Genetic² interactions（相互作用） between avian（鸟类的） H5N1 influenza³ and human seasonal⁴ influenza viruses have the potential to create hybrid⁵（杂种的，混合的） strains combining the virulence⁶（毒性） of bird flu with the pandemic（流行的，普遍的） ability of H1N1, according to a new study. In laboratory experiments in mice, a single gene¹ segment from a human seasonal flu virus, H3N2, was able to convert the avian H5N1 virus into a highly pathogenic（致病的，病原的） form. The findings are reported the week of Feb. 22 in the online early edition of the Proceedings⁷ of the National Academy of Sciences.

"Some hybrids⁸ between H5N1 virus and seasonal influenza viruses were more pathogenic than the original H5N1 viruses. That is worrisome（令人烦恼的，麻烦的） ," says Yoshihiro Kawaoka, a virologist（滤过性病原体学者） at the University of Wisconsin-Madison and senior author of the new study.

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 Organization. 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 virus may provide that nature in the background of this highly pathogenic H5N1 virus," says Kawaoka, a professor of pathobiological sciences at the UW-Madison School of Veterinary Medicine.

Two viruses infecting a single host cell can swap⁹ genetic material, or reassort（再分类，再搭配） , creating hybrid strains with characteristics of each parent virus.

Before the current study, hybrid viruses generated in lab studies had always been less virulent¹⁰ 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 contagious¹¹（蔓延的，感染性的） .

The increased virulence seen in the new study seems to arise from one of the eight genes¹² in the viral genome, called PB2, which is known to affect how well the bird flu virus grows in mammalian（哺乳动物的） hosts, including humans. When tested in mice, the human virus version of PB2 swapped¹³ into H5N1 converted the avian virus to a highly pathogenic form.

The researchers say surveillance of viral populations is critical to monitor the potential emergence¹⁴ of highly pathogenic viral variants¹⁵ due to reassortment of avian and human influenza viruses. Their results, including identification of the PB2 segment as a key to enhanced virulence, offer information likely to be useful in the event of a pandemic caused by a hybrid avian-human influenza strain.

"With the new pandemic H1N1 virus, people sort of forgot about H5N1 avian influenza. But the reality is that H5N1 avian virus is still out there," Kawaoka says. "Our data suggests that it is possible there may be reassortment between H5 and pandemic H1N1 that can create a more pathogenic H5N1 virus."