transmission between humans, suggesting that, “Aggressive intervention to block further animal-to-person transmission in live poultry markets, as has previously been done in Hong Kong, should be considered. Temporary closure of live bird markets and comprehensive programmes of surveillance, culling, improved biosecurity, segregation of different poultry species, and possibly vaccination programmes to control H7N9 virus infection in poultry seem necessary to halt evolution of the virus into a pandemic agent.”