Swine Influenza Virus Reassortants in Pigs | CDC EID

EID Journal Home > Volume 16, Number 7–July 2010

Volume 16, Number 7–July 2010
Dispatch
Novel Swine Influenza Virus Reassortants in Pigs, China
Yuhai Bi,1 Guanghua Fu,1 Jing Chen,1 Jinshan Peng, Yipeng Sun, Jingjing Wang, Juan Pu, Yi Zhang, Huijie Gao, Guangpeng Ma, Fulin Tian, Ian H. Brown, and Jinhua Liu
Author affiliations: China Agricultural University, Beijing, People's Republic of China (Y. Bi, G. Fu, J. Chen, J. Peng, Y. Sun, J. Wang, J. Pu, Y. Zhang, H. Gao, J. Liu); Shandong Animal Disease Control Center, Jinan, People's Republic of China (J. Chen, F. Tian, J. Liu); Veterinary Laboratories Agency–Weybridge, Addlestone, Surrey, UK (I.H. Brown); China Rural Technology Development Center, Beijing (G. Ma); and Chongwen Animal Health Inspection Institute, Beijing (J. Peng)

Suggested citation for this article

Abstract
During swine influenza virus surveillance in pigs in China during 2006–2009, we isolated subtypes H1N1, H1N2, and H3N2 and found novel reassortment between contemporary swine and avian panzootic viruses. These reassortment events raise concern about generation of novel viruses in pigs, which could have pandemic potential.
Genetic characterization of pandemic (H1N1) 2009 virus has indicated that it may have derived from swine (1,2). However, because of the lack of systematic swine influenza surveillance, the generation pathway of the novel virus is uncertain. Therefore, we attempted to obtain more information about swine influenza viruses isolated from pigs.

The Study
During December 2006–February 2009 in the People's Republic of China, 3,546 samples from 3 main swine industry provinces—Fujian (765 samples), Guangdong (1,276 samples), and Shandong (1,505 samples)—were collected for influenza surveillance. Nasal and tracheal swab samples were collected from apparently healthy domestic pigs at abattoirs. Virus isolation and identification were performed as described (3). Of 29 strains of influenza A virus obtained, 19 were subtype H1N1, 1 subtype H1N2, and 9 subtype H3N2. Subtype H1N2 was isolated from diseased pigs in Guangdong Province in 2006; the others were isolated from healthy pigs. Isolation rates for subtypes H1N1 and H3N2 were 0.54% and 0.25%, respectively, indicating that subtype H1N1 viruses were predominant in the sampled pig population.

To determine genetic and antigenic characteristics, we conducted phylogenetic and antigenic analysis of all isolates. Cross–hemagglutination-inhibition showed that the subtype H3N2 viruses could be divided into 2 distinct antigenic groups (Appendix Table 1). Viruses of subtype H1 (swine/Shandong/101/2008, swine/Shandong/327/2008, and swine/Shandong/275/2009) reacted well with antiserum to the European avian-like swine virus, swine/FJ/204/2007, but not with antiserum to classical swine (H1N1) virus, swine/Guangdong/1/2005. The other 6 subtype H1N1 isolates reacted strongly with antiserum to swine/Guangdong/1/2005 (Appendix Table 1; Appendix Table 2), indicating that the antigenicity of the subtype H1N1 viruses could also be divided into 2 distinct antigenic groups. Subtype H1N2 virus (swine/Guangdong/1222/2006) had low reactivity with swine/Guangdong/1/2005 and swine/Fujian/204/2007 (Appendix Table 2), indicating that the antigenicity of subtype H1N2 isolate differed from that of classical and European avian-like swine viruses.

