- Open Access
Host shifts and molecular evolution of H7 avian influenza virus hemagglutinin
Virology Journal volume 8, Article number: 328 (2011)
Evolutionary consequences of host shifts represent a challenge to identify the mechanisms involved in the emergence of influenza A (IA) viruses. In this study we focused on the evolutionary history of H7 IA virus in wild and domestic birds, with a particular emphasis on host shifts consequences on the molecular evolution of the hemagglutinin (HA) gene. Based on a dataset of 414 HA nucleotide sequences, we performed an extensive phylogeographic analysis in order to identify the overall genetic structure of H7 IA viruses. We then identified host shift events and investigated viral population dynamics in wild and domestic birds, independently. Finally, we estimated changes in nucleotide substitution rates and tested for positive selection in the HA gene. A strong association between the geographic origin and the genetic structure was observed, with four main clades including viruses isolated in North America, South America, Australia and Eurasia-Africa. We identified ten potential events of virus introduction from wild to domestic birds, but little evidence for spillover of viruses from poultry to wild waterbirds. Several sites involved in host specificity (addition of a glycosylation site in the receptor binding domain) and virulence (insertion of amino acids in the cleavage site) were found to be positively selected in HA nucleotide sequences, in genetically unrelated lineages, suggesting parallel evolution for the HA gene of IA viruses in domestic birds. These results highlight that evolutionary consequences of bird host shifts would need to be further studied to understand the ecological and molecular mechanisms involved in the emergence of domestic bird-adapted viruses.
Over the last decade, an increasing number of studies has focused on the effects of human activities on pathogen evolution [1, 2]. Historically, agriculture and domestication of wild animals have been linked to the emergence of several human pathogens. Ecological changes related to modern agricultural practices also are likely to affect emergence of both human and animal diseases [3, 4]. Influenza A (IA) virus provides a good example of a pathogen that can move from wild bird reservoir to domestic animal systems and adapt to humans and other mammals. These viruses have invaded, and in some cases have become established in a diversity of agrosystems, ranging from integrated rice-duck farming , to live poultry markets  and industrial and intensive farming systems [7, 8]. The emergence and long-term circulation of the highly pathogenic (HP) H5N1 virus in domestic birds in Southeastern Asia [9, 10], as well as the recent emergence of the swine-origin H1N1 virus in humans [11, 12], highlights the ability of IA viruses to spread beyond species barriers and adapt rapidly to new host and environmental conditions .
Wild waterbirds in the orders Anseriformes (ducks, geese and swans) and Charadriiformes (gulls, terns and waders) are recognized to be natural hosts for low pathogenic (LP) IA viruses . In these two avian orders, a large diversity of IA subtypes have been described, based on genetic and antigenic characteristics of the hemagglutinin (HA) and the neuraminidase (NA) proteins. In wild waterbirds, IA viruses do not cause significant disease but may have subtle physiological and behavioral effects [15, 16]. In ducks, ecological factors affecting the prevalence of infection include host species, age, behavior, population density, and persistence of viruses in the environment.
In domestic birds (e.g. chickens, ducks, turkeys, quails), IA viruses replicate in both the intestinal and respiratory tracts, but in the case of HP H5 and H7 viruses, can result in multi-organ systemic infections with high mortality [17, 18]. Several HP IA virus outbreaks have been described and were responsible for large economic losses for poultry producers . With the exception of the Asian strains of the HP H5N1, HP viruses are rarely reported in wild waterbirds [19, 20].
Genetic exchanges between viruses circulating in wild and domestic birds have been documented, especially when wild bird origin LP viruses infect poultry [21–23]. The regular spillover of Asian HP H5N1 viruses from domestic to wild birds also demonstrate the potential for reverse flow [24–26]. However, the time, location, frequency, effect on the population genetic diversity, and on the molecular evolution of IA viruses of such genetic exchanges remain unclear. The evolutionary consequences of these host and resulting environmental shifts represent a challenge to understand the mechanisms involved in the emergence of IA viruses, in particular regarding molecular changes involved in the increase of virulence and host specificity.
We focused on the evolutionary history of H7 IA viruses circulating in wild and domestic birds. Compared to other HA subtypes, numerous H7 viruses have been isolated from both wild waterbirds and poultry. In domestic birds, H7 IA viruses have been responsible for severe outbreaks sometimes with long-term circulation of LP and HP viruses; in Italy [21, 27–29], Germany , the Netherlands , Australia [32, 33], China , Pakistan , Canada [36, 37], United States of America (USA) [38, 39] and Chile .
The aims of this study were: (i) to provide an overview of subtype combinations, host diversity and phylogeographic structure of H7 IA virus isolated in wild and domestic birds, worldwide; (ii) to investigate the population dynamic of H7 HA, with a particular emphasis on virus emergence and extinction dates, genetic diversity and exchanges between wild and domestic hosts; and (iii) to estimate changes in nucleotide substitution rates and test for positive selection in the HA gene. We discuss the consequences of host shifts on the population dynamics of H7 IA viruses in wild and domestic birds and the ecological and molecular mechanisms potentially involved in the emergence of domestic bird-adapted viruses.
Complete nucleotide and protein sequences of the HA gene were downloaded from the Influenza Sequence Database (ISD; ), on October 22, 2009. For each sequence, the following information was collected: accession number, strain name, subtype, geographic origin (i.e., continent: Europe, Asia, Australia, Africa, North America and South America), bird host species, virulence (LP or HP) and year of virus collection. Host status (domestic or wild) was determined based on bird species or from the description of the isolate provided in reference papers. Sequences from viruses for which host was not identified (e.g., 'duck' or 'mallard', without any additional information concerning their origin) were not included in the analyses. Duplicate sequences (from the same strain) as well as sequences previously identified as reflecting potential laboratory errors  also were removed. When not available in the ISD, virulence (LP or HP) of viruses was assigned by comparing the HA amino acid sequence pattern of the cleavage site with viruses previously characterized as LP or HP. Differences in the frequency of subtype combinations between wild and domestic birds were investigated using a χ2 test implemented in the R 2.10.1 software .
The coding region of nucleotide sequences was aligned with ClustalW 2.0.10 . Four viruses representing two different subtypes (H3N8 and H6N2) and geographic origins (North America and Europe) were included to root trees: A/Green-winged teal/Ohio/1289/2005 (H3N8), A/Mallard/Maryland/2022/2005 (H6N8), A/Mallard/Finland/12072/06 (H3N8) and A/Bewick's swan/Netherlands/1/2005 (H6N2). Phylogenetic trees were constructed by maximum likelihood with the software PhyML 3.0 . The evolutionary model was selected by Model Generator 0.85 . Nodal supports were assessed with 100 bootstrap replicates.
Viral population dynamics were investigated with a Bayesian Markov Chain Monte Carlo coalescent approach, implemented in the program BEAST 1.5.3 . Time of the most recent common ancestor (TMRCA) as well as rates of nucleotide substitution were obtained from analyses performed with BEAST. The Shapiro-Rambaut-Drummond-2006 (SRD06) nucleotide substitution model was used in all simulations as this model is recognized to provide better resolution for coding regions , and has recently been used in population dynamic studies of other IA subtypes [49, 50].
We focused on the viral population dynamics of two main genetic lineages identified by the phylogeographic analysis (cf. results section): (i) the North American-South American lineage (NA-SA) and (ii) the Eurasian-African-Australian lineage (EURAS-AF-AU). Three molecular clock models were tested for each genetic lineage. The strict clock (SC) that assumes a single evolutionary rate in the phylogenetic trees, and two relaxed clocks: the uncorrelated exponential (UE) and the uncorrelated lognormal (UL), which allow evolutionary rates to vary along branches, within an exponential or lognormal distribution . Molecular clock models were evaluated and tested with the Bayes Factor (BF) [52, 53] implemented in the program TRACER 1.5.0 [47, 54]. The ratio of marginal likelihoods were compared between models and BF significance was determined from the values of 2ln(BF), as described by Brandley et al. . A Bayesian skyline coalescent tree prior was used in all simulations as it makes the fewest a priori assumptions about the data  and has been shown to be more appropriate to describe the population dynamics of IA virus . Analyses were performed (i.e. with SC, UE and UL molecular clocks) for each genetic lineage, with a chain length of 120 million generations sampled every 1 000 iterations. Results were analyzed with TRACER: for each simulation, the first 5-10% trees were discarded as burn-in and an effective sample size superior to 200 was obtained to ensure adequate sample size for the posterior, prior, likelihood, mean rate of nucleotide substitution (clock rate for SC), and skyline. Phylogenetic consensus trees were produced using the program TREEANNOTATOR 1.5.3 and edited for generation of figure captions with the program FigTree 1.3.1.
