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Genetic Analysis Indicates Likely Person-to-Person Transmission of Avian Flu

NEW YORK (GenomeWeb) – Genetic analysis performed by a team of Chinese researchers indicates that the avian flu was likely transmitted between two unrelated people in a hospital ward.

A 49-year-old man in China was admitted to the hospital and ultimately diagnosed with the H7N9 influenza virus, and one of his ward mates, admitted for another condition, subsequently came down with the flu. To trace how these men became infected, the Beijing Institute of Microbiology and Epidemiology's Wu-Chun Cao and his colleagues turned to a combination of epidemiological and genetic approaches, finding that the two patients harbored nearly identical viral sequences, as they reported yesterday in the British Medical Journal.

"Although we cannot completely rule out an unidentified environmental exposure that might explain the H7N9 infection in the second patient, our findings suggest that H7N9 virus transmission occurred in the hospital setting between two unrelated people," Cao and his colleagues wrote in their paper.

Previous instances of suspected human-to-human transmissions of avian flu occurred within families and may have been due to shared exposure or shared genetic susceptibility to infection, they noted.

Human infections of H7N9 influenza emerged in February 2013, with three epidemic waves affecting 17 provinces and cities in China. As of earlier this year, the researchers noted there have been 672 laboratory-confirmed cases with 271 deaths reported.

The index patient in their study became ill with a fever, cough, and sore throat last February. After visiting two village clinics, he was admitted to a district hospital for treatment. He was then transferred to a provincial hospital where he was diagnosed with H7N9 through an RT-PCR test. After confirmation, he was moved to a specialist hospital.

Meanwhile, the second patient was admitted to the same district hospital for his chronic obstructive pulmonary disease. There, he and a third patient with diabetes shared a room with the index patient. The second patient was discharged following an improvement in his COPD, but he soon became ill and was admitted to the negative pressure ward of a fourth hospital, as he'd been exposed to H7N9.

Cao and his colleagues performed contact tracing for both patients and monitored their close contacts. They also collected environmental samples for analysis.

As the index patient recently bought two chickens from a local live poultry market for his daughter's wedding ceremony, they collected fecal and cage swab samples from them as well as samples from that market and nearly a dozen other environmental samples.

The second patient hadn't been in contact with live poultry, though the researchers collected samples from eggs in his refrigerator as well as other surfaces in his house and from his neighbor's chickens and a chicken farm a kilometer away.

The researchers then analyzed these samples using a combination of rRT-PCR assays targeting the influenza matrix gene and sequencing on the Ion Torrent PGM.

The fecal samples obtained from the chickens at the index patient's house as well as five of the 11 samples taken from the live poultry market he visited were positive for H7N9. None of the samples, meanwhile, taken from the second patient's home or nearby neighborhood tested positive.

The virus obtained from a throat swab of the index and second patients were fully sequenced and compared. Based on this, the researchers noted a one-basepair difference — a difference that didn't lead to an amino acid change — in the polymerase segment between the patients' viruses.

Similarly, there was a one-basepair difference between their hemagglutinin segments and a two-basepair difference between their neuraminidase segments.

The samples from the poultry market exhibited additional differences, though a maximum likelihood phylogenetic analysis found that those samples clustered together in a single clade. Further, they reported that that clade was genetically similar to viruses that circulated in chickens in 2014.

Receptor binding specificity assays also indicated that the strain could infect both people and birds, the researchers added.

"Our findings strongly suggest that the live poultry market is the most probable source of influenza H7N9 virus infection for the index patient, because no other animals were kept at his home and he had no exposure to poultry before he bought the two chickens from the market," Cao and his colleagues wrote.

They added that the second patient's proximity and underlying COPD likely contributed to him contracting influenza.

This person-to-person transmission also underscores the threat to public health, the researchers said.

"We must remain alert for (re)emerging infections, including avian influenza, particularly when we still cannot tell how risks to humans will evolve," added Marianne van der Sande and Wim van der Hoek from the Dutch National Institute for Public Health and the Environment in an accompanying editorial. "We also need to invest more in clinical, epidemiological, and virological research to unravel the risks posed by sporadic human infections with any avian influenza virus.

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