Experiments involving an avian H2N2 flu virus that caused a pandemic in the late 1950s and still circulates in birds found that it can infect mammalian cells and spread in ferrets, though it lacks the genetic markers for human adaptation, according to a study in the Journal of Virology. The goal was to assess the risk of the H2N2 virus, which hasn't been seen in humans since 1968.
Researchers from St. Jude Children's Research Hospital in Memphis analyzed 22 H2N2 viruses obtained from domestic and wild poultry from 1961 to 2008, the most comprehensive analysis so far of the virus. They found that the viruses haven't changed much since when H2N2 circulated as a pandemic strain. Most of the isolates they looked at replicated in mouse and human bronchial epithelial cells but showed little ability to replicate in swine tissues.
Several isolates replicated well in ferrets, and three spread through direct contact in the animals. Researchers classified one of the three strains, which was isolated in 1979 from a duck in Hong Kong, as having high pandemic potential. But no ferret-transmissible H2N2 strains spread by the airborne route.
Isolates still had a preference for cells in avian respiratory tracts, the team wrote, and the team's antiviral susceptibility tests found that all of the viruses were susceptible to neuraminidase inhibitors and adamantanes.
Co-author Jeremy Jones, PhD, said in a St. Jude press release that although the viruses appear avian, the study found that they can behave like mammalian flu strains. "That is troubling, because some of the H2N2 pandemic viruses looked avian when the pandemic began in 1957, but in a few short months, all of the isolated viruses had picked up the genetic signatures of adaptation to humans," he said. Jones added that the pattern could recur if the viruses jump from birds to humans again.
The group concluded that the greatest threat would be to people under age 50 who have no previous exposure to the virus. They also observed that isolates' close similarity to an H2N2 pandemic vaccine candidate could suggest that another outbreak in humans might be mitigated by a vaccine, though the changes in the virus warrant close surveillance.