Phylogenetic analysis showed that the H3 hemagglutinin (HA) tree separated into avian and human lineages (Technical Appendix [ 1,054 KB, 11 pages], panel A), implying that at least 2 independent H3 sublineages of virus prevail in pigs in China. Neuraminidase (NA) genes of the 9 subtype H3N2 and 1 subtype H1N2 isolates were located in distinct lineages (Technical Appendix [ 1,054 KB, 11 pages], panel B). A cluster was formed by 4 strains of H3N2—swine/Fujian/43/2007, swine/Guangdong/811/2006, swine/Shandong/106/2007, and swine/Shandong/133/2007—and the cluster grouped with Eurasian avian (H9N2) viruses. Four H3N2 strains—swine/Guangdong/211/2006, swine/Guangdong/423/2006, swine/Guangdong/223/2006, and swine/Guangdong/968/2006—were located in the intermediate human sublineage represented by A/Beijing/39/75 (H3N2). One subtype H3N2 isolate, swine/Guangdong/7/2006, grouped closely with A/Moscow/10/99 (H3N2), and the subtype H1N2 isolate swine/Guangdong/1222/2006 shared close similarities with North American swine triple reassortant viruses (Technical Appendix [ 1,054 KB, 11 pages], panel B). These findings showed that viruses of avian, intermediate human, and recent human N2 sublineages were prevalent in pigs in China.

Phylogenic analysis of subtype H1 HA showed that the 9 subtype H1N1 isolates were located in either the classical or European avian-like swine lineages (Technical Appendix [ 1,054 KB, 11 pages], panel C). Swine/Guangdong/1222/2006, together with subtype H1N2 isolates from Hong Kong and subtype H1N2 strains from Guangxi, have a sister-like relationship with those of pandemic (H1N1) 2009 virus (Technical Appendix [ 1,054 KB, 11 pages], panel C). Consistent with characteristics of HA genes, NA genes of the 6 influenza (H1N1) strains isolated belong to classical swine lineages (Technical Appendix [ 1,054 KB, 11 pages], panel D). The other 3 isolates—swine/Shandong/101/2008, swine/Shandong/275/2008, and swine/Shandong/327/2008—together with pandemic (H1N1) 2009 virus, were located in the European avian-like swine group.

The polymerase acidic protein (PA) gene of swine/Guangdong/7/2006 (H3N2) was closely related to that of duck/Guangdong/12/2000 (H5N1) (Technical Appendix [ 1,054 KB, 11 pages], panel G), and other internal genes of swine/Guangdong/7/2006 were located in the recent human subtype H3N2 lineages (Technical Appendix [ 1,054 KB, 11 pages], panels E, F, and H–J). The matrix (M) gene of the 3 isolates—swine/Guangdong/211/2006 (H3N2), swine/Guangdong/223/2006 (H3N2), and swine/Guangdong/423/2006 (H3N2)—grouped in classical swine lineage (Technical Appendix [ 1,054 KB, 11 pages], panel I), and other internal genes were located in intermediate human subtype H3N2 lineage (Technical Appendix [ 1,054 KB, 11 pages], panels E–H and J). Except for the fact that the PA gene of swine/Guangdong/968/2006 and NA gene of swine/Guangdong/811/2006 are of the Eurasian H9N2 avian virus lineage (Technical Appendix [ 1,054 KB, 11 pages], panels B and G), the other internal genes are located in the same lineages with the 3 viruses swine/Guangdong/211/2006, swine/Guangdong/223/2006, and swine/Guangdong/423/2006 (Technical Appendix [ 1,054 KB, 11 pages], panels E–J). The polymerase basic protein 1 (PB1), PA, NP, and nonstructural (NS) genes of swine/Shandong/106/2007 (H3N2) and swine/Shandong/133/2007 (H3N2) belong to the Eurasian avian lineage grouping with the H9N2 viruses (Technical Appendix [ 1,054 KB, 11 pages], panels F–H and J). The PB2 and M genes of the 2 isolates group in human subtype H1N1 lineage (Technical Appendix [ 1,054 KB, 11 pages], panels E and I). The PA and M genes of swine/Fujian/43/2007 (H3N2) belong to recent human-like H3N2 virus lineages (Technical Appendix [ 1,054 KB, 11 pages], panels G and I); the NS gene originates from European avian-like virus (Technical Appendix [ 1,054 KB, 11 pages], panel J), and the PB2, PB1, and NP genes were located in the Eurasian avian lineages with subtype H9N2 viruses (Technical Appendix [ 1,054 KB, 11 pages], panels E, F, and H). Except for the M gene, all other internal genes of swine/Guangdong/1222/2