H7N9 virus is more transmissible and harder to detect than H5N1, say experts

BMJ 2013; 346 doi: (Published 22 April 2013) Cite this as: BMJ 2013;346:f2568
  1. Jane Parry
  1. 1Hong Kong

In the two months since it was first detected, the H7N9 avian flu virus has resulted in almost twice as many confirmed infections in China as the H5N1 bird flu virus caused in a decade. As at 19 April the Chinese authorities had confirmed 87 cases in humans of H7N9 infection, including 17 deaths.

“The first case of H5N1 was confirmed in 2003, and there have been only 45 confirmed cases in the whole of China. H7N9 is much more transmissible to humans, and it’s much more difficult to track down,” said Ho Pak-leung, associate professor in the Department of Microbiology at the University of Hong Kong.

Of the 47 800 samples that have been taken from more than 1000 farms and poultry markets and other habitats, only 39 samples tested positive for H7N9, said a statement by China’s Ministry of Agriculture. “We don’t understand why it’s so difficult to find,” said Ho.

“I don’t think it’s related to sampling technique or testing level. It could be that the infected animals might not shed virus for more than a few days, so it’s a matter of chance if you test them and find it. It might be that they are not sampling enough animal species, and they may have to take a look at the less common species of birds being sold in Chinese markets,” he said.

The fact that there has been evidence of limited human to human transmission, as was the case with H5N1, was also worrying, said Ho. Also, some cases could not be tied to exposure to poultry. “This is one of the puzzles still to be solved and therefore argues for a wider investigation net,” a World Health Organization spokesman, Gregory Hartl, told the Reuters news agency.

WHO has sent an international team of experts to Beijing to help the Chinese authorities investigate the outbreak, and the UN Food and Agriculture Organization is also assisting the authorities in the investigation.

Migratory birds have been implicated in the spread of the virus. He Hongxuan, a principal investigator at the Institute of Zoology with the Chinese Academy of Sciences, told state media that the spread of the virus from the Yangtze River delta northwards follows the migration pattern of wild waterfowl. As these birds migrated south, additional cases in Hong Kong and other parts of the world were very likely, said Ho.

However, it is the sale of live poultry that puts people at risk, he said. “We have to continue to watch the situation closely. Once human cases are identified, the most effective measure to stop more infections is to suspend the sale of live poultry.”

Besides putting four complete genome sequences of the virus into the public domain on an online genome database, the Chinese authorities have also published an article in the New England Journal of Medicine presenting the first three human cases.1


Cite this as: BMJ 2013;346:f2568