H7N9 as defined by Wikipedia, is a bird flu strain of the species influenza A (avian influenza virus or bird flu virus). Avian Influenza A H7 viruses normally circulate amongst avian populations with some variants known to occasionally infect humans.

An analysis of viruses from the second wave of H7N9 avian flu in China that began in late 2013 shows a rapid expansion both geographically and in genetic diversity, which poses a challenge to disease control and demonstrates the potential of H7N9 to emerge as a pandemic strain in humans, according to a study summarized in a letter to Nature today.

An international group of researchers obtained swabs at live-poultry markets in 15 cities of five provinces in eastern China from October 2013 to July 2014.  H7N9 was detected in swabs from seven cities, all of which have confirmed human cases. The virus was found in an average of 3% of samples, with a high occurrence of 15.4% one month in the city of Nanchang in Jiangxi province.

The investigators also sequenced the complete genomes of 438 isolates and found that as H7N9 spreads south, it will evolve into three main phylogenetic branches, or clades, each with multiple sub-branches.

They were able to identify many new genetic variants that have become established in chickens and have spread across the country, most likely from poultry movement along trade routes. The researchers propose that control measures such as permanently closing live-poultry markets and inhibiting poultry transport during disease outbreaks will reduce the number of human cases of H7N9.

“Repeated introductions of viruses from Heilongjiang to other provinces and the presence of H7N9 viruses at live-poultry markets have fueled the recurrence of human infections. This rapid expansion of the geographical distribution and genetic diversity of the H7N9 viruses poses a direct challenge to current disease control systems,” the authors summarize.

And they conclude that, “Our results also suggest that H7N9 viruses have become enzootic in China and may spread beyond the region, following the pattern previously observed with H5N1 and H9N2 influenza viruses. . . . Therefore, H7N9 viruses should be considered as a major candidate to emerge as a pandemic strain in humans.”