New Strain of Influenza A Virus (H5N1), Thailand

To the Editor: During 2004– 2005, 3 major waves of avian influenza outbreaks occurred in Thailand (1). The first wave was reported in early January 2004, the second in July 2004, and the third in October– December 2005. In total, 22 persons were infected and 14 died. Recently, a fourth wave began on July 23, 2006. The Thai Ministry of Public Health reported that avian influenza A (H5N1) virus killed 2 infected persons. The first patient, a 17-year-old man in Phichit Province, began to experience symptoms on July 15, 2006, and died on July 24, 2006 (2). The second patient, a 27-year-old man in Uthai Thani Province, began to experience symptoms on July 24, 2006, and died on August 3, 2006 (3). The fourth wave of these outbreaks involved chickens and encompassed 2 distinct areas: Phichit Province, identified on July 23, 2006 (4), and Nakhon Phanom Province, identified on July 28, 2006 (5). We sequenced all 8 gene segments of the 2 viruses isolated from Phichit and 1 virus isolated from Nakhon Phanom and then submitted to GenBank as follows: A/chicken/Thailand/PC-168/ 2006 (DQ999879–86) and A/chicken/ Thailand/PC-170/2006 (DQ999887– 94) from Phichit and A/chicken/ Thailand/NP-172/2006 (DQ999871– 8) from Nakhon Phanom. Whole genome analysis showed that all 3 samples had undergone minor mutations that are typical of circulating influenza A viruses. Unexpectedly, this outbreak was associated with 2 strains of the virus. The 2 samples from Phichit closely resembled H5N1 strains that had circulated in Thailand during 2004 and 2005. The sample from Nakhon Phanom was newly observed in Thailand and more closely related to H5N1 strains that had been circulating since 2005 in southeast People’s Republic of China. The whole genome phylogenetic analysis also showed that the viruses isolated from Phichit belonged to genotype Z, whereas virus isolated from Nakhon Phanom belonged to genotype V, which differs from genotype Z in the PA gene (6) (Figure, panel A). The phylogenetic tree of the hemagglutinin (HA) gene (Figure, panel B) showed that the Phichit samples were similar to the cluster of samples isolated during 2004 and 2005 in Thailand and Vietnam. In contrast, the Nakhon Phanom sample was clustered into the same group with viruses isolated from southeast People’s Republic of China, including Zhejiang, Shantou, Hunan, Fujian, Guangxi, and Lao People’s Democratic Republic (7) with the differences in the cleavage site, SPLRERRRK-R/G (underline and dash indicate differences), which had never been found in Thailand. The N-link glycosylation sites (positions 154–156) of the Pichit isolates were NST residues, whereas in the Nakhon Phanom isolate, NNT residues were observed. However, the receptorbinding site of HA (positions 222 and 224) was unchanged. In the neuraminidase (NA) gene, the new isolates contain 20 amino acid deletions within the stalk region, the same as previously described (1). The ESEV residues in the C-terminal and Asp92 of NS1 were observed in the 2006 isolates and in viruses that have been isolated from Thailand, Vietnam, and People’s Republic of China. This finding indicates that the new isolates were highly virulent but sensitive to treatment with interferon and tumor necrosis factor-α (8). The 2006 isolates contain Glu627 of PB2, identical to the previous isolates from Thailand and Indonesia, which may indicate that the new isolates had less efficient replication capability in