As China faces its largest bird flu epidemic on record, a group of predominantly Chinese researchers last week published a paper in British medical journal The Lancet in which they concluded that H7N9 is spreading from urban to rural areas and increasingly affecting younger victims, suggesting potentially worrying changes in the virus’s epidemiology.
Avian influenza viruses can be carried by poultry and wild aquatic birds and transmitted to humans, often through contact with infected chickens or ducks. Cases of human-to-human transmission are rare. Infections can cause conjunctivitis, flu-like symptoms, pneumonia, and — in about 40 percent of cases — death. According to the World Health Organization, 1,486 laboratory-confirmed cases have been reported in China since early 2013, and this winter the country saw a record number of infections. The authors of the Lancet article believe H7N9, which is found almost exclusively in China, has undergone genetic mutations that change the way it affects humans.
Initially, H7N9 disproportionately affected the elderly and their less robust immune systems, as well as city-dwellers who frequented poultry markets, where birds that carried the virus were still sold because they did not show symptoms of infection. The duration of epidemic waves, which roughly overlap with each winter, was shorter, and the risk of death lower.
In recent years, however, more middle-aged adults (57 percent in the 2016-2017 wave compared with 41 percent in 2013) and more people in rural or semi-urban areas (over 60 percent since 2015-2016 compared with 39 percent in 2013) are becoming infected.
“A large epidemic in 2016-17 prompted concerns that the epidemiology of the virus might have changed, increasing the threat of a pandemic,” wrote the authors of the paper. One of these changes, according to Lisa Schnirring of the Center for Infectious Disease Research and Policy at the University of Minnesota, is a mutation that confers resistance to neuraminidase inhibitors, a common anti-viral flu treatment, and which may explain the highly pathogenic strain found today.
The research team compiled their data from an electronic database managed by the China Center for Disease Control and Prevention. They found that there were 447 laboratory-confirmed cases of H7N9 between Oct. 1, 2016 — the date marking the beginning of the fifth wave — and Feb. 23 of this year. While the fifth wave is still ongoing, this figure already exceeds the 306 infections detected during the 2013-2014 epidemic, the second-highest on record.
Chinese law requires that every case of bird flu be reported to the China CDC within 24 hours. In May, China’s National Health and Family Planning Commission, the agency that oversees health services for residents of and visitors to the mainland, reported an additional 23 cases of H7N9 to the WHO. Occurring over almost four weeks from April to May, the cases spanned 11 provinces, autonomous regions, and municipalities. Seven resulted in death and 15 in pneumonia of varying degrees of severity. Only one case was deemed mild.
Ben Cowling, a professor at Hong Kong University’s School of Public Health and one of the co-authors of the Lancet study, told Sixth Tone that because some cases of bird flu in humans are mild or untested, the number of confirmed infections is somewhat misleading. “There could be many more unreported infections by at least one order of magnitude,” he said. By this logic, the actual number of fifth-wave infections is likely closer to 4,470 than 447.
To Cowling, the most plausible explanation for the rise in confirmed infections this season has to do with the weather. H7N9 prefers colder temperatures, and with the Chinese winter beginning early in 2016, the virus has been able to “spread more widely, earlier, and reach a higher prevalence in poultry,” with the higher risk to humans being a secondary effect.
When cases of bird flu are detected in China, it is not uncommon for local authorities to close down poultry markets, where many victims become exposed to the virus. But this may actually be part of the problem, as research has shown that vendors denied the chance to sell their blighted birds in one place may simply move on to another where oversight is less strict, potentially spreading the virus further.
Virulence of H7N9 in humans has remained relatively stable in recent years, but in birds, it seems to have increased — a trend experts like Cowling say may actually be a boon. “This [newly emerging strain] has been easier to control because outbreaks of the virus are obvious to poultry farmers and traders,” he said, referring to the now-observable symptoms in some infected birds. “If all circulating H7N9 viruses became highly pathogenic, they would be much easier for health authorities to deal with.”
Until recently, infected chickens and ducks exhibited few if any symptoms of the virus, allowing it to spread stealthily through poultry populations. In fact, infections were rarely detected until farmers performed random checks on their stock of birds. Even when farmers became aware of the virus, there was little to stop them from selling the infected but asymptomatic animals.
Richard Webby and Yang Zifeng, authors of a Lancet editorial published alongside the research team’s paper, wrote that while there is still no evidence of sustained human-to-human transmission of H7N9, signs of the virus’s propagation in China are mounting. “If one considers human beings the canaries in the coal mine,” they wrote, “these findings imply that the virus is more widespread in poultry.”
“Experts agree that it is not a question of if, but when, the virus will adapt in ways that facilitate sustained human-to-human transmission,” said Bernhard Schwartländer, the WHO’s representative in China, echoing the concerns of Webby, Yang, and others. He told Sixth Tone it is “imperative” that the country’s policymakers identify effective methods to contain the virus, such as market closures and poultry vaccinations, and implement them “before the seasonal peak next winter.”
The WHO has advised people in affected areas to wash their hands often and avoid direct contact with birds at places like markets and poultry farms. Chinese state news agency Xinhua has recommended that consumers make sure the poultry they buy comes with a quarantine certificate.
Avian viruses like H7N9 will remain an ongoing concern for countries like China, where they are able to cause a large number of human infections by spreading mostly undetected in poultry, said Cowling. With such pervasive and prolonged exposure to people, he added, the virus is being given ample opportunity to stay one step ahead of human immune systems. If a substantial genetic shift were to occur, he said, the implications would be “potentially disastrous.”
Editor: Kevin Schoenmakers.
(Header image: A quarantine officer inspects a live chicken at a poultry farm in Xiangyang, Hubei province, Feb. 3, 2017. Rui Mu/VCG)