A new strain of bird flu that has already claimed 37 lives in China has traits that make it a potential global threat, say scientists.
Researchers tested the ability of the H7N9 virus to infect several mammal species including ferrets and monkeys. They found that as well as readily invading the lungs, it could be spread like seasonal flu by coughing and sneezing.
Professor Yoshihiro Kawaoka, from the University of Wisconsin-Madison, US, and Tokyo University in Japan, who led the international team, said: "H7N9 viruses have several features typically associated with human influenza viruses and therefore possess pandemic potential and need to be monitored closely."
Most bird flu viruses do not infect humans. But H7N9 has so far infected at least 132 people, more than a fifth of whom have died. Several instances of human-to-human infection are suspected.
In monkeys the H7N9 virus efficiently infected cells in both the upper and lower respiratory tract, the scientists reported in the journal Nature. Most human flu viruses are restricted to the upper airway of non-human primates.
Transmission studies were conducted in ferrets, which like humans possess the ability to cough and sneeze. One of the H7N9 sub-strains isolated from humans was found to transmit via respiratory droplets, though not as efficiently as human flu viruses.
"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," said Prof Kawaoka.
A major problem associated with the virus is that it infects poultry birds without killing them, making surveillance difficult.
"We cannot simply watch out for sick or dead birds," said Prof Kawaoka. "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."
A more positive outcome from the research was the discovery that most H7N9 strains are sensitive to the antiviral drugs effective against seasonal flu. One strain, which appeared to be a mix of two H7N9 variants, showed signs of resistance against the drug Tamiflu.