A new study of the H7N9 bird flu that has spread throughout China does little to ease fears that the virus could be transmissible from person to person.

Chinese health officials have confirmed a total of 132 cases of human infection with H7N9 and 43 fatalities as of June 30. Scientists aren't sure whether or not this new strain of bird flu is a “dead end” infection that could only jump from animals to humans. Now, a new paper published in the journal Science on Thursday shows that one strain of the virus isolated from a human patient passes efficiently between ferrets via respiratory droplets. The discovery is further evidence of the virus’ ability to jump from mammal to mammal, documented by another study published in Science in May.

While there’s no hard evidence yet that the virus can pass from one human to another, scientists worry that it could gain that ability in the future.

“If these viruses acquire the ability to efficiently transmit among humans, there is a high chance of an influenza pandemic, because humans have no immunity to H7N9 viruses,” the authors of the new study wrote.

The news comes on the heels of another study that suggests some strains of H7N9 already found in people are becoming resistant to the only antiviral drugs available to treat the infection. Drug resistance may be particularly problematic if the virus develops the ability to jump from person to person.

For their experiments, researchers from the Chinese Academy of Agricultural Sciences in Harbin and Gansu Agricultural University in Lanzhou collected 10,703 samples from poultry markets, farms, slaughterhouses, wild bird habitats, and pig slaughterhouses between March 30 and May 2 of this year. They sequenced the genomes of 37 representative H7N9 viruses from birds, and compared them to the genomes of five H7N9 strains taken from human patients.

All of the H7N9 viruses the scientists found came from birds sampled in live poultry markets – save for one case at a homing pigeon farm. Both the human and bird-derived strains of H7N9 could bind to human airway receptors, and some strains still retained the ability to attach themselves to bird airway receptors as well. The viruses isolated from birds didn't cause any noticeable ill effects in chickens, ducks or mice. But H7N9 viruses taken from human patients caused laboratory mice to lose up to 30 percent of their body weight.

The crux of the study, though came with the discovery that several viruses isolated from birds and humans could be passed between laboratory ferrets, with one particular human isolate, dubbed AH/1, able to pass between the mammals “highly efficiently.” The ferrets themselves suffered relatively mild effects from the viruses, with no marked changes in body temperature, and a weight change ranging from 1.1 to 7.6 percent.

“Our findings indicate nothing to reduce the concern that these viruses can transmit between humans,” the authors wrote.

Since the virus doesn't cause noticeable illness in birds, it can replicate itself and spread among the avian population silently, making it harder to track. While poultry market shutdowns in China may help curb the virus’s spread from birds to humans, H7N9 may continue to mutate in other animals and in people.

“Its replication in humans will provide further opportunities for the virus to acquire more mutations and become more virulent and transmissible in the human population,” the authors wrote.

SOURCE: Zhang et al. “H7N9 Influenza Viruses Are Transmissible in Ferrets by Respiratory Droplet.” Science published online 18 July 2013.