Chinese pigs are more and more frequently becoming infected with a strain of influenza that has the potential to jump to humans, could mutate further so that it can spread easily from person to person and trigger a global outbreak, a new study has found.
The study, which was published in the Proceedings of the National Academy of Sciences, focuses on an influenza virus named G4. G4 is genetically descended from the H1N1 strain that caused a pandemic in 2009.
Scientists at Chinese universities and China’s Center for Disease Control and Prevention say it possesses “all the essential hallmarks of being highly adapted to infect humans” and needs close monitoring.
“It is of concern that human infection of G4 virus will further human adaptation and increase the risk of a human pandemic,” the researchers have said.
The virus is a unique blend of three lineages: one similar to strains found in European and Asian birds, the H1N1 strain that caused the 2009 pandemic, and a North American H1N1 that has genes from avian, human, and pig influenza viruses.
The G4 variant is especially concerning because its core is an avian influenza virus—to which humans have no immunity—with bits of mammalian strains mixed in.
A team led by Liu Jinhua from the China Agricultural University (CAU) analysed nearly 30,000 nasal swabs taken from pigs at slaughterhouses in 10 Chinese provinces and 1,000 swabs from pigs with respiratory symptoms seen at their school’s veterinary teaching hospital from 2011 to 2018.
It was a part of a project to identify potential pandemic influenza strains.
The swabs collected yielded 179 swine influenza viruses, the vast majority of which were G4 or one of five other G strains from the Eurasian avianlike lineage.
The researchers carried out various experiments including on ferrets, which are widely used in flu studies because they experience similar symptoms to humans such as fever, coughing and sneezing.
They found that G4 was highly infectious, replicating in human cells and causing more serious symptoms in ferrets than other viruses. Tests also showed that any immunity humans gain from exposure to seasonal flu does not provide protection from G4.
According to blood tests, which showed up antibodies created by exposure to the virus, 10.4% of swine workers had already been infected. The tests showed that as many as 4.4% of the general population also appeared to have been exposed.
The virus has therefore already passed from animals to humans but there is no evidence yet that it can be passed from human to human — the scientists’ main worry.
They have called for strengthening the surveillance of Chinese pigs for influenza viruses and for urgent measures to monitor people working with pigs.