August 7, 2013
Human-To-Human Transmissibility Discovered In H7N9 Bird Flu
Lawrence LeBlond for redOrbit.com - Your Universe Online
A team of Chinese researchers has discovered the first probable case of human to human transmission of H7N9 bird flu in China. While the news is startling, the researchers still stress that it does not spread efficiently in this manner.Published today in the British Medical Journal, the study findings provide the strongest evidence yet of H7N9 transmission between humans, a disease that has so far sickened 134 people and resulted in 43 deaths. The most recent case -- that of a 61-year-old woman -- was confirmed to the World Health Organization on July 20 by China's National Health and Family Planning Commission.
Nearly all cases of H7N9 bird flu have appeared to have visited live poultry markets or had been in close contact with live poultry seven to 10 days prior to developing the illness. While there is no definitive evidence linking sustained human-to-human transmission of this virus, the news of a probable case is no less unsettling.
Xian Qi, a virologist with the Department of Acute Infectious Disease Control and Prevention, Jiangsu Province Center for Disease Control and Prevention, Nanjing, Jiangsu, China, and colleagues reported a family cluster of two patients that had become infected with H7N9 bird flu in eastern China in March 2013.
The first patient -- a 60-year-old man -- had been known to visit live poultry markets on a regular basis and had become ill within six days after his last exposure. He was admitted to hospital on March 11. When his symptoms worsened by March 15, he was transferred to the hospital's ICU. He was then transferred to another ICU on March 18 where he remained until his death of organ failure on May 4.
The second patient -- the man's 32-year-old daughter -- had no known exposure to live poultry before her onset of illness. However, she had provided direct and unprotected care for her father in the hospital before he was moved to the ICU.
The woman developed her symptoms six days after her last contact with her father and was admitted to the hospital on March 24. She was transferred to the ICU on March 28 and died of organ failure on April 24.
Health experts, who were tasked with isolating the virus strains associated with each death, determined that both were nearly genetically identical, suggesting transmission from father to daughter. After 43 close contacts of both cases were investigated and tested for flu virus, one other person was determined to be infected; a son-in-law who had helped care for the father. He was found to have a mild form of the illness and later recovered. All other contacts tested negative for H7N9 infection.
In an accompanying editorial of the study, Dr. James Rudge, of the London School of Hygiene and Tropical Medicine, said that limited transmission between humans is not surprising as it has been seen in other bird flu viruses, notably H5N1.
Since the H7N9 virus first emerged in China, the most primary concern was whether it may be transmissible between humans. But the vast majority of the cases reported have so far seemed to be "epidemiologically unconnected," according to Rudge.
Nearly all cases seen have been due to reported contact with live poultry within days before the onset of illness. While the research team did find limited evidence of person-to-person transmission, it is not known if the 32-year-old woman did in fact become ill due to close contact with her father or if she had some exposure to a common animal source of infection.
However, added Rudge, occasional transmission between humans is the norm for most viruses rather than an exception where influenza only infects humans via close contact with an infected animal source. Limited human-to-human transmission was reported in the avian influenza H5N1 as well as bird flu subtype H7N7, which caused a mild outbreak in 2003 in The Netherlands. So to observe some human-to-human transmission in H7N9 is not a surprising discovery. However, this discovery does not set the stage for the virus to take a course toward sustained transmission between humans.
But then again, a recent study using a mammalian model showed that H7N9 does have the potential for a sustained human-to-human transmission.
Researchers from three distinct laboratories conducted H7N9 bird flu transmission studies on groups of ferrets, which have been regarded as the most accurate animal model in assessing human-to-human transmission of disease, and found similar results of transmissibility between ferrets.
For their study, Qi and colleagues tested environmental samples from poultry cages, water at two local markets and swans from a nearby residential area. One strain was isolated but it was genetically different to the two strains isolated in the father and daughter patients.
The researchers acknowledge that there are some study limitations, but maintain that the most likely explanation for the family cluster of two cases is that the virus "transmitted directly from the index patient to his daughter." But they stress that "the virus has not gained the ability to transmit itself sustained from person to person efficiently."
They concur that the most likely source of infection for the father was via the live poultry market.
"To our best knowledge, this is the first report of probable transmissibility of the novel virus person to person with detailed epidemiological, clinical, and virological data. Our findings reinforce that the novel virus possesses the potential for pandemic spread," concluded the authors.
In a video abstract summarizing the findings of this study, Dr M.H. Zhou noted that the reason researchers carried out this study was because there was "no definite evidence to show that the novel virus can transmit person-to-person" and wanted to find out whether H7N9 possesses the capability for human-to-human transmission.
She concluded that "the infection of the daughter is likely to have resulted from her father during unprotected exposure," but maintains that the infection was "limited and non-sustainable as there is no outbreak following the two cases."