“That’s certainly not good news,” said Fouchier, who reviewed a gene sequencing of H7N9 published by Chinese health authorities. “This virus really doesn’t look like a bird virus anymore; it looks like a mammalian virus.”

To curb the spread of H7N9, officials in Shanghai have halted trading in live poultry, closed bird markets and slaughtered more than 20,000 birds. The outbreak, in which 16 people have been infected, caused soybean futures and airline stocks to fall yesterday on concern the virus may spark a pandemic.

While there’s no evidence yet of human-to-human transmission, scientists are scrutinizing the virus’s genetic makeup for clues to the threat it may pose.

Fouchier authored a study last year that showed five genetic tweaks to the deadly H5N1 virus, which has killed more than 600 people since 2003, made it airborne in ferrets, the mammals whose response to flu most closely resembles that of humans.

‘More Concern’

One of the mutations he made is in an enzyme called polymerase; another was in a protein called hemagglutinin on the surface of the virus. H7N9 has both mutations, he said.

“This virus is certainly of more concern than the vast majority of bird flu viruses,” Fouchier said. “Most bird flu viruses that we know do not have these mutations.”

Whether those mutations alone are enough to make the virus easily transmissible isn’t clear, and should be “high on the research agenda,” Fouchier said. Still, there’s no evidence yet that the virus is more likely to become more dangerous than H5N1, he said.

“Even if we see relatively high numbers of human cases, it doesn’t mean a pandemic is imminent,” he said. “H5N1 has circulated for 16 years and not become mammal-to-mammal transmissible.”

Heightened Vigilance

As part of its surveillance effort, China needs to conduct blood serum testing among all people who have been in contact with confirmed cases to look for antibodies that form in response to an infection, said Roy Anderson, a professor of infectious disease epidemiology at Imperial College London. That will indicate how many people have been infected with the virus, including those who aren’t showing symptoms, he said.

“Heightened vigilance needs to be in place at the moment,” said Anderson. While no human-to-human transmission has yet occurred, once an epidemic gets established, the doubling time can be very fast. “That’s why much needs to be done very thoroughly at the beginning to ascertain whether this is a risk or not.”

Unlike H5N1, which is highly lethal for birds, H7N9 is a so-called low-pathogenic virus in birds, meaning it may be widespread without causing severe sickness, Fouchier said. That would make it difficult to eradicate, he said. It doesn’t necessarily follow that the virus will be mild in humans, he said.

Spanish Flu

The Spanish flu of 1918, which killed about 50 million people worldwide, wasn’t highly pathogenic in birds, he said. He and colleagues plan to test the new virus in ferrets to see how deadly and how easily transmissible it is, and to test vaccines and antiviral drugs against it.

H7N9 also is more difficult to track because it’s not highly lethal to birds, said Alex Thiermann, technical adviser to the director general of the World Organisation for Animal Health in Paris.

“That indicates we need to take very careful surveillance measures because it will not be as obvious as in 2001,” Thiermann said in a telephone interview yesterday. “Symptoms are not going to help us. The Chinese are doing an intensive surveillance on poultry, pigs and wildlife. We need to continue to do that intensively.”