Seven of these demonstrated only H5-specific HI activity, whereas, one serum (G10-195) inhibited HA activity induced by the influenza A virus carrying either H5 or H3 hemagglutinin (Table 2). Of the seven sera with only H5-specific HI activity, five (G10-192, G44-1, G44-2, G44-5, and G44-20)

solely inhibited N1-specific neuraminidase activity. In addition to the N1-specific NI activity, however, the remaining two sera simultaneously inhibited neuraminidase activities induced by the viruses carrying N2 or N4 (G10-209), and N2 or N4 or N8 (G10-218) protein (Table 2). Taken together, five sera (G10-192, G44-1, G44-2, AZD1152-HQPA G44-5, and G44-20) were demonstrated to contain H5N1-specific HI and NI antibodies together with anti-NS1 and anti-NP/M antibodies. These five sera were subjected to the HI test using HPAI H5N1 virus, which was isolated from a healthy duck in northern Vietnam in 2008 (14), and showed titers comparable to those observed against A/whistling swan/499/83 (H5N3). The serological analyses indicated that at least five ducks had naturally been infected with H5N1 viruses. The NS1 is synthesized in infected cells during the replication of the influenza A virus but is not incorporated into the mature virion (15, 16); hence, poultry vaccinated with an inactivated whole H5 influenza A virus failed to develop NS1-specific

antibodies (17, 18). Therefore, these five ducks, one raised in Hanoi and the remaining four raised

in Nam Dinh province, had probably been infected with H5N1 viruses. Sera DNA-PK inhibitor from five ducks (G10-188, -195, -199, -209, -218) in farm G10 and a duck (G51-14) in farm G51 inhibited HA or NA activities induced by more than one subtype (Table 2). It probably indicated that more than one influenza A subtype had been circulating simultaneously or at a different time among ducks reared in those farms. In the current study, the prevalence of H5N1 infections among ducks was estimated at least as 0.45% (5/1106) overall and as 0.22% (1/447) in Hanoi and 1.1% (4/360) in Nam Dinh province. When a farm was considered as the unit of calculation, the detection rate observed in Hanoi and Nam Dinh province was at least 4.5% (1/22) and 5.5% (1/18), respectively. L-gulonolactone oxidase None of the ducks raised in Vinh Phuc province tested positive for H5N1. A nationwide survey conducted in Vietnam between 2004 and 2007 revealed the H5N1 virus-positive rate to be 10% (1). Although it is not plausible to compare our data directly with that reported by Wan et al. (1), which was obtained with samples collected from backyard flocks, live bird markets, and even from sick or dead birds, the low prevalence of H5N1 infection revealed in the present study might reflect the effectiveness of the disease control activities enforced by the Vietnamese government (1, 2). Moreover, subtype H5N1 viruses were not isolated in the present study.