We also performed independent analyses including only viruses isolated in wild or domestic birds for each genetic lineages (NA-SA and EURAS-AF-AU) and for the main genetic sub-lineages (we limited the analysis to groups including more than ten sequences obtained from viruses isolated over a time period of at least a three years). Analyses were carried out with a Bayesian skyline coalescent tree prior and a UE molecular clock (most appropriate according to the results of the previous analysis).
Molecular evolution of the HA gene
Signatures of selection in the HA nucleotide sequences were evaluated using the codon-based approach as implemented in HyPhy , available at the Datamonkey webserver [59, 60]. Estimations of the dN (nonsynonymous substitutions) and dS (synonymous substitutions) were obtained with the single likelihood ancestor counting (SLAC), fixed-effects likelihood (FEL) and random effects likelihood (REL) methods . Analyses were conducted under the HKY85 nucleotide substitution model, with a statistical significance set to p = 0.05. Finally, in order to identify the potential function of positively selected amino acids, we located N-linked glycosylation sites in the HA protein sequences using the NetNGlyc 1.0 Server.
Among the 508 sequences downloaded from the ISD, 94 (18.5%) were excluded from the analysis because: (i) they represented duplicate sequences of the same virus (N = 21), (ii) they were previously identified as potential laboratory errors (N = 3; ) or (iii) information concerning the bird species and/or its status (domestic or wild) was not available (N = 70).
The overall proportions of viruses isolated from domestic and wild birds were 84% and 16%, respectively. The most represented bird species was chicken (53%) for domestic birds and Mallard (41%) for wild birds (Additional file 1: Figure S1). Ninety percent of isolated viruses came from North American and European birds (Additional file 2: Table S1). Subtype combinations with the NA were significantly different between domestic and wild birds (χ2 = 71.6, df = 6, p <0.001, excluding N6 and N8; cf. Additional file 3: Figure S2 for details). For wild birds, H7N3 and H7N7 were the most represented subtypes (49% and 35% respectively), while in domestic birds H7N2 was mainly represented (52%), with H7N3 (20%) and H7N1 (19%). Seventy-six viruses were HP (18%); all of them were isolated from domestic birds (Additional file 2: Table S1).
A strong association between the geographic origin and genetic structure was observed, with four main clades including viruses isolated in (i) North America, (ii) South America, (iii) Australia and (iv) Europe, Asia and Africa (Figure 1 and Additional file 4: Figure S3). The two latter groups were less supported by bootstrap values; however, they were excluded from a 'superclade' including all American viruses (North and South America; Additional file 4: Figure S3). Several sub-groups were observed in each geographic clades, including viruses isolated in the same location or time period (Additional file 4: Figure S3 for details). Most of these clades have previously been identified in studies that focused on a limited number of viruses, responsible for outbreaks in poultry, locally. Finally, we observed that HP H7 viruses isolated in domestic birds in Eurasia-Africa, Australia, North and South America formed unrelated genetic lineages.
The North American-South American (NA-SA) and the Eurasian-African-Australian (EURAS-AF-AU) lineages were analyzed independently, because of their strong genetic divergence as well as for computational reasons. For both lineages, the UE molecular clock model was significantly better than the SC and UL models, concordant with other studies [49, 50]. Based on the maximum clade credibility tree topologies, we identified at least 14 potential events of host shift (Figures 2 & 3 and Additional file 5 & 6: Figures S4 & S5 - events A to N). Ten of these corresponded to viruses that circulated in domestic birds and shared a common ancestor with a virus isolated from wild birds prior to the detection in domestic birds (i.e. introduction from wild to domestic birds; Additional file 7: Table S2). The direction of virus introduction (i.e. wild-to-domestic or domestic-to-wild) for the four other host shift events (A, B, K and L; Figures 2 & 3 and Additional file 5 & 6: Figures S4 & S5) couldn't be determined because of the limited number of sequences available (3 to 6), sometimes over extended time periods (2 to 13 years).
The EURAS-AF-AU lineage was structured into ten major genetic sub-lineages, strongly supported by posterior probability values (Figure 2 and Additional file 5: Figure S4). The Australian H7 viruses were clearly differentiated from Eurasian-African viruses and the time of the most recent common ancestor (TMRCA) between the two lineages was 1873 (95% HPD: [1811-1914]). The TMRCA estimated for the Australian lineage was 1968 [1952-1975] with a circulation of HP viruses for more than 29 years in domestic birds, and genetic reassortment with different NA subtypes.
The Eurasian-African (EURAS-AF) genetic lineage was divided into 9 major clades including viruses isolated in different geographical locations and time period (Figure 2 and Additional file 5: Figure S4). The presence of a sub-lineage including viruses isolated between 1927 and 1945 suggested an ancient circulation of genetically differentiated HP H7 IA virus in domestic birds in Europe and Africa. These viruses did not shown genetic relatedness with modern HP H7 IA viruses and could have circulated in domestic birds for 21 to 48 years in Europe and Africa (TMRCA: 1912 [1897-1924]). The existence of such an ancient lineage of H7 HA viruses has been suggested by other studies; however, our results demonstrate that they are included in the EURAS-AF genetic lineage and are not at the basis of the EURAS-AF-AU lineage, as previously suggested (e.g. ).
A lineage including LP and HP viruses isolated in England and Germany, between 1977 and 1982, could have emerged in domestic birds in 1972-1973 (TMRCA: 1973 [1969-1976]). This lineage is probably extinct as no genetically related H7 HA viruses have been detected after 1982. In addition, the genetic structure of this clade supports the circulation of two sub-lineages of LP H7 in England, one at the origin of the HP H7N7 outbreak in domestic birds in Germany in 1979 (A/Turkey/England/647/1977) as previously reported by Banks et al. .
A distinct clade of HP H7N3 viruses was evidenced with domestic birds in Pakistan, with a co-circulation of two genetically different sub-lineages. According to the TMRCA of this clade (1993 [1991-1995]), HP H7 viruses could have circulated for 11 years in domestic birds. These viruses were genetically unrelated to other HP H7 viruses circulating during the same time period in Europe (Italy, Germany and the Netherlands), suggesting an independent emergence and circulation, as recently proposed by Abbas et al. .
Two unrelated lineages were observed in Italy, in domestic birds. The first one included both LP and HP H7N1 viruses that circulated between 1999 and 2001, with a strong clustering of HP viruses (TMRCA: 1997 [1996-1998]). The second one included only LP H7N3 viruses isolated between 2002 and 2007 (TMRCA: 2001 [2001-2002]). Our analysis also supported the introduction of LP H7N3 from wild birds to domestic poultry in Italy (host shift H; cf.  for complete description).
A paraphyletic group including H7 IA viruses isolated from wild birds, in Sweden and the Netherlands was also observed (TMRCA: 1999 [1999-2000]). A genetic clade including HP viruses isolated from domestic birds was branched within this group (TMRCA: 2002 [2002-2003]). As reported by Munster et al. , the HA of A/Mallard/Netherlands/12/2000 was genetically related to the four HP H7 HA from domestic birds in the Netherlands and Germany in 2003 (Additional file 5: Figure S4), suggesting that this HP H7 outbreak could have arisen from the circulation of LP H7 IA viruses in wild waterbirds (host shift D).
