Figure 2. Neuraminidase (NA) inhibition by virus neutralization (VN)–positive raccoon serum samples. Each of the N1 to N9 viruses, consisting of A/whooper swan/Mongolia/4/05 (H5N1), A/Hong Kong/1073/99 (H9N2), A/whooper swan/Shimane/499/83 (H5N3), A/turkey/Ontario/6188/68 (H8N4), A/duck/Alberta/60/76 (H12N5), A/duck/England/56 (H11N6), A/seal/Massachusetts/1/80 (H7N7), A/duck/Ukraine/1/63 (H3N8), and A/duck/Memphis/546/74 (H11N9), was incubated with a VN-positive serum sample (A-6, B-2, or C-1) for 1 h at 37°C, and viral NA activity was then measured (6). Data are shown as percentage of activities compared with incubation with VN-negative serum samples (100%). Three independent tests were performed, and significant reduction of NA activity (p<0.05, t test with 2-tailed analysis) was observed only for N1 virus (*). HA, hemagglutinin. Error bars indicate SDs of 3 independent tests.