The new H7N9 avian influenza, which has infected more than 120 people and killed at least 27, appears to have evolved as wild birds mingled with one another and with domestic flocks, Chinese researchers reported on Wednesday.
A genetic analysis of the virus shows strains from at least four different sources – all birds – mixed together to make a new strain that likely arose in January of this year, the Chinese team reports in the Lancet medical journal.
Their report underscores that new flu viruses can pop up quickly and unpredictably. While most never gain the ability to infect humans, sometimes they do, and very occasionally they cause pandemics that kill millions.
“We hypothesize that the H7N9 viruses that infected human beings resulted from a reassortment of avian influenza viruses of at least four origins,” George Gao of the Chinese Center for Disease Control and Prevention and colleagues wrote in their report.
They found at least two duck and two chicken genes in the virus. Scientists regularly test birds and other animals for viruses and publish them in libraries that other scientists can access. Flu viruses have just eight genes, so the analysis can be fairly simple.
The new H7N9 is a blend of several – some from wild birds in both Asia and Europe, the researchers said. They look especially closely at two genes that give influenza A viruses their names - -the hemagglutinin gene (the “H” in a virus name) and the neuraminadase gene (the “N”).
As best they can tell, the “H” gene come from wild birds and ducks in Asia and the “N” gene comes from ducks that migrated from Europe and infected wild birds. “Wild ducks, especially mallards, were frequently infected with or carried H7 viruses and mallards and spot-billed ducks often mix together in a very large colony for molting and wintering,” the researchers noted.
Another bird flu virus called H9N2 was circulating in chickens and ducks in eastern China, and the viruses all mixed as the birds mingled. This happens all the time with flu viruses -- they infect birds and pigs, often causing no symptoms.
“After these reassortment events, the new viruses started to circulate in chickens, with low pathogenicity. Furthermore, we propose that these reassortment events most probably took place in Shanghai or the adjacent provinces, such as Zhejiang or Anhui,” the researchers wrote.
It’s the low pathogenicity that worries many experts. This virus doesn’t seem to make birds sick, making it hard to track. The other bird flu virus that health officials have been watching, H5N1, kills chickens pretty quickly, so it’s clear when it is circulating.
The findings show it’s important to step up testing of flocks of wild and domestic birds alike, to keep an eye out for this and other viruses that can infect people, the researchers said.
No one is sure precisely how people are becoming infected. Not all the sick have directly handled poultry, although at least one study suggests patients have had direct or indirect contact with birds and the live bird markets that are so common in China. And wild birds are, of course, everywhere.
The new H7N9 so far hasn’t infected anyone outside China, and it’s mostly found eastern China. The World Health Organization says one patient who was diagnosed in Taiwan almost certainly was infected in China and almost certainly did not infect anyone else on his flight home to Taiwan.
But H5N1 managed to spread from South Korea in 2003 to 15 other countries. It has now infected more than 600 people and killed more than 370 of them.
From 2003 to 2009, only 38 cases of H5N1 were reported in China. After two years of spread, by the end of 2005, it had infected 130 people in five countries, all in Asia, and killed 69 of them. H7N9 has infected more people more quickly, but that does not necessarily mean it is spreading faster or more easily.
“Influenza viruses constantly change and it’s possible that this virus could become able to easily and sustainably spread between people, triggering a pandemic,” the U.S. Centers for Disease Control and Prevention says on its website. CDC is already working on a vaccine against H7N9, but flu vaccines take months to prepare.
The agency has warned U.S. doctors to treat anyone with influenza who’s been to China with one of two flu drugs – Tamiflu or Relenza – right away, because the risk is so high.
The best way for people to protect themselves from any virus, the CDC says, is to wash hands frequently.