We revealed the existence of a genetic lineage recently isolated from both wild and domestic birds in Europe. Two sub-lineages were identified and provided evidence of virus transmissions from wild to domestic birds (host shifts E, F and G). For both sub-lineages (including: A/Mallard/Italy/299/2005 and A/Mute swan/Hungary/5973/2007; cf. Additional file 5: Figure S4 for details) the TMRCA was 2004 [2003-2005].
In Asia (China and Japan), we identified a clade that includes viruses isolated from wild and domestic birds. This result suggested a regional circulation of genetically differentiated H7 viruses, with a likely host shift from wild to domestic birds (host shift B). The TMRCA of this clade (1998 [1997-1999]), together with the detection of viruses in domestic birds in 2003, suggest that this genetic sub-lineage could have circulated in domestic birds in China for five years.
Finally, we highlighted the existence of two 'atypical' genetic clades. The first included H7N1 IA viruses isolated in caged birds in Asia and Europe, between 1994 and 1995 (Figure 2 and Additional file 5: Figure S4). The geographical origin of these genetically related viruses highlights the potential for rapid spread of H7 IA viruses due to illegal caged bird trade (cf.  for discussion). The second clade included two viruses: A/Parrot/Northern Ireland/VF7367/1973 and A/Chicken/Chakwal/NARC-35/2001 (Additional file 5: Figure S4). The isolation of these viruses from distant geographic areas (Northern Ireland and Pakistan) and over a 28 years period, could support a long term circulation of an undetected H7 lineage in domestic birds, in Eurasia (cf.  for discussion). However, given the extremely high level of identities between the two HA sequences (99% homology) they probably reflect laboratory errors (cf.  for discussion).
Temporal changes in genetic diversity were plotted independently for viruses isolated in wild and domestic birds, with a Bayesian Skyline reconstruction (Figure 4). Although not significant, the recent co-circulation of several genetically different lineages in wild birds was illustrated by a slight increase in the relative genetic diversity in 2001 and after 2004 (Figure 4A). After introduction in domestic birds, no significant changes in the relative genetic diversity were observed (Figure 4C, 4D, 4E, 4F) although these patterns may reflect the low number of sequences available for certain genetic lineages (e.g. Australia).
The NA-SA genetic lineage was also structured into sub-lineages (Figure 3 and Additional file 6: Figure S5). Viruses isolated from North and South America were strongly differentiated (TMRCA: 1954 [1928-1969]). The TMRCA of the South American sub-lineage was 1999 [1997-2001] and included a LP H7N3 virus isolated in a wild bird (A/Cinnamon Teal/Bolivia/4537/2001), presumably at the origin of the circulation of LP and HP H7N3 viruses detected in domestic birds in Chile during the following year (host shift I; cf.  for discussion).
The North American lineage was divided into three major sub-lineages. An ancient sub-lineage included viruses isolated from wild and domestic birds, between 1980 and 1993. Based on the TMRCA (1979 [1977-1980]), this genetic sub-lineage circulated for at least 14 years in the USA and Canada. Several host shifts would have occurred (K), however the direction of virus introductions (i.e. wild-to-domestic or domestic-to-wild) remains to be determined.
A second genetic sub-lineage included LP H7 IA viruses isolated from wild and domestic birds in North America, between 2001 and 2008. Two genetic groups were observed and suggested the co-circulation of genetically different H7 IA viruses in wild waterbirds between 2005 and 2006 (TMRCA: 2005 [2005-2006] for both clades, including A/Ruddy Turnstone/Delaware/752/2006 and A/Blue-winged Teal/Ohio/566/2006, respectively; Figure S5). Genetically related viruses isolated in domestic birds suggested two host shifts (M and N; Figure 3, Additional file 6: Figure S5 and Additional file 7: Table S2), with a rapid evolution toward an increased virulence (cf. [37, 63] for discussion).
We underlined the long-term circulation of H7 HA in domestic birds in the USA and dated the emergence of this sub-lineage in 1992-1993 (TMRCA: 1993 [1989-1994]). These viruses would have circulated for 13 years in domestic birds with co-circulation of genetically different viruses at the same time and location (e.g. in Chicken in New-York in 2005; Additional file 6: Figure S5).
In wild birds, temporal changes in the relative genetic diversity showed an important decrease after 2000, and a recent peak in 2006 (Figure 4B). In the USA, after introduction in domestic birds, important fluctuations were observed from 1992 onward with peaks of diversity reached every 2-3 years (Figure 4G), although these changes were not significant.
Mean nucleotide substitution rates were calculated for viruses isolated in wild and domestic birds, separately, for each major genetic lineage (NA-SA and EURAS-AF-AU) and sub-lineages (Table 1). Overall, high rates of substitutions were obtained, ranging from 0.37 × 10-3 to 11.74 × 10-3 substitution/site/years. Higher rates were obtained for viruses circulating in wild birds than for those isolated from domestic birds. Differences were also obtained in domestic birds between genetic lineages.
We tested evidence of positive or negative selection for the same genetic lineages (Table 1). For viruses isolated in wild birds, a strong evidence of purifying selection was found with low mean dN/dS ratio and numerous negatively selected sites. When all viruses isolated from domestic birds were considered, for each geographic lineage (EURAS-AF-AU and NA-SA), strong evidence of purifying selection also was found (i.e. sites under negative selection); however several sites were detected to be under positive selection. When analyzing the main genetic sub-lineages independently (Table 1), high dN/dS ratios were also found, with a lower number of sites under negative selection and evidence for positive selection for several amino acids.
The majority of positively selected sites were located in the HA1 coding region of the HA (position 1 to 339, H7 HA numbering; Table 1). Only two amino acids, at positions 340 and 341, were located outside the HA1 coding region, in the cleavage site of the protein. We determined the N-linked glycosylation sites in the HA protein for viruses isolated in wild birds. Three sites were identified (position 30, 46 and 249) in the HA1 coding region, for 100% of the sequences analyzed. Among the positively selected sites in domestic birds, we found that the amino acid mutation at position 143 (from Ala, Val or Lys, to Thr) led to the generation of a fourth sequon (140-Asn-Gly-Thr-144) in the HA1 coding region. In wild birds, only one of the 68 sequences analyzed showed the presence of this fourth sequon (A/Northern shoveler/California/HKWF1026/2007). For domestic bird viruses, there were also evidence of positive selection for amino acid changes leading to the removal of a highly conserved sequon, at position 46 (45-Asn-Ala-Thr-49 to 45-Asp-Ala-Thr-49). This observation needs to be confirmed as only one of the three methods detected evidence of positive selection and the analysis was based on a low number of sequences (Table 1). The consequence of amino acid mutation at other sites under positive selection is not clear, although most were located in, or at the edge of, the receptor binding domain of the HA protein (positions 139, 148, 150 and 152).
In this study, we provided an extensive phylogenetic analysis of 414 H7 IA viruses that have been isolated worldwide in wild and domestic birds, and investigated the consequences of host shifts on the molecular evolution of the HA gene. First, we underlined that precise information related to host is critical. The lack of accuracy related to the species or 'host status' (domestic or wild) in public sequence databases limits our ability to effectively study host shifts and their consequences on viral evolution. We indeed excluded 14% of the sequences composing the initial dataset because of missing or unclear information related to the bird species or status. In the context of increasing isolations of IA virus and availability of sequences, this issue needs immediate attention.
Evolutionary history of H7 IA virus in wild birds
The overall phylogenetic structure of H7 HA was concordant with results reported in previous studies, based on a limited number of sequences (e.g. ): a clear phylogeographic pattern was evidenced with distinct genetic lineages in the Eastern and Western hemispheres. High divergence was observed, in particular for the Australian and South American clades, but also at a smaller spatial scale for viruses responsible for outbreaks in domestic birds. Genetic exchanges between biogeographical regions have been documented for IA viruses, especially between North America and Europe, and North America and Asia (e.g. ). These exchanges are likely to be favored by habitat sharing in breeding areas between waterbirds using different migratory flyways, and can have important consequences on the population dynamics of IA viruses in wild birds, such as the replacement of endemic genetic lineages . For H7 IA viruses, the strong phylogeographic structure supports that no intercontinental exchanges resulting in the replacement of an endemic lineage have occurred recently between Eurasia and the Americas.
