Human, bird flu virus interaction can create more virulent strains

WASHINGTON - Genetic interactions between avian flu virus and human seasonal flu viruses can potentially create hybrid strains, combining the virulence of bird flu with the pandemic ability of swine flu, according to a new study.

Two viruses infecting a single host cell can swap genetic material, or reassort, creating hybrid strains with characteristics of each parent virus.

In lab experiments with mice, a single gene segment from a human seasonal flu virus, H3N2, was able to convert the avian H5N1 virus into a highly pathogenic (causing infections) form.

“Some hybrids between H5N1 virus and seasonal influenza viruses were more pathogenic than the original H5N1 viruses. That is worrisome,” says Yoshihiro Kawaoka, virologist at the University of Wisconsin-Madison (UW-M) and senior study co-author.

The H5N1 bird flu virus has spread worldwide through bird populations and has caused 442 confirmed human cases and 262 deaths, according to the World Health Organisation. To date, however, bird flu has not been able to spread effectively between people.

“H5N1 virus has never acquired the ability to transmit among humans, which is why we haven’t had a pandemic. The worry is that the pandemic H1N1 (swine flu) virus may provide that nature in the background of this highly pathogenic H5N1 virus,” says Kawaoka.

Before the current study, hybrid viruses generated in lab studies had always been less virulent than parent strains.

However, the new findings raise concerns that H5N1 and pandemic H1N1 viruses could reassort in individuals exposed to both viruses and generate an influenza strain that is both highly virulent and contagious.

The increased virulence seen in the new study seems to arise from one of the eight genes in the viral genome, called PB2, which is known to affect how well the bird flu virus grows in mammalian hosts, including humans.

When tested in mice, the human virus version of PB2 swapped into H5N1 converted the avian virus into a highly pathogenic form.

The researchers say surveillance of viral populations is critical to monitor the potential emergence of highly pathogenic viral variants due to reassortment of avian and human influenza viruses, said an UW-M.

The findings were reported in Monday’s online edition of the Proceedings of the National Academy of Sciences.