Persistent circulation of highly pathogenic influenza H5N1 virus in Lake Qinghai area of China.

To trace the endemic situation of highly pathogenic influenza H5N1 virus in wildlife in the Lake Qinghai area of northwest China, a continuous surveillance program was implemented from September 2005 to September 2007. A total of 2699 field samples were collected, and 95 of the samples were positive by a reverse transcription-polymerase chain reaction (RT-PCR) test of subtype H5 influenza virus. To some extent, the detection rate correlated with the species, the location, and the seasons of collection. In 2007, two H5N1 isolates were identified from two species of migratory birds. The two 2007 isolates showed high similarities of the hemagglutinin and neuraminidase genes with those of the 2005 and 2006 Qinghai isolates (98.6-99.8% for HA and 98.7-98.9% for NA at the nucleotide level). Both isolates fell into clade 2.2.2 and were experimentally highly pathogenic to chickens and mice. According to our surveillance results, HPAI H5N1 viruses still exist in this region.