Lab experiments on a new strain of the H7N9 bird flu circulating in China suggest the virus can transmit easily among animals and can cause lethal disease, raising alarms that the virus has the potential for triggering a global human pandemic, researchers reported online October 19 in Cell Host & Microbe.
The H7N9 virus has been circulating in China since 2013, causing severe disease in people exposed to infected poultry. Last year, however, human cases spiked, and the virus split into two distinct strains that are so different they no longer succumb to existing vaccines.
One of these has also become highly pathogenic, meaning it has gained the ability to kill infected birds, posing a threat to agriculture markets.
U.S. and Japanese researchers studied a sample of this new highly pathogenic strain to see how well it spread among mammals, including ferrets, which are considered the best animal model for testing the transmissibility of influenza in humans.
In the study, flu expert Yoshihiro Kawaoka of the University of Wisconsin and colleagues tested a version of the new H7N9 strain taken from a person who died from their infection last spring.
They found that the virus replicated efficiently in mice, ferrets, and non-human primates, and that it caused even more severe disease in mice and ferrets than a low pathogenic version of the same virus that does not cause illness in birds.
To test transmissibility, the team placed healthy ferrets next to infected animals and found the virus spread easily from cage to cage, suggesting the virus can be transmitted by respiratory droplets such as those produced by coughing and sneezing.
Two out of three healthy ferrets infected in this way died, which Kawaoka said is “extremely unusual,” suggesting that even a small amount of virus can cause severe disease.
“The work is very concerning in terms of the implications for what H7N9 might do in the days ahead in terms of human infection,” said Michael Osterholm, an infectious disease expert from the University of Minnesota.
Since 2013, the H7N9 bird flu virus has already sickened at least 1,562 people in China and killed at least 612. Some 40 percent of people hospitalized with the virus die.
In the first four epidemics, the virus showed few changes. But last flu season, there were some 764 cases, nearly half of the 1,562 total.
“The whole world is worried about it,” Osterholm said.