Regular emergence and co-circulation of genetically different H7 IA virus lineages were identified in wild waterbird populations. In Eurasia, the circulation of four lineages between 1999 and 2001 was followed by a slight decrease in the genetic diversity and the emergence of a new and diversified lineage since 2004-2005. In North America, one dominant lineage has circulated since 2006. Rapid genetic diversification and extinction processes seem to occur for H7 IA viruses, concordant with surveillance studies that have related strong temporal differences of H7 subtype prevalence in waterbird populations in both Europe and North America [65, 66]. Along with previous studies, we suggest that population immunity could play an important role for these epidemiological fluctuations and be responsible for rapid changes in the relative genetic diversity of H7 IA viruses in wild waterbirds [65, 67].
Virus spillover to wild birds has been documented many times in association with the circulation of the HP H5N1 IA viruses in Southeastern Asia [24–26]. Introduction of HP viruses from domestic to peri-domestic and wild bird species often occurs in areas where the circulation of these viruses is endemic in domestic birds, and has been suggested as a mechanism of perpetuation of HP IA during and potentially between outbreaks [68, 69]. This hypothesis however lacks epidemiological evidence, especially when considering the potential role of domestic ducks as reservoir for HP IA viruses [70, 71]. After spillover to wild waterbirds, HP IA virus spread has been shown to be limited in space and time and dependent on various host and environmental factors, such as distance traveled between migratory stopovers, population density, air temperature or virus persistence in aquatic habitats [72–76].
For H7 IA viruses (both LP and HP), we did not detected domestic-to-wild bird transmission with long-term circulation of the introduced virus in wild waterbird populations. Although this could be related to the relatively low number of sequences available from wild birds, this could also reflect adaptations to domestic hosts that result in reduce fitness after spillover in wild waterbird populations . Such hypothesis has been proposed to explain the absence of long-term circulation of HP viruses in wild waterbird populations  and could be extended to LP IA viruses that had undergone significant adaptations to domestic hosts and environments, precluding persistence in the wild when a spillover occurs.
Host shifts and population dynamics in domestic birds
Wild-to-domestic bird transmission has been previously documented and includes the detection of LP ancestral virus of the Asian HP H5N1 viruses . In the present study, ten independent host shifts (wild-to-domestic birds) were detected for H7 IA viruses. Local circulation of a LP H7 viruses in wild waterbirds has been shown to occurs prior to the introduction in poultry (e.g. [21, 37]). Wallensten et al. also suggested that the increase LP H7 virus prevalence in waterbirds could favor introduction in poultry and affect the risk for emergence of HP strains . In this study, we observed a slight increase of the genetic diversity of viruses circulating in wild waterbirds in Europe since 2004-2005, followed by three independent introductions in domestic birds. Interactions between IA virus prevalence in wild waterbirds, changes in the genetic diversity of viral populations and introduction in domestic birds need to be further studied; such information could provide a basis to estimate the risks of emergence of IA virus in poultry. In addition, local surveillance of LP H7 viruses in wild waterbirds has the potential to provide critical information for the development of efficient vaccines protecting poultry against the emergence of highly pathogenic viruses, as proposed by Sakabe et al. .
Highly pathogenic H7 IA viruses have circulated for extended periods of time in domestic birds in Pakistan and Australia. In Australia, there was evidence of genetic reassortment, in particular with different NA subtypes [32, 33]. Because of the very low number of isolates available in this long time period, it is not possible to determine if the H7 HA genetic lineage represents an endemic circulation of virulent viruses or is the result of multiple introductions of LP viruses that evolved into HP forms.
In Pakistan, HP H7 IA viruses have also circulated for an extended period of time, as the result of poor vaccination or eradication strategies before 2002-2003 . The emergence of these viruses have occurred after 1993 with a co-circulation of two or three major genetic sub-lineages. In addition, H9N2 IA virus also have been detected in poultry infected with HP H7N3 virus with evidence of genetic reassortment between the two viruses . Such a pattern of emergence of HP viruses, with the co-circulation of several genetic lineages and reassortments with other circulating LP viruses (in particular H9N2) is similar to the one observed for HP H5N1 in Southeastern Asia [23, 49, 80, 81].
In live bird markets in the USA, rapid changes in the relative genetic diversity have occurred, with peaks of diversity reached every 2-3 years, suggesting diversification and extinction processes for different sub-lineages of H7 HA. These changes could have been driven by eradication procedures that were initiated to control the outbreak. The case of the H5N2 outbreak in poultry in Mexico, with the diversification and co-circulation of multiple genetic sub-lineages after vaccination, has shown that control and eradication procedures can act as a strong selective pressure for IA evolution . In the USA, the H7 IA virus genetic diversity has also been maintained by genetic reassortments with the NA and NS (nonstructural) gene segments of other IA viruses circulating in live bird markets [38, 39]. Finally, no case of HP viruses were detected in this genetic lineage, although evidence of mutations at the cleavage sites were described , contrasting with the rapid evolution toward an increased virulence that was observed in Italy, Australia, Pakistan and Canada.
Emergence of domestic bird-adapted IA viruses
Influenza A viruses exhibit high rates of nucleotide substitutions allowing continuous antigenic changes, favoring escape from population immunity [49, 50, 83]. In this study, we highlighted that differences in this rate can be obtained when viruses isolated from wild and domestic birds are analyzed separately. Overall, we found higher rates for viruses circulating in wild birds than in domestic birds, however, important differences were observed between genetic lineages in domestic birds (from 0.37 x10-3 to 5.59 x10-3 substitution/site/year). Previous works has suggested that after introduction in poultry the rate of nucleotide substitution could increases significantly (e.g. for H5 HA; ). For H7 HA, the opposite trend was observed but we speculate that this rate is highly variable in poultry, most likely depending on the selective pressures from the domestic environment. In wild waterbirds, naturally acquired immunity has a strong importance on IA virus evolution, favoring continuous antigenic changes. In domestic bird populations, rates of nucleotide substitutions could be strongly affected by immunity acquired through vaccination. This would explain the differences observed between outbreaks where no vaccination was performed for several years (e.g. Pakistan) as compared to the ones where intensive control strategies were developed (e.g. USA, Italy), although such trend would needs further investigations to be validated.
During the last century, at least 11 unrelated genetic lineages of HP H7 IA virus would have circulated in domestic birds. As for H5 IA viruses, evolution from LP to HP viruses arose at several occasions, independently . The amino acid sequences of the H7 HA cleavage site is highly polymorphic between genetic lineages. For instance, evidence of recombination between the HA and the NP genes has been shown to be at the origin of the nucleotide insertion in the cleavage site for the Chilean outbreak . In addition to these reports from outbreaks in poultry, experimental infections with different HP H7 strains have confirmed that different amino acid sequences in the cleavage site can lead to similar levels of virulence [30, 68, 87, 88]. Amino acid insertion in the H7 HA cleavage site could therefore be selected for but with little evidence for a specific sequence motif, as the resulting increase in virulence is likely to increase viral fitness compared to wild-bird origin LP viruses. The absence of detection of HP viruses in H7 genetic lineages that circulated for extended periods of time (e.g. in the USA between 1994 and 2006) however suggests that high virulence is not likely to be selected for under all type of bird production systems, and may not always represents an optimum for viral fitness.
