Emergence and predominance of an H5N1 influenza variant in China

Abstract

The development of highly pathogenic avian H5N1 influenza viruses in poultry in Eurasia accompanied with the increase in human infection in 2006 suggests that the virus has not been effectively contained and that the pandemic threat persists. Updated virological and epidemiological findings from our market surveillance in southern China demonstrate that H5N1 influenza viruses continued to be panzootic in different types of poultry. Genetic and antigenic analyses revealed the emergence and predominance of a previously uncharacterized H5N1 virus sublineage (Fujian-like) in poultry since late 2005. Viruses from this sublineage gradually replaced those multiple regional distinct sublineages and caused recent human infection in China. These viruses have already transmitted to Hong Kong, Laos, Malaysia, and Thailand, resulting in a new transmission and outbreak wave in Southeast Asia. Serological studies suggest that H5N1 seroconversion in market poultry is low and that vaccination may have facilitated the selection of the Fujian-like sublineage. The predominance of this virus over a large geographical region within a short period directly challenges current disease control measures.

Fig. 1.

Comparison of H5N1 influenza virus isolation rate (%) in chicken (A), duck (B) and goose (C) from southern China, July 2004 to June 2006. Surveillance was conducted in live-poultry markets in Fujian, Guangdong, Guangxi, Guiyang, Hunan, and Yunnan.

Fig. 2.

Numerical analysis of HI titers (see Table 4) by using hierarchical agglomerative clustering (A) and nonmetric multidimensional ordination in two dimensions (B). Colors indicate viruses from the FJ-like (red), GY1 (blue), GY2 (green), YN2 (purple), and GD/06 (orange) sublineages. MYS, Malaysia; Ph, pheasant; VTM, Vietnam/Thailand/Malaysia.

Fig. 3.

Phylogenetic relationships of the HA genes of representative influenza A viruses isolated in Asia. Trees were generated by the neighbor-joining method in the PAUP* program (27). Numbers above or below branches indicate neighbor-joining bootstrap values. Not all supports are shown because of space constraints. Analysis was based on nucleotides 1–1011, and the tree was rooted to A/tern/South Africa/61. Colors indicate viruses from the FJ-like (red), GY1 (blue), GY2 (green), YN2 (purple), and GD/06 (orange) sublineages. VTM, Vietnam/Thailand/Malaysia. (Scale bar, 0.01 substitutions per site.)

Fig. 4.

Phylogenetic relationships of the HA (A) and PB2 (B) genes of representative influenza A viruses isolated in Asia. Trees were generated by the neighbor-joining method in the PAUP* program (27) (Bayesian analysis revealed the same relationships.) Numbers above and below branches indicate neighbor-joining bootstrap values and Bayesian posterior probabilities, respectively. Not all supports are shown because of space constraints. Analysis was based on nucleotides 1–1011 of the HA gene and 985-2233 of the PB2 gene. The HA and PB2 trees were rooted to A/tern/South Africa/61 and A/equine/Prague/1/56, respectively. Colors indicate viruses from the FJ-like (red), GY1 (blue), GY2 (green), YN2 (purple), and GD/06 (orange) sublineages. Recent human isolates from China are underlined. ∗, Viruses included in antigenic analysis (Fig. 2 and Table 4). JX, Jiangxi; MDK, migratory duck; MYS, Malaysia; Ph, pheasant; Qa, quail; SCK, silky chicken; VTM, Vietnam/Thailand/Malaysia. (Scale bar, 0.01 substitutions per site.)