Introduction

In April 2009, a novel strain of the influenza A (H1N1) virus emerged in Mexico, the United States, and multiple other countries. By early June, the World Health Organization reported that the virus had spread to 66 countries with 19,273 confirmed cases including 117 deaths (http://www.who.int/csr/disease/swineflu/en). In the United States, the Centers for Disease Control had reported 11,054 cases, including 17 deaths, spanning all 50 states. The outbreak strain has been identified as a swine-origin influenza virus that resulted from a reassortment of two previously circulating strains: a “triple-reassortant” swine influenza that has been circulating in North America since 1998 and an H1N1 strain that has been circulating for decades in swine populations in Europe and Asia. The new strain contains six segments from the North American lineage and two segments from the Eurasian lineage [1]. An estimate based on coalescent analysis using 23 public hemagglutinin sequences from distinct locations placed the beginning of the outbreak in early January 2009 [2], although incomplete sampling leads to considerable uncertainty in this date.

Results

In this note, we report the results of the application of a comprehensive, computational search for reassortments between the Eurasian and North American swine influenza lineages. Using the genome sequence of one of the isolates from the current S-OIV outbreak, we conducted a comprehensive computational search among all hemagglutinin (HA) and neuraminidase (NA) sequences from H1N1 swine isolates to identify all reassortments matching the outbreak strain. We found that two similar reassortments had occurred among H1N1 isolates collected in Thailand between 2000 and 2006, resulting in multiple infections among pigs and a single sequenced human case, A/Thailand/271/2005 [3], [4]. The human case and two of the swine isolates in Thailand (A/Sw/Chonburi/NIAH9469/2004 and A/Sw/Chonburi/NIAH977/2004) bring together the HA segment from the North American “classical” lineage and the NA segment from the Eurasian lineage, as in the 2009 S-OIV outbreak strain (Figure 1). In the three Thai isolates and the new S-OIV isolates, the segment containing the M protein derives from the Eurasian lineage, and the segment containing the nonstructural protein (NS) derives from the classical lineage. Thus the Thai and S-OIV reassortants have four segments that share similar broad evolutionary history, though there are a number of differences between their sequences. The other four segments — NP, PA, PB1, and PB2 — have a different phylogenetic origin in the Thai sequences than in the 2009 outbreak strain sequences. In the Thai isolates, these internal segments are most closely related to the Eurasian lineage, while in the S-OIV strains they derive from the North American triple-reassortant lineage. The H1N2 triple-reassortant and classical H1N1 lineages share similar HA, M, NS, and NP segments. An additional three isolates, A/swine/Chonburi/05CB1/2005, A/swine/Chonburi/06CB2/2006 [3], [4], and A/swine/Thailand/HF6/2005 [5] have HA and NA segments similar to the Thai sequences described above, but sequences for the internal segments are not available.

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Figure 1. Reassortment history of the 2009 S-OIV outbreak strain and the Thai reassortants.

Colors indicate whether the segment derived from human H3N2, classical H1N1 swine, Eurasian H1N1, or avian influenza. The isolates from Thailand represent the only sequenced examples, prior to S-OIV, of reassortment strains containing the HA segment from classical H1N1 swine and the NA segment from the Eurasian H1N1 swine lineage. The reassortment history of the H1N2 “triple reassortment” was described by Olsen [9]. Arrows indicate ancestor relationships; additional, unobserved reassortment events may have occurred.

https://doi.org/10.1371/journal.pone.0006402.g001

A second set of reassortants, also from Thailand, involved only the HA segment (A/Sw/Chonburi/NIAH589/2005, A/Sw/Chachoengsao/NIAH587/2005, A/Sw/Ratchaburi/NIAH550/2003, and A/Sw/Ratchaburi/NIAH1481/2000 [4]). Our analysis reconfirms the previous identification [3], [4], [5], [6] of the two sets of Thai isolates as classical-Eurasian reassortants, while highlighting their importance in the context of the S-OIV strains. As with the first set of Thai strains above, the second set of strains brought a classical-lineage HA into contact with a Eurasian-lineage NA, but these isolates have a Eurasian NS segment in addition to Eurasian-derived NP, PA, PB1, and PB2 segments. Despite extensive recent phylogenetic analyses of swine influenza [1], [2], [7], [11], the similarities in these reassortment architectures remained unreported.