As reported by previous studies [49, 50, 83], the low mean dN/dS ratios and the detection of numerous negatively selected sites supports that the H7 HA is under strong purifying selection. For viruses circulating in domestic birds, we found several sites under positive selection, most of them located in the receptor binding domain. In particular, the amino acid mutation at position 143 (135 in H3 numbering; ) led to the generation of an additional N-linked glycosylation site in the HA1 coding region. N-linked glycosylation sites are recognized to be involved in host cell entry, proteolytic processing and protein trafficking . The amino acid mutation at position 143 was positively selected in three unrelated genetic lineages (USA, Pakistan and Italy during the 1999-2001 outbreak). In addition, a previous study also reported the acquisition of this glycosylation site in unconnected farms in Italy and suggested that strong selective pressure in poultry could favor the simultaneous and rapid acquisition of this amino acid mutation in separate genetic lineages . The location of this amino acid mutation, at the right edge of the receptor binding domain of the HA protein , suggests an importance for host infection and provides evidence of selection for increased host specificity in domestic birds. In addition, it has been shown that important amino acid mutations in the receptor-binding site can be fully functional , supporting that such structural modifications in the HA protein can potentially increase IA viral fitness in domestic birds. Positive selection for amino acid changes in genetically unrelated lineages supports evidence of parallel evolution in the HA gene of IA viruses in domestic birds. This is likely to result from host shifts and broadly to be a response to the new selective pressures composing domestic bird production systems (e.g. host density, host genetic diversity, population immunity, environmental transmission), and underlines the need to consider host ecology as well as the consequences of ecosystem shifts in IA virus evolution.
Palumbi SR: Humans as the world's greatest evolutionary force. Science 2001, 293: 1786-1790. 10.1126/science.293.5536.1786
Lebarbenchon C, Brown SP, Poulin R, Gauthier-Clerc M, Thomas F: Evolution of pathogens in a man-made world. Mol Ecol 2008, 17: 475-484. 10.1111/j.1365-294X.2007.03375.x
Barrett R, Kuzawa C, McDade T, Armelagos G: Emerging and re-emerging infectious diseases: the third epidemiological transition. Annu Rev Anthropol 1998, 27: 247-271. 10.1146/annurev.anthro.27.1.247
Mennerat A, Nilsen F, Ebert D, Skorping A: Intensive farming: evolutionary implications for parasites and pathogens. Evol Biol 2010, 37: 59-67. 10.1007/s11692-010-9089-0
Gilbert M, Xiao X, Pfeiffer DU, Epprecht M, Boles S, Czarnecki C, Chaitaweesub P, Kalpravidh W, Minh PQ, Otte MJ, Martin V, Slingenbergh J: Mapping H5N1 highly pathogenic avian influenza risk in Southeast Asia. Proc Natl Acad Sci USA 2008, 105: 4769-4774. 10.1073/pnas.0710581105
Woo PC, Lau SK, Yuen K: Infectious diseases emerging from Chinese wet-markets: zoonotic origins of severe respiratory viral infections. Curr Opin Infect Dis 2006, 19: 401-407. 10.1097/01.qco.0000244043.08264.fc
Leibler JH, Otte J, Roland-Holst D, Pfeiffer DU, Soares Magalhaes R, Rushton J, Graham JP, Silbergeld EK: Industrial food animal production and global health risks: exploring the ecosystems and economics of avian influenza. Ecohealth 2009, 6: 58-70. 10.1007/s10393-009-0226-0
Harder TC, Teuffert J, Starick E, Gethmann J, Grund C, Fereidouni S, Durban M, Bogner KH, Neubauer-Juric A, Repper R, Hlinak A, Engelhardt A, Nöckler A, Smietanka K, Minta Z, Kramer M, Globig A, Mettenleiter TC, Conraths FJ, Beer M: Highly pathogenic avian influenza virus (H5N1) in frozen duck carcasses, Germany, 2007. Emerg Infect Dis 2009, 15: 272-279. 10.3201/eid1502.080949
Hogerwerf L, Wallace RG, Ottaviani D, Slingenbergh J, Prosser D, Bergmann L, Gilbert M: Persistence of Highly Pathogenic Avian Influenza H5N1 Virus Defined by Agro-Ecological Niche. Ecohealth 2010, 7: 213-225. 10.1007/s10393-010-0324-z
Peiris JSM, de Jong MD, Guan Y: Avian influenza virus (H5N1): a threat to human health. Clin Microbiol Rev 2007, 20: 243-267. 10.1128/CMR.00037-06
Garten RJ, Davis CT, Russell CA, Shu B, Lindstrom S, Balish A, Sessions WM, Xu X, Skepner E, Deyde V, Okomo-Adhiambo M, Gubareva L, Barnes J, Smith CB, Emery SL, Hillman MJ, Rivailler P, Smagala J, de Graaf M, Burke DF, Fouchier RAM, Pappas C, Alpuche-Aranda CM, López-Gatell H, Olivera H, López I, Myers CA, Faix D, Blair PJ, Yu C, et al.: Antigenic and genetic characteristics of swine-origin 2009 A(H1N1) influenza viruses circulating in humans. Science 2009, 325: 197-201. 10.1126/science.1176225
Smith GJD, Vijaykrishna D, Bahl J, Lycett SJ, Worobey M, Pybus OG, Ma SK, Cheung CL, Raghwani J, Bhatt S, Peiris JSM, Guan Y, Rambaut A: Origins and evolutionary genomics of the 2009 swine-origin H1N1 influenza A epidemic. Nature 2009, 459: 1122-1125. 10.1038/nature08182
Holmes EC: Evolution in health and medicine Sackler colloquium: The comparative genomics of viral emergence. Proc Natl Acad Sci USA 2010,107(S1):1742-1746.
Olsen B, Munster VJ, Wallensten A, Waldenström J, Osterhaus ADME, Fouchier RAM: Global patterns of influenza a virus in wild birds. Science 2006, 312: 384-388. 10.1126/science.1122438
van Gils JA, Munster VJ, Radersma R, Liefhebber D, Fouchier RAM, Klaassen M: Hampered foraging and migratory performance in swans infected with low-pathogenic avian influenza A virus. PLoS ONE 2007, 2: e184. 10.1371/journal.pone.0000184
Latorre-Margalef N, Gunnarsson G, Munster VJ, Fouchier RAM, Osterhaus ADME, Elmberg J, Olsen B, Wallensten A, Haemig PD, Fransson T, Brudin L, Waldenström J: Effects of influenza A virus infection on migrating mallard ducks. P Roy Soc B-Biol Sci 2009, 276: 1029-1036. 10.1098/rspb.2008.1501
Swayne DE, Suarez DL: Highly pathogenic avian influenza. Rev Sci Tech 2000, 19: 463-482.
Alexander DJ: An overview of the epidemiology of avian influenza. Vaccine 2007, 25: 5637-5644. 10.1016/j.vaccine.2006.10.051
Becker WB: The isolation and classification of Tern virus: influenza A-Tern South Africa--1961. J Hyg 1966, 64: 309-320. 10.1017/S0022172400040596
Gaidet N, Cattoli G, Hammoumi S, Newman SH, Hagemeijer W, Takekawa JY, Cappelle J, Dodman T, Joannis T, Gil P, Monne I, Fusaro A, Capua I, Manu S, Micheloni P, Ottosson U, Mshelbwala JH, Lubroth J, Domenech J, Monicat F: Evidence of infection by H5N2 highly pathogenic avian influenza viruses in healthy wild waterfowl. PLoS Pathog 2008, 4: e1000127. 10.1371/journal.ppat.1000127
Campitelli L, Mogavero E, De Marco MA, Delogu M, Puzelli S, Frezza F, Facchini M, Chiapponi C, Foni E, Cordioli P, Webby R, Barigazzi G, Webster RG, Donatelli I: Interspecies transmission of an H7N3 influenza virus from wild birds to intensively reared domestic poultry in Italy. Virology 2004, 323: 24-36. 10.1016/j.virol.2004.02.015
Munster VJ, Wallensten A, Baas C, Rimmelzwaan GF, Schutten M, Olsen B, Osterhaus ADME, Fouchier RAM: Mallards and highly pathogenic avian influenza ancestral viruses, northern Europe. Emerg Infect Dis 2005, 11: 1545-1551.
