Genetic changes and a leap in case numbers are fueling concerns that China might be on the brink of a major outbreak of avian flu.
The current Chinese flu season, which kicked off in October last year, has so far seen at least 450 confirmed cases of influenza A(H7N9), a virulent and often lethal form that first emerged in 2013, and is strongly linked to poultry markets.
According to virologist Ian Mackay of the Australian Infectious Diseases Research Centre at the University of Queensland, the period “has seen more human cases than any of the previous three seasons”.
Using data from the Hong Kong Centre for Health Protection, MacKay established that most of the confirmed cases were from five provinces, with Jiangsu the worst hit. Almost all are associated with bird markets, although rare cases of human-to-human transmission have been recorded.
In the first outbreak of A(H7N9) the initial spread was explosive, growing from just three to 111 in two months. Of those, more than 70% became seriously ill and around 27% died.
While acknowledging problems with the quality and reliability of the data, MacKay estimates more recent surges of the same flu have delivered a mortality rate of around 40 per cent. Compared to the last season, he notes, the current tally of confirmed cases “dwarfs by quite a lot”.
Because they have the capacity to spread rapidly, flu viruses are always likely to undergo mutation. While most such changes are likely to be either neutral or harmful to the virus itself, exacerbating worries at present is the fact that A(H7N9) seems to have altered in two significant ways.
“Just lately two viral changes have added to concerns,” says MacKay. “One has seen a viral variant that now causes serious illness in birds. Previously, H7N9 spread through flocks unseen because it didn’t cause avian disease.
“Another change has seen up to 9% of tested variants with mutations suggesting they may resist a common class of anti-flu drugs.”
Thankfully, he adds, neither of these changes is known to alter how the virus transmits to humans.
Because of them, though, health authorities are recommending existing vaccines be tweaked to take the new variants into account.
As in past outbreaks the Chinese authorities have moved to close down poultry markets in affected areas. However, says MacKay, there are concerns this time that the spread of infection may have outpaced public health.
“Closing live bird markets has, in the past, been followed by a precipitous decline in human cases,” says. “This season of H7N9 cases spread perhaps too quickly for the response to keep up. Closure of markets confirmed to harbour H7N9-infected poultry may not have occurred in sufficient numbers or fast enough to prevent so many human exposures.”
New figures on flu cases are released each week. Given that the World Health Organisation has described H7N9 as “an unusually dangerous virus for humans”, MacKay is by no means the only scientist keeping a careful eye on the tally.