By Maggie Fox, Senior Writer, NBC News
Six?more people have been diagnosed with a new strain of bird flu in China ,?officials said, and one of them has died, bringing the death toll from the new outbreak to three.
That makes nine human cases of H7N9 bird flu, all in the eastern part of the country around Shanghai. This is a different strain of bird flu from H5N1 avian influenza, which has killed 371 people out of 622 infected in 15 countries since 2003.
The new cases are reported in China's eastern Jiangsu Province, including a woman in Nanjing who works killing poultry. All are in critical condition, according to the official Chinese news agency Xinhua. It quotes officials as saying more than 300 people who were in close contact with?the six new victims don?t appear to have any flu symptoms.
That suggests -- but doesn?t prove -- that the virus isn?t passing from person to person.
The World Health Organization says genetic studies suggest the virus has mutated slightly, to make it more easily able to infect mammals, including humans.
On Sunday, China reported three H7N9 bird flu infections: two in Shanghai and one in Anhui. The two Shanghai victims died and the third patient is in critical condition, Xinhua says. There?s no suggestion yet that they infected any of their friends or relatives, either, Chinese officials have said.
"These are the first reported cases of A(H7N9) in humans. That makes it a unique event, which the World Health Organization is taking seriously," WHO said in a statement posted on its website. "WHO is working closely with the national authorities to better understand the situation and will communicate important updates as they become available."
Other H7 type viruses have infected people on very rare occasions, WHO says.
"From 1996 to 2012, human infections with H7 influenza viruses (H7N2, H7N3, and H7N7) were reported in Netherlands, Italy, Canada, USA, Mexico and the United Kingdom. Most of these infections occurred in association with poultry outbreaks," WHO said in a statement posted on its website Wednesday.
"The infections mainly resulted in conjunctivitis and mild upper respiratory symptoms, with the exception of one death, which occurred in the Netherlands. Until now, no human infections with H7 influenza viruses have been reported in China."
Doctors keep a very close eye on cases of animal flu that pass into humans. Seasonal flu causes an annual pandemic that kills tens of thousands of people globally every year. But if a new virus starts passing from animals to humans, it can cause far more serious disease.
For instance, H5N1 kills about 60 percent of the people it infects. Luckily, it doesn?t pass easily from person to person, either, and most people who got it appear to have been directly infected by sick chickens.
But flu can mutate very quickly and it?s possible that a bird or animal strain of flu could develop the ability to pass quickly from one person to another. H1N1 swine flu appears to have done this in 2009. It killed a greater than usual number of young adults and children that year, and has now joined the mix of annual human flu strains.
And H1N1 is an indirect descendant of the 1918 ?Spanish flu?, which killed anywhere between 50 million and 100 million people. So public health experts take the risk of another such pandemic seriously.
Chinese officials have not said what they are doing to see if the H7N9 virus is spreading among poultry flocks. H5N1 pops up among chickens regularly, forcing mass culls.
?Since its widespread re-emergence in 2003 and 2004, this avian virus has spread from Asia to Europe and Africa and has become entrenched in poultry in some countries, resulting in millions of poultry infections, several hundred human cases, and many human deaths,? the World Health Organization says.
?Outbreaks in poultry have seriously impacted livelihoods, the economy and international trade in affected countries.?
H5N1 infects ducks without causing any symptoms. As long as an influenza virus exists in an animal, it will be steadily mutating and also swapping genes with other viruses.
That?s what happened with H1N1 swine flu in 2009. It was a never-before-seen mixture of human, pig and bird viruses. It wasn?t nearly as deadly as other new flu viruses that cause pandemics, something that could lead people to believe that flu pandemics aren?t that big a deal. But H5N1 and H7N9 are completely new to the human body and appear to be more deadly than seasonal flu.
While there are vaccines against seasonal flu, there are only experimental human vaccines for H5N1 and none for H7N9. Flu mutates so quickly that it?s impossible to formulate vaccines until a strain is actually circulating. Researchers are working to develop a universal flu vaccine that could protect people against a range of strains, or even all strains of influenza.
But most types of flu, including H7N9,?appear to be helped at least a little by two antiviral drugs -- oseltamivir or Tamiflu, and zanamivir or Relenza. Neither is a cure, but given early enough can relieve the worst symptoms, WHO says.
Influenza A viruses are named based on two of their genes - the hemagglutinin, or H gene, and neuraminaiase, or N. Human seasonal flu has been?caused by H1N1, H3N2 and H2N2 type viruses, as well as influenza B.
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