The Thai reassortants were identified here by an automatic, exhaustive, computational search that compares ensembles of trees sampled from a Markov chain Monte Carlo (MCMC) walk. This search, implemented in a novel reassortment discovery program, exhaustively enumerates all HA-NA tree incompatibilities that have sufficient statistical support to be considered likely reassortments despite the ambiguities in phylogenetic reconstruction. Experiments [8] on collections of human, avian, and artificial sequences indicate that the algorithm has a low false-negative rate, suggesting that the Thai sequences listed above, along with the 2009 S-OIV outbreak sequences, are the only sequenced reassortments that bring together classical H1N1 HA segments and Eurasian H1N1 NA segments. Additional reassortments are present among non-H1N1 swine isolates, including, for example, A/Sw/Saraburi/NIAH13021/2005(H1N2), A/Sw/Ratchaburi/NIAH874/2005(H3N2), A/Sw/Nakhon pathom/NIAH586-1/2005(H3N2), A/Sw/Chachoengsao/2003(H3N2), A/Sw/Ratchaburi/NIAH59/2004(H3N2).

The evolutionary relationship among the S-OIV outbreak strain, the Thai isolates, and other influenza strains is illustrated for the HA and NA segments in Figures 2 and 3. (Trees for the other segments are shown in Figures S1–S6.) The HA tree shows that the most similar sequences to HA for both S-OIV and the Thai reassortants are descendants of classical H1N1, including triple-reassortant H1N2. The sequence similarity between the Thai and S-OIV HA segments ranged from 85–88% for the HA segment and 90–91% for the NA segment. The 7+1 Thai reassortants are split into two clades in the HA tree, probably indicating several reassortment events. Trees for the M, NP, NS, PA, PB1, and PB2 segments are provided in supplemental data.

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Figure 2. Majority consensus tree for the HA segment computed with MrBayes [14].

Numbers on branches give posterior probabilities. S-OIV (H1N1) 2009 isolates are colored red. To simplify the figure, only three S-OIV isolates are shown; all other sequences from S-OIV are nearly identical and would appear in the same location in the trees. Thai isolates representing the 6+2 and 7+1 reassortments are colored green, with Thai (6+2) reassortants marked by arrows. The human cases of infection with swine-origin influenza that appear in the trees are shown in blue.

https://doi.org/10.1371/journal.pone.0006402.g002

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Figure 3. Majority consensus tree for the NA segment, computed and labeled as in Figure 2.

https://doi.org/10.1371/journal.pone.0006402.g003

Discussion

The evolutionary history of swine influenza A (H1N1) over the past decade is complex. For many decades, H1N1 influenza in North American swine (also called “classical” H1N1) mutated relatively slowly [9], but in 1997, a novel reassortant emerged that contained three segments (HA, NA, and PB1) from the human H3N2 virus, and the remaining five from the classical North American H1N1. Soon thereafter, the first “triple reassortant” viruses were documented [10], in which avian-derived PA and PB2 polymerase genes replaced two of the classical segments in the H3N2 reassortant (Figure 1). These triple reassortants rapidly spread through pig populations, and additional reassortments occurred, including one that created an H1N2 triple reassortant by re-combining the classical HA (H1) segment with an H3N2 triple reassortant (Figure 1). Until the current S-OIV outbreak, there were only a few sporadic cases of human infection by triple-reassortant swine influenza A; the first case in the United States was reported to the Centers for Disease Control in December 2005, and only 11 cases were reported subsequently until February 2009 [11].

The results presented here catalog the complete collection of sequenced reassortments for which a combination of the HA and NA segments similar to the S-OIV outbreak has occurred. Our main novel result is that no other sequenced examples of this pattern besides the discussed Thai isolates could be found. We can confidently say then that among publicly available sequences these isolates represent the complete catalog of such events. The collection shows that this has happened at least twice within the past ten years and that all previous such sequenced reassortments were collected in Thailand. Due to the lack of detailed surveillance of swine populations around the world, this almost certainly represents an underestimate in the frequency of classical-Eurasian reassortment.

Comparisons between these previous reassortant strains and the S-OIV strain may shed light on the cause of S-OIV's virulence. The previous reassortants did not cause a major human outbreak, despite bringing together somewhat similar surface proteins. One hypothesis is that subsequent mutations in the HA or NA proteins was sufficient to facilitate human-to-human transmission of S-OIV. Alternatively, it may be that some combination of the internal proteins not shared with the Thai strains (PA, NP, PB1, and PB2), possibly interacting with changes in the surface proteins, has given S-OIV its ability to cause human outbreaks. Owing to our comprehensive search through the available H1N1 swine isolates, we know the Thai isolates described above represent the complete set of currently available reassortant sequences that are available to answer this question.

Methods

Supporting Information