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Among winter migratory waterfowl, Northern pintails (Anas acuta), in one of the largest flocks in Tohoku district, northeast Japan, were surveyed for influenza A viruses at five wintering sites in three prefectures, viz., Aomori, Akita, and Miyagi. A total of 38 influenza A viruses were isolated from 2066 fecal samples collected during November 2006 through March 2007. The overall isolation rate was 1.84%. Eleven different subtypes were isolated, including nine H5N2, seven H6N8, seven H10N1, four H4N6, three H6N1, three H11N9, and one each of H1N1, H6N2, H6N5, H10N9, H11N1. Only the H4N6 subtype was detected during two successive months, November and December, from Lake Ogawara of Aomori prefecture. One wintering site, Lake Izunuma of Miyagi prefecture, was negative for virus isolation throughout the study period. During the sampling period, the highest virus isolation rate was in December (4.90%) followed by November (2.18%), January (0.91%), and February (0.30%). Virus isolation was negative for samples collected in March 2007. These results suggest that influenza viruses are introduced by Northern pintail when they migrate into Japan, but the viruses are not maintained in the flocks, most likely because the birds are not breeding during the winter. We believe that this relatively large data set creates a strong foundation for future studies of avian influenza virus (AIV) prevalence, evolution, and ecology in wintering sites, along with the role of Northern pintails in the spread of AIV during their migration from northern Russia and Asia to Japan.

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Pintails constitute an important host of avian influenza viruses (AIVs). Genetic, molecular, and antigenic characteristics of H5 and H7 AIVs, which we isolated from northern pintails (Anas acuta) wintering in Japan, were analyzed and found to be linked to various ecological features, chiefly in terms of gene geography, as shaped by various migratory aquatic host species. Although all the isolates were found to be of low pathogenicity (LP), we explored gene predispositions that may potentially underlie tentative transition to high pathogenicity (HP). Evolutionarily, the HA and NA genes of the isolates affiliated mostly with Eurasian lineage. The viruses closely related to ours were derived from China, Korea, Mongolia, Japan, and Australia. Comprehensive ecophylogenetic evaluations revealed that the pintail populations we sampled might have given rise to or been involved in the emergence of a LPAI H7N6 subtype that caused outbreaks in quail (Coturnix japonica) farms in Japan, as well as of the first H5N9 subtype ever isolated in Asia. The latter strain isolated by us showed, yet, notable affinity to certain North American and Australian strains, thereby signifying apparent intercontinental interfaces accounted for by extensive water-bird flyways. Noticeable conservation of certain antigenic sites within both Eurasian and North American H7 HAs is apparently an outcome of their advantageous survival value, in terms of restricted immunogenicity. Besides, the Japanese-Korean-Siberian regional axis seems to be particularly important for ongoing generation of novel viral strains due to conveyance of certain genes and genomes by migratory ducks, including such that circulate among pigs and human.