Duan L, Campitelli L, Fan XH, Leung YHC, Vijaykrishna D, Zhang JX, Donatelli I, Delogu M, Li KS, Foni E, Chiapponi C, Wu WL, Kai H, Webster RG, Shortridge KF, Peiris JSM, Smith GJD, Chen H, Guan Y: Characterization of low-pathogenic H5 subtype influenza viruses from Eurasia: implications for the origin of highly pathogenic H5N1 viruses. J Virol 2007, 81: 7529-7539. 10.1128/JVI.00327-07
Chen H, Smith GJD, Zhang SY, Qin K, Wang J, Li KS, Webster RG, Peiris JSM, Guan Y: Avian flu: H5N1 virus outbreak in migratory waterfowl. Nature 2005, 436: 191-192. 10.1038/nature03974
Ellis TM, Dyrting KC, Wong CW, Chadwick B, Chan C, Chiang M, Li C, Li P, Smith GJD, Guan Y, Malik Peiris JS: Analysis of H5N1 avian influenza infections from wild bird surveillance in Hong Kong from January 2006 to October 2007. Avian Pathol 2009, 38: 107-119. 10.1080/03079450902751855
Feare CJ: Role of wild birds in the spread of highly pathogenic avian influenza virus H5N1 and implications for global surveillance. Avian Dis 2010, 54: 201-212. 10.1637/8766-033109-ResNote.1
Banks J, Speidel ES, Moore E, Plowright L, Piccirillo A, Capua I, Cordioli P, Fioretti A, Alexander DJ: Changes in the haemagglutinin and the neuraminidase genes prior to the emergence of highly pathogenic H7N1 avian influenza viruses in Italy. Arch Virol 2001, 146: 963-973. 10.1007/s007050170128
Di Trani L, Bedini B, Cordioli P, Muscillo M, Vignolo E, Moreno A, Tollis M: Molecular characterization of low pathogenicity H7N3 avian influenza viruses isolated in Italy. Avian Dis 2004, 48: 376-383. 10.1637/7088
Campitelli L, Di Martino A, Spagnolo D, Smith GJD, Di Trani L, Facchini M, De Marco MA, Foni E, Chiapponi C, Martin AM, Chen H, Guan Y, Delogu M, Donatelli I: Molecular analysis of avian H7 influenza viruses circulating in Eurasia in 1999-2005: detection of multiple reassortant virus genotypes. J Gen Virol 2008, 89: 48-59. 10.1099/vir.0.83111-0
Röhm C, Süss J, Pohle V, Webster RG: Different hemagglutinin cleavage site variants of H7N7 in an influenza outbreak in chickens in Leipzig, Germany. Virology 1996, 218: 253-257. 10.1006/viro.1996.0187
Stegeman A, Bouma A, Elbers ARW, de Jong MCM, Nodelijk G, de Klerk F, Koch G, van Boven M: Avian influenza A virus (H7N7) epidemic in The Netherlands in 2003: course of the epidemic and effectiveness of control measures. J Infect Dis 2004, 190: 2088-2095. 10.1086/425583
Selleck PW, Arzey G, Kirkland PD, Reece RL, Gould AR, Daniels PW, Westbury HA: An outbreak of highly pathogenic avian influenza in Australia in 1997 caused by an H7N4 virus. Avian Dis 2003, 47: 806-811. 10.1637/0005-2086-47.s3.806
Bulach D, Halpin R, Spiro D, Pomeroy L, Janies D, Boyle DB: Molecular analysis of H7 avian influenza viruses from Australia and New Zealand: genetic diversity and relationships from 1976 to 2007. J Virol 2010, 84: 9957-9966. 10.1128/JVI.00930-10
Li Y, Li C, Liu L, Wang H, Wang C, Tian G, Webster RG, Yu K, Chen H: Characterization of an avian influenza virus of subtype H7N2 isolated from chickens in northern China. Virus Genes 2006, 33: 117-122. 10.1007/s11262-005-0042-8
Abbas MA, Spackman E, Swayne DE, Ahmed Z, Sarmento L, Siddique N, Naeem K, Hameed A, Rehmani S: Sequence and phylogenetic analysis of H7N3 avian influenza viruses isolated from poultry in Pakistan 1995-2004. Virol J 2010, 7: 137. 10.1186/1743-422X-7-137
Pasick J, Handel K, Robinson J, Copps J, Ridd D, Hills K, Kehler H, Cottam-Birt C, Neufeld J, Berhane Y, Czub S: Intersegmental recombination between the haemagglutinin and matrix genes was responsible for the emergence of a highly pathogenic H7N3 avian influenza virus in British Columbia. J Gen Virol 2005, 86: 727-731. 10.1099/vir.0.80478-0
Berhane Y, Hisanaga T, Kehler H, Neufeld J, Manning L, Argue C, Handel K, Hooper-McGrevy K, Jonas M, Robinson J, Webster RG, Pasick J: Highly pathogenic avian influenza virus A (H7N3) in domestic poultry, Saskatchewan, Canada, 2007. Emerg Infect Dis 2009, 15: 1492-1495.
Suarez DL, Garcia M, Latimer J, Senne D, Perdue M: Phylogenetic analysis of H7 avian influenza viruses isolated from the live bird markets of the Northeast United States. J Virol 1999, 73: 3567-3573.
Spackman E, Senne DA, Davison S, Suarez DL: Sequence analysis of recent H7 avian influenza viruses associated with three different outbreaks in commercial poultry in the United States. J Virol 2003, 77: 13399-13402. 10.1128/JVI.77.24.13399-13402.2003
Spackman E, McCracken KG, Winker K, Swayne DE: H7N3 avian influenza virus found in a South American wild duck is related to the Chilean 2002 poultry outbreak, contains genes from equine and North American wild bird lineages, and is adapted to domestic turkeys. J Virol 2006, 80: 7760-7764. 10.1128/JVI.00445-06
Bao Y, Bolotov P, Dernovoy D, Kiryutin B, Zaslavsky L, Tatusova T, Ostell J, Lipman D: The influenza virus resource at the National Center for Biotechnology Information. J Virol 2008, 82: 596-601. 10.1128/JVI.02005-07
Krasnitz M, Levine AJ, Rabadan R: Anomalies in the influenza virus genome database: new biology or laboratory errors? J Virol 2008, 82: 8947-8950. 10.1128/JVI.00101-08
R Development Core Team: R: A language and environment for statistical computing. Vienna, Austria; 2009.
Larkin MA, Blackshields G, Brown NP, Chenna R, McGettigan PA, McWilliam H, Valentin F, Wallace IM, Wilm A, Lopez R, Thompson JD, Gibson TJ, Higgins DG: Clustal W and Clustal X version 2.0. Bioinformatics 2007, 23: 2947-2948. 10.1093/bioinformatics/btm404
Guindon S, Gascuel O: A simple, fast, and accurate algorithm to estimate large phylogenies by maximum likelihood. Syst Biol 2003, 52: 696-704. 10.1080/10635150390235520
Keane TM, Creevey CJ, Pentony MM, Naughton TJ, Mclnerney JO: Assessment of methods for amino acid matrix selection and their use on empirical data shows that ad hoc assumptions for choice of matrix are not justified. BMC Evol Biol 2006, 6: 29. 10.1186/1471-2148-6-29
Drummond AJ, Rambaut A: BEAST: Bayesian evolutionary analysis by sampling trees. BMC Evol Biol 2007, 7: 214. 10.1186/1471-2148-7-214
Shapiro B, Rambaut A, Drummond AJ: Choosing appropriate substitution models for the phylogenetic analysis of protein-coding sequences. Mol Biol Evol 2006, 23: 7-9.
Vijaykrishna D, Bahl J, Riley S, Duan L, Zhang JX, Chen H, Peiris JSM, Smith GJD, Guan Y: Evolutionary dynamics and emergence of panzootic H5N1 influenza viruses. PLoS Pathog 2008, 4: e1000161. 10.1371/journal.ppat.1000161
Bahl J, Vijaykrishna D, Holmes EC, Smith GJD, Guan Y: Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts. Virology 2009, 390: 289-297. 10.1016/j.virol.2009.05.002
Drummond AJ, Ho SYW, Phillips MJ, Rambaut A: Relaxed phylogenetics and dating with confidence. PLoS Biol 2006, 4: e88. 10.1371/journal.pbio.0040088
Kass RE, Raftery AE: Bayes Factors. J Am Stat Assoc 1995, 90: 773-795. 10.2307/2291091
Suchard MA, Weiss RE, Sinsheimer JS: Bayesian selection of continuous-time Markov chain evolutionary models. Mol Biol Evol 2001, 18: 1001-1013.
