โดย kiatisak » 18 เม.ย. 2013, 07:58
April 17, 2013
H7N9 silently spreads in humans and birds
The photo shows a H7N9-infected patient receiving treatment at an ICU in Huaihe Hospital of Henan University in Kaifeng, central China's Henan Province. The current H7N9 case fatality rate is approximately 20 per cent. Photo: AP
Only about 40 per cent of the 77 infected people had contact with poultry
The novel H7N9 avian flu virus that is currently circulating in certain regions in China has bewildered public health officials within and outside the country.
To start with, H7N9 is a product of reassortment of three avian influenza virus strains that “infect only birds.” Reassortment happens when gene swapping takes place between two or more viruses present at the same time in a host.
The influenza, which was initially restricted to Shanghai and neighbouring regions, has now reached Beijing — two people have so far been infected with the virus.
Till date, 77 people have been infected and 16 have died. But this number may be a gross underestimation of the actual spread of the infection. Therein begin the many puzzling and worrying characteristics of the bird flu.
Unlike the initial cases where the infection proved to be deadly, cases now being detected have wide ranging virulence. A 4-year-old boy has been tested positive for the virus on April 15, but shows no symptoms of infection. This is the first time that an asymptomatic case has been found. Unlike other avian flu infections and initial H7N9 infection cases, people appear to exhibit the entire range of infection — critical, mild and completely asymptomatic.
According to Gregory Hartl, Head of Media for WHO, the current H7N9 case fatality rate is “approximately 20 per cent,” and may end up even lower if the actual number of infected people is known.
Widespread?
But knowing the denominator is the biggest challenge. This is because, the presence of asymptomatic and mild cases raises the real possibility that the virus may be more widespread than believed and difficult to find. Though people with mild/asymptomatic infection may not be dying, such cases are, in fact, “very worrying,” notes Nature.
According to WHO, there is no way of knowing whether the number of cases identified represents “some or all of the cases actually occurring.” The occurrence of some “relatively mild cases” raises the possibility that there are “other such cases that have not been identified and reported.”
Reduced virulence may be facilitating “further genetic adaptation of the virus to infection of human beings — and thus greater potential to spread.”
According to a paper published on April 11 in The New England Journal of Medicine, genome sequencing of the first three cases of H7N9 infected people who died revealed that it is “better adapted” than other bird flu viruses to “infecting mammals.”
But the peculiar feature of the virus is that it causes only asymptomatic or mild disease even in birds. This allows the virus to silently spread among birds. The reason for this is now clear: the NEJM study indicated that the haemmagglutinin sequence data is associated with “low” pathogenicity in birds.
In the case of H5N1, birds falling sick after infection were clearly seen, and this helped in knowing the spread of the infection.
Exacerbating this enigma is not knowing which animals act as viral hosts. This is despite intense surveillance of animals to find out the reservoirs. “We can't be 100 per cent sure how anyone has contracted H7N9. Many patients had contact with poultry, but not all. So [it is] still a puzzle,” Hartl of WHO tweeted on April 13.
According to reports, about 40 per cent of infected people have had no contact with poultry.
The routes of transmission from animals to humans are not fully known either. But the NEJM paper provides certain clues. An amino acid substitution in H7N9 may facilitate transmission through respiratory droplets, just the way the highly pathogenic H5N1 bird flu spread from birds to humans.
Genome sequence of the first three cases showed that there have been “at least two introductions” from animals to humans.
Another peculiar aspect is that the number of people infected with H7N9 shot up from 24 to 63 within a short span of seven days. A reported increase of 14 infected cases on April 16 was the biggest ever for a single day.
Though sustained human-to-human transmission has not been found, two such “suspicious” cases have been found.
“We are not near a H7N9 pandemic yet but we need to understand better how the virus works in order to control the outbreak,” Hartl tweeted. “It is premature to dismiss the possibility of an H7N9 pandemic or to say the outbreak is under control.”
Keywords: H7N9, avian flu virus, China, virus infection
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