H7N9, the emerging avian influenza virus responsible for at least 37 deaths in China, has the potential to spark a global outbreak, according to a new study published in the journal Nature.

A comprehensive analysis of two of the first human isolates of the virus from patients in China by an international team of researchers reveal that the virus can both infect and replicate in several species of mammals, including ferrets and monkeys, as well as transmit to ferrets, suggesting that the virus holds the potential to become a worldwide threat to humans.

"H7N9 viruses have several features typically associated with human influenza viruses and therefore possess pandemic potential and need to be monitored closely," Yoshihiro Kawaoka, one of the world's leading experts on avian flu and a professor at the University of Wisconsin-Madison and the University of Tokyo, said in a press release.

With the exception of the highly pathogenic H5N1 strains, avian influenza viruses typically do not infect humans. However, the H7N9 virus has so far infected at least 132 humans, killing more than 20 percent of those infected, with several suspected instances of human-to-human infection, according to the researchers.

Based on the results of their study, the scientists hypothesize that the ability of the H7N9 virus to infect and replicate in human cells may be due to just a few amino acid changes in the genetic sequence of the virus.

"These two features are necessary, although not sufficient, to cause a pandemic," Kawaoka said, explaining that the influenza virus depends on host cells, which it hijacks to make new virus particles and thus sustain the chain of infection.

In the case of monkeys, the H7N9 virus was shown to efficiently infect cells in the upper and lower respiratory tract, while conventional human flu viruses are usually restricted to just the upper airway of infected primates.

"If H7N9 viruses acquire the ability to transmit efficiently from person to person, a worldwide outbreak is almost certain since humans lack protective immune responses to these types of viruses," Kawaoka said.

In studying the transmission of the disease between ferrets, which, like humans, infect each other through coughing and sneezing, the researchers found that one of the H7N9 strains isolated from humans can transmit via respiratory droplets, though not as efficiently as human influenza viruses. Though limited, this aerosol transmission only adds to concerns about the potential threat as avian flu viruses typically lack this capacity, Kawaoka notes.

"H7N9 viruses combine several features of pandemic influenza viruses, that is their ability to bind to and replicate in human cells and the ability to transmit via respiratory droplets," Kawaoka says.

The fact that the virus fails to kill the birds it infects promises to make surveillance much more difficult.

"We cannot simply watch out for sick or dead birds," Kawaoka said. "Rather, tests have to be performed to determine whether or not a bird is infected. Considering the vast number of poultry, this is a daunting task."

The report was not completely without positive news, however: the scientists found that most of the H7N9 strains tested were somewhat sensitive to antiviral drugs effective against the seasonal flu virus, although one isolate, which appears to be a mix of two variants of the H7N9 virus, seemed to resist neuraminidase inhibitors like Tamiflu.

Ultimately, in order to understand the virus and its adeptness at infecting humans, as well as to develop vaccines to guard against it, further research is needed.