Rambaut A, Drummond AJ: Tracer v1.5.[http://tree.bio.ed.ac.uk/software/tracer]
Brandley MC, Schmitz A, Reeder TW: Partitioned Bayesian analyses, partition choice, and the phylogenetic relationships of scincid lizards. Syst Biol 2005, 54: 373-390. 10.1080/10635150590946808
Drummond AJ, Rambaut A, Shapiro B, Pybus OG: Bayesian coalescent inference of past population dynamics from molecular sequences. Mol Biol Evol 2005, 22: 1185-1192. 10.1093/molbev/msi103
Rambaut A, Pybus OG, Nelson MI, Viboud C, Taubenberger JK, Holmes EC: The genomic and epidemiological dynamics of human influenza A virus. Nature 2008, 453: 615-619. 10.1038/nature06945
Kosakovsky Pond SL, Frost SDW, Muse SV: HyPhy: hypothesis testing using phylogenies. Bioinformatics 2005, 21: 676-679. 10.1093/bioinformatics/bti079
Kosakovsky Pond SL, Frost SDW: Datamonkey: rapid detection of selective pressure on individual sites of codon alignments. Bioinformatics 2005, 21: 2531-2533. 10.1093/bioinformatics/bti320
Delport W, Poon AFY, Frost SDW, Kosakovsky Pond SL: Datamonkey 2010: a suite of phylogenetic analysis tools for evolutionary biology. Bioinformatics 2010, 26: 2455-2457. 10.1093/bioinformatics/btq429
Kosakovsky Pond SL, Frost SDW: Not So Different After All: A Comparison of Methods for Detecting Amino Acid Sites Under Selection. Mol Biol Evol 2005, 22: 1208-1222. 10.1093/molbev/msi105
Banks J, Speidel EC, McCauley JW, Alexander DJ: Phylogenetic analysis of H7 haemagglutinin subtype influenza A viruses. Arch Virol 2000, 145: 1047-1058. 10.1007/s007050050695
Pasick J, Berhane Y, Hisanaga T, Kehler H, Hooper-McGrevy K, Handel K, Neufeld J, Argue C, Leighton F: Diagnostic test results and pathology associated with the 2007 Canadian H7N3 highly pathogenic avian influenza outbreak. Avian Dis 2010, 54: 213-219. 10.1637/8822-040209-Reg.1
Krauss S, Obert CA, Franks J, Walker D, Jones K, Seiler P, Niles L, Pryor SP, Obenauer JC, Naeve CW, Widjaja L, Webby RJ, Webster RG: Influenza in migratory birds and evidence of limited intercontinental virus exchange. PLoS Pathog 2007, 3: e167. 10.1371/journal.ppat.0030167
Krauss S, Walker D, Pryor SP, Niles L, Chenghong L, Hinshaw VS, Webster RG: Influenza A viruses of migrating wild aquatic birds in North America. Vector Borne Zoonotic Dis 2004, 4: 177-189. 10.1089/vbz.2004.4.177
Wallensten A, Munster VJ, Latorre-Margalef N, Brytting M, Elmberg J, Fouchier RAM, Fransson T, Haemig PD, Karlsson M, Lundkvist A, Osterhaus ADME, Stervander M, Waldenström J, Björn O: Surveillance of influenza A virus in migratory waterfowl in northern Europe. Emerg Infect Dis 2007, 13: 404-411. 10.3201/eid1303.061130
Sharp GB, Kawaoka Y, Wright SM, Turner B, Hinshaw V, Webster RG: Wild ducks are the reservoir for only a limited number of influenza A subtypes. Epidemiol Infect 1993, 110: 161-176. 10.1017/S0950268800050780
Nestorowicz A, Kawaoka Y, Bean WJ, Webster RG: Molecular analysis of the hemagglutinin genes of Australian H7N7 influenza viruses: role of passerine birds in maintenance or transmission? Virology 1987, 160: 411-418. 10.1016/0042-6822(87)90012-2
Smith GJD, Vijaykrishna D, Ellis TM, Dyrting KC, Leung YHC, Bahl J, Wong CW, Kai H, Chow MKW, Duan L, Chan ASL, Zhang LJ, Chen H, Luk GSM, Peiris JSM, Guan Y: Characterization of avian influenza viruses A (H5N1) from wild birds, Hong Kong, 2004-2008. Emerg Infect Dis 2009, 15: 402-407. 10.3201/eid1503.081190
Hulse-Post DJ, Sturm-Ramirez KM, Humberd J, Seiler P, Govorkova EA, Krauss S, Scholtissek C, Puthavathana P, Buranathai C, Nguyen TD, Long HT, Naipospos TSP, Chen H, Ellis TM, Guan Y, Peiris JSM, Webster RG: Role of domestic ducks in the propagation and biological evolution of highly pathogenic H5N1 influenza viruses in Asia. Proc Natl Acad Sci USA 2005, 102: 10682-10687. 10.1073/pnas.0504662102
Sturm-Ramirez KM, Hulse-Post DJ, Govorkova EA, Humberd J, Seiler P, Puthavathana P, Buranathai C, Nguyen TD, Chaisingh A, Long HT, Naipospos TSP, Chen H, Ellis TM, Guan Y, Peiris JSM, Webster RG: Are ducks contributing to the endemicity of highly pathogenic H5N1 influenza virus in Asia? J Virol 2005, 79: 11269-11279. 10.1128/JVI.79.17.11269-11279.2005
Lebarbenchon C, Albespy F, Brochet A, Grandhomme V, Renaud F, Fritz H, Green AJ, Thomas F, van der Werf S, Aubry P, Guillemain M, Gauthier-Clerc M: Spread of avian influenza viruses by Common Teal ( Anas crecca ) in Europe. PLoS ONE 2009, 4: e7289. 10.1371/journal.pone.0007289
Brochet A, Guillemain M, Lebarbenchon C, Simon G, Fritz H, Green AJ, Renaud F, Thomas F, Gauthier-Clerc M: The potential distance of highly pathogenic avian influenza virus dispersal by mallard, common teal and Eurasian pochard. Ecohealth 2009, 6: 449-457. 10.1007/s10393-010-0275-4
Bourouiba L, Wu J, Newman S, Takekawa J, Natdorj T, Batbayar N, Bishop CM, Hawkes LA, Butler PJ, Wikelski M: Spatial dynamics of bar-headed geese migration in the context of H5N1. J R Soc Interface 2010, 7: 1627-1639. 10.1098/rsif.2010.0126
Reperant LA, Fuckar NS, Osterhaus ADME, Dobson AP, Kuiken T: Spatial and temporal association of outbreaks of H5N1 influenza virus infection in wild birds with the 0 degrees C isotherm. PLoS Pathog 2010, 6: e1000854. 10.1371/journal.ppat.1000854
Gaidet N, Capelle J, Takekawa J, Prosser D, Iverson S, Douglas D, Perry W, Mundkur T, Newman S: Potential spread of highly pathogenic avian influenza H5N1 by wildfowl: dispersal ranges and rates determined from large-scale satellite telemetry. J Appl Ecol 2010, 47: 1147-1157. 10.1111/j.1365-2664.2010.01845.x
Dugan VG, Chen R, Spiro DJ, Sengamalay N, Zaborsky J, Ghedin E, Nolting J, Swayne DE, Runstadler JA, Happ GM, Senne DA, Wang R, Slemons RD, Holmes EC, Taubenberger JK: The evolutionary genetics and emergence of avian influenza viruses in wild birds. PLoS Pathog 2008, 4: e1000076. 10.1371/journal.ppat.1000076
Lebarbenchon C, Feare CJ, Renaud F, Thomas F, Gauthier-Clerc M: Persistence of highly pathogenic avian influenza viruses in natural ecosystems. Emerg Infect Dis 2010, 16: 1057-1062.
