Aquatic birds are recognized as natural gene reservoirs of influenza A viruses around the world (1
). Interspecies transmission usually happens from aquatic birds to terrestrial birds or to mammals such as pigs, horses, and even humans. These interspecies transmissions establish a very complex ecological system in nature. Since most of these interspecies transmission events were identified and associated with zoonotic outbreaks in new hosts, influenza gene flow was usually considered to occur from aquatic birds to other animal species. This idea inhibited our understanding of the whole influenza ecology system and the emergence of new influenza viruses with pandemic propensity.
Antigenic and genetic analyses of the H9N2 influenza viruses isolated from domestic ducks in southern China during 2000-2001 provide convincing evidence that the H9N2 influenza virus lineages established since the mid-1990s in chicken and quail (8
) have been transmitted back to domestic ducks, generating double or triple reassortants with influenza viruses already resident in ducks. Rather than indicating a one-way flow of H9N2 viruses and their genes from aquatic to terrestrial poultry as previously recognized (7
), these findings indicated reverse transmission back to duck, i.e., there is a two-way transmission between terrestrial and aquatic birds (Fig. ). This resulted in the generation of multiple genotypes of H9N2 viruses containing internal genes of aquatic avian origin. Further interspecies transmission of these reassortants to other hosts may subsequently occur. This parallels the reemergence of H5N1 influenza viruses in Hong Kong in 2001 (5
) and 2002 (authors' unpublished data). The lessons learned from these H5N1 incidents are that when an established virus lineage such as the A/Goose/Guangdong/1/96-like virus is reintroduced into the duck (6
), it leads to the generation of multiple reassortants with new aquatic gene segments (5
). Novel reassortants are biologically unstable (12
) and may transmit further to other hosts—in this case terrestrial poultry, lower mammals, and possibly humans (see below). This may be one method by which the next pandemic virus will emerge.
FIG. 3. Current understanding of the ecology of influenza virus in southern China that favors the emergence of H9N2 as a possible new pandemic influenza virus. The concept of the classic gene pool and the interplay between aquatic birds and influenza viruses (more ...)
Phylogenetic analyses of the internal genes of the H9N2 virus showed that some of them are closely related to the internal gene complex found in H5N1/01 viruses. This information suggests that the precursors of H5N1/01 viruses are still circulating in the gene pool in this region and are reassorting promiscuously. This finding also provides an example of the interaction between those established lineages in terrestrial or semiaquatic poultry and the domestic duck. In the long term, this kind of interaction will increase virus diversity in the ecosystem in southern China.
The findings of the present investigation suggest that these H9N2 reassortant viruses may have pandemic potential since they contain a receptor-binding profile (10
) that favors infection of humans and that some (e.g., the Qa/HK/G1/97-like lineage) have gene segments previously associated with human disease (14
). As H9N2 influenza viruses are not highly pathogenic for poultry, it makes them more, rather than less, likely to be of pandemic relevance. In fact, viruses that are less pathogenic for poultry have a greater opportunity to become widespread since they do not raise concern and permit their hosts to survive unhindered. Thus, they are free to continue to reassort and are more likely to have the opportunity to find the best gene constellation (19
) that permits infection of humans and facilitates further person-to-person transmission. The connecting peptides of the HA of the 1968 H3N2 and 1957 H2N2 pandemic viruses indicate that they are unlikely to have been highly pathogenic for poultry. Two-way transmissions between different types of poultry in southern China increase the opportunity to generate influenza viruses with pandemic potential. Our findings demonstrate that such viruses may be directly generated from ducks.
It was noteworthy that the NS genes of two H9N1 viruses are closely related to the counterparts in the Gs/Gd/1/96 virus. This fact suggests that Gs/Gd/1/96-like H5N1 influenza virus might also be generated from ducks in this region. As more sequence data have been accumulated, precursors of important emerging influenza viruses can be identified. Southern China is considered to be an influenza epicenter mainly because the region was the site of two previous pandemics (Asian and Hong Kong influenza) (22
). The current status of the influenza ecosystem in this region makes it very likely that the next pandemic virus will also be generated in this region (Fig. ).
One remaining question is why the HA and NA genes of Qa/HK/G1/97-like H9N2 viruses have not been recently detected in ducks even though some of their internal gene segments were found incorporated into some Ck/Bei/1/94-like duck viruses. One explanation is that the HA and NA genes of the Qa/HK/G1/97-like virus are no longer adaptable to aquatic birds since the viruses have become more adapted to terrestrial poultry. Another possible reason is that we sampled ducks from specific regions where Qa/HK/G1-like viruses were not locally prevalent. It is believed that these gene segments were acquired in terrestrial birds, since reassortment events are frequently observed between two established H9N2 lineages in quail isolates (authors' unpublished data).
Virus gene movements in both directions between different types of poultry seem to be not uncommon in this region in the recent past. The species barriers between the birds have become much more permeable than previously anticipated. Increasing the heterogeneity of influenza viruses in these hosts results in an enlarged and dynamic influenza gene pool in continuous flux rather than one that is limited to aquatic birds and therefore in evolutionary stasis (Fig. ). The diversity of genotypes, gene constellations, and host receptor specificities will provide these viruses and their progeny with multiple options of hosts (11
). Such a situation would conceivably increase the opportunity for the generation of a new pandemic influenza virus.