Reassortant H5N1 Avian Influenza Virus Bearing PB2 Gene From a 2009 Pandemic H1N1 Exhibits Increased Pathogenicity in Mice

Reassortment is a key driving force of the evolution and host adaptation of the influenza virus. A(H1N1)pdm2009 (pdm09), a novel H1N1 influenza viral subtype, caused a pandemic in 2009. The strain was established in pig herds and cocirculated with the highly pathogenic H5N1 avian influenza virus. The coexistence of pdm09 with H5N1 raises concerns that reassortment may cause the development of novel viral strains with unpredictable virulence. Given that the viral polymerase subunit PB2 is a determinant of host range and pathogenicity, and that the substantial amino acid differences in PB2 between pdm09 and H5N1, including positions 590/591 and 271, which are shown to play key roles in enhanced polymerase activity in mammalian host cells, we generated a reassortant virus containing PB2 derived from a pdm09 (A/Liaoning/1/2009, LN/09) to investigate if pdm09-derived PB2 can function in a heterologous avian virus isolate as an adaptive strategy, with H5N1 (A/duck/Hubei/hangmei01/2006, HM/06) as the backbone. We assessed the biological characteristics, including pathogenicity, replication, and polymerase activity, of the reassortant. Compared with HM/06 and LN/09, H5N1 hybrid virus containing PB2 from LN/09 exhibited significantly increased pathogenicity in mice and proliferation activity in mammalian cell lines, as well as markedly enhanced polymerase activity. Our results indicate that the coexistence of H5N1 and pdm09 may pose a great threat to public health through reassortment. Moreover, our results highlight the importance of monitoring the emergence of H5N1 reassortants containing pdm09-derived PB2.

Nosocomial Co-Transmission of Avian Influenza A(H7N9) and A(H1N1)pdm09 Viruses between 2 Patients with Hematologic Disorders

Transmission of these viruses was limited to 2 immunocompromised patients in the same ward.

A nosocomial cluster induced by co-infections with avian influenza A(H7N9) and A(H1N1)pdm09 (pH1N1) viruses occurred in 2 patients at a hospital in Zhejiang Province, China, in January 2014. The index case-patient was a 57-year-old man with chronic lymphocytic leukemia who had been occupationally exposed to poultry. He had co-infection with H7N9 and pH1N1 viruses. A 71-year-old man with polycythemia vera who was in the same ward as the index case-patient for 6 days acquired infection with H7N9 and pH1N1 viruses. The incubation period for the second case-patient was estimated to be <4 days. Both case-patients died of multiple organ failure. Virus genetic sequences from the 2 case-patients were identical. Of 103 close contacts, none had acute respiratory symptoms; all were negative for H7N9 virus. Serum samples from both case-patients demonstrated strong proinflammatory cytokine secretion but incompetent protective immune responses. These findings strongly suggest limited nosocomial co-transmission of H7N9 and pH1N1 viruses from 1 immunocompromised patient to another.