SHANGHAI, CHINA—An avian influenza virus that emerged in 2013 is suddenly spreading widely in China, causing a sharp spike in human infections and deaths. Last month alone it sickened 192 people, killing 79, according to an announcement this week by China's National Health and Family Planning Commission in Beijing.
The surge in human cases is cause for alarm, says Guan Yi, an expert in emerging viral diseases at the University of Hong Kong in China. "We are facing the largest pandemic threat in the last 100 years," he says.
As of 16 January, the cumulative toll from H7N9 was 918 laboratory-confirmed human infections and 359 deaths, according to the World Health Organization (WHO). Despite its high mortality rate, H7N9 had gotten less attention of late than two other new strains—H5N8 and H5N6—that have spread swiftly, killing or forcing authorities to cull millions of poultry. But so far, H5N8 has apparently not infected people; H5N6 has caused 14 human infections and six deaths.
All human H7N9 cases have been traced to exposure to the virus in mainland China, primarily at live poultry markets. The strain likely resulted from a reshuffling of several avian influenza viruses circulating in domestic ducks and chickens, Guan's group reported in 2013. Studies in ferrets and pigs have shown that H7N9 more easily infects mammals than H5N1, a strain that sparked pandemic fears a decade ago. There have been several clusters of H7N9 cases in which human-to-human transmission "cannot be ruled out," but there is "no evidence of sustained human-to-human transmission," according to an analysis of recent developments that WHO posted online last week. WHO’s analyses of viral samples so far "do not show evidence of any changes in known genetic markers of virulence or mammalian adaptation," WHO's China Representative Office in Beijing wrote in an email to Science.
Still, there are worrisome riddles. One is that H7N9 causes severe disease in people but only mild or even no symptoms in poultry. The only previous example of that pattern, Guan says, is the H1N1 strain responsible for the 1918 flu pandemic, which killed 50 million to 100 million people.
Because poultry infected with H7N9 show few symptoms, the virus has spread stealthily, coming to the attention of authorities only after human victims appeared. Determining where the virus is circulating requires testing chickens and collecting environmental samples from live poultry markets.
Human infections have followed a consistent pattern, dropping to zero during summer, picking up in the fall, and peaking in January. During the fifth wave of H7N9 that began last fall, authorities noticed an early and sudden uptick in cases, with 114 human infections from September to December 2016, compared with 16 cases during the same months in 2015 and 31 in 2014, according to a surveillance report. The report notes that the virus has spread geographically, with 23 counties in seven eastern Chinese provinces reporting their first human cases last fall.
"It is too late to contain the virus in poultry," Guan says. He predicts that the virus will continue to spread in China's farms, possibly evolving into a strain that would be pathogenic for poultry. Authorities have culled more than 175,000 birds this winter to stamp out local outbreaks of H7N9 and other avian flu strains. Further spread of H7N9 "will naturally increase human infection cases," Guan says.
H7N9 may also spread beyond China's borders, either through the poultry trade or through migratory birds. The virus has not been reported in poultry outside China. However, warns WHO's Beijing office, "continued vigilance is needed."