Emerging multiple reassortant H5N5 avian influenza viruses in ducks, China, 2008

Three highly pathogenic H5N5 avian influenza viruses (HPAI), A/duck/Guangdong/wy11/2008 (WY11), A/duck/Guangdong/wy19/2008 (WY19), and A/duck/Guangdong/wy24/2008 (WY24) were isolated from ducks in southern China in April 2008. Here, we characterized these viruses by performing sequencing and phylogenetic analyses of their viral genes, assessing their virulence in ducks and mice, and performing cross-protection experiments in chickens. Sequence analysis revealed that the HA genes of these H5N5 viruses showed 97.1-97.8% homology to A/wild duck/Hunan/211/2005 (H5N1) influenza virus and that their NA genes showed 96.4-96.8% nucleotide identity to the NA gene of A/duck/Hunan/5613/2003 (H6N5) influenza virus, which belongs to the Eurasian lineage. Genotypic analysis indicated that these H5N5 viruses were multiple reassortants among H5N1, H5N2, H6N2, and H6N5 viruses. The analysis of HA clade showed that these H5N5 viruses are clustering into clade 2.3.4. In animal experiments, these H5N5 viruses caused 50% mortality in ducks and 100% mortality in chickens. In cross-protection experiments, the clade 2.3.2 avian influenza vaccine could provide only 75% protection with chickens against H5N5 virus challenge. Moreover, the H5N5 virus replicated efficiently in the lungs of mice, which suggested that the H5N5 viruses have the potential to infect mammalian hosts. Since ducks have served as reassortant vessels, playing pivotal roles in the generation of new subtypes of influenza viruses, it is important to monitor the emergence of this novel subtype of influenza viruses in waterfowl to understand their ecology and evolution and to control the spread of new viruses.