Sakabe S, Sakoda Y, Haraguchi Y, Isoda N, Soda K, Takakuwa H, Saijo K, Sawata A, Kume K, Hagiwara J, Tuchiya K, Lin Z, Sakamoto R, Imamura T, Sasaki T, Kokumai N, Kawaoka Y, Kida H: A vaccine prepared from a non-pathogenic H7N7 virus isolated from natural reservoir conferred protective immunity against the challenge with lethal dose of highly pathogenic avian influenza virus in chickens. Vaccine 2008, 26: 2127-2134.
Guan Y, Shortridge KF, Krauss S, Webster RG: Molecular characterization of H9N2 influenza viruses: were they the donors of the "internal" genes of H5N1 viruses in Hong Kong? Proc Natl Acad Sci USA 1999, 96: 9363-9367. 10.1073/pnas.96.16.9363
Hoffmann E, Stech J, Leneva I, Krauss S, Scholtissek C, Chin PS, Peiris M, Shortridge KF, Webster RG: Characterization of the influenza A virus gene pool in avian species in southern China: was H6N1 a derivative or a precursor of H5N1? J Virol 2000, 74: 6309-6315. 10.1128/JVI.74.14.6309-6315.2000
Lee C, Senne DA, Suarez DL: Effect of vaccine use in the evolution of Mexican lineage H5N2 avian influenza virus. J Virol 2004, 78: 8372-8381. 10.1128/JVI.78.15.8372-8381.2004
Chen R, Holmes EC: Avian influenza virus exhibits rapid evolutionary dynamics. Mol Biol Evol 2006, 23: 2336-2341. 10.1093/molbev/msl102
García M, Suarez DL, Crawford JM, Latimer JW, Slemons RD, Swayne DE, Perdue ML: Evolution of H5 subtype avian influenza A viruses in North America. Virus Res 1997, 51: 115-124. 10.1016/S0168-1702(97)00087-7
Röhm C, Horimoto T, Kawaoka Y, Süss J, Webster RG: Do hemagglutinin genes of highly pathogenic avian influenza viruses constitute unique phylogenetic lineages? Virology 1995, 209: 664-670. 10.1006/viro.1995.1301
Suarez DL, Senne DA, Banks J, Brown IH, Essen SC, Lee C, Manvell RJ, Mathieu-Benson C, Moreno V, Pedersen JC, Panigrahy B, Rojas H, Spackman E, Alexander DJ: Recombination resulting in virulence shift in avian influenza outbreak, Chile. Emerg Infect Dis 2004, 10: 693-699.
Wood GW, Parsons G, Alexander DJ: Replication of influenza A viruses of high and low pathogenicity for chickens at different sites in chickens and ducks following intranasal inoculation. Avian Pathol 1995, 24: 545-551. 10.1080/03079459508419093
Swayne DE, Slemons RD: Using mean infectious dose of high- and low-pathogenicity avian influenza viruses originating from wild duck and poultry as one measure of infectivity and adaptation to poultry. Avian Dis 2008, 52: 455-460. 10.1637/8229-012508-Reg.1
Wilson IA, Skehel JJ, Wiley DC: Structure of the haemagglutinin membrane glycoprotein of influenza virus at 3 A resolution. Nature 1981, 289: 366-373. 10.1038/289366a0
Vigerust DJ, Shepherd VL: Virus glycosylation: role in virulence and immune interactions. Trends Microbiol 2007, 15: 211-218. 10.1016/j.tim.2007.03.003
Nobusawa E, Aoyama T, Kato H, Suzuki Y, Tateno Y, Nakajima K: Comparison of complete amino acid sequences and receptor-binding properties among 13 serotypes of hemagglutinins of influenza A viruses. Virology 1991, 182: 475-485. 10.1016/0042-6822(91)90588-3
Yang H, Chen L, Carney PJ, Donis RO, Stevens J: Structures of receptor complexes of a North American H7N2 influenza hemagglutinin with a loop deletion in the receptor binding site. PLoS Pathog 2010, 6: e1001081. 10.1371/journal.ppat.1001081
For helpful discussions and comments on previous versions of the manuscript, we thank Muriel Dietrich, Shamus Keeler, Andrew Park and Erica Spackman. This work was funded by the National Institute of Allergy and Infectious Diseases, National Institutes of Health (NIH), Department of Health and Human Services, under Contract No. HHSN266200700007C. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIH. CL was supported by a 'Fondation pour la Recherche Médicale' post-doctoral fellowship.
The authors declare that they have no competing interests.
CL conceived the study and analyzed the data. CL and DES wrote the paper. All authors read and approved the final manuscript.
Electronic supplementary material
Additional file 1:Figure S1 Host diversity of the 414 H7 IA viruses included in the analysis. Unidentified hosts included 'avian', 'fowl', 'non-psitaccine' and 'psitaccine' for domestic birds and 'shorebird', 'gull' and 'duck' for wild birds. 'Others' domestic birds included 'African starling', 'common iora', 'conure', 'fairy bluebird', 'macaw', 'parakeet', and 'parrot'. (PDF 51 KB)
Additional file 2:Table S1 Geographic origin of the 414 H7 IA viruses analyzed. Numbers in parenthesis report HP viruses included in each group. (PDF 17 KB)
Additional file 3:Figure S2 Subtype combination frequency of H7 IA viruses for wild (green) and domestic (orange) birds. (PDF 23 KB)
Additional file 4:Figure S3 Detailed maximum likelihood consensus phylogram. Tip color represents the geographic origin of the viruses: dark blue: Europe; light blue: Australia; green: Asia; yellow: Africa; purple: South America; orange: North America. Internal branches were also colored for monophyletic groups. Viral strain names were colored in red for HP viruses and annotated with a W or D for viruses isolated in wild and domestic birds, respectively. Bootstrap values are noted when superior to 90%. (PDF 162 KB)
Additional file 5:Figure S4 Maximum clade credibility tree for the HA of H7 IA viruses isolated in Eurasia, Africa and Australia. Tip and branch colors represent host origin (wild in green, domestic in orange). Viral strain names were colored in red for HP IA viruses. Main genetic lineages, with information related to virus origin, were highlighted in the right part of the tree. Nodes with posterior probability values superior to 0.9 were annotated, as well as 95% highest posterior density for times of the most recent common ancestor (blue bars). Blue letters (A to H) represent potential host shift events. (PDF 51 KB)
Additional file 6:Figure S5 Maximum clade credibility tree isolated for the HA of H7 IA viruses North and South America. Tip and branch colors represent host origin (wild in green, domestic in orange). Viral strain names were colored in red for HP IA viruses. Main genetic lineages, with information related to virus origin, were highlighted in the right part of the tree. Nodes with posterior probability values superior to 0.9 were annotated, as well as 95% highest posterior density for times of the most recent common ancestor (blue bars). Blue letters (I to N) represent potential host shift events. (PDF 56 KB)
Additional file 7:Table S2 Host shift events suggesting an introduction of a H7 IA virus from wild to domestic birds. 1 evidenced from the analysis of the maximum clade credibility trees (Figures 2-3 and S4-S5); 2 most related virus or genetic lineage identified in wild birds; 3 virus or genetic lineage circulating in domestic birds following host shift; 4 time of the most recent common ancestor (TMRCA) between the domestic and wild lineages; 5 estimated time period of circulation in domestic birds, in years (based on the difference between the last virus isolation event and the TMRCA); 6 TMRCA for the 'A/Chicken/Netherlands HP H7N3' clade. (PDF 34 KB)
Authors’ original submitted files for images
Below are the links to the authors’ original submitted files for images.
Rights and permissions
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Lebarbenchon, C., Stallknecht, D.E. Host shifts and molecular evolution of H7 avian influenza virus hemagglutinin. Virol J 8, 328 (2011). https://doi.org/10.1186/1743-422X-8-328
- Influenza A
- receptor binding domain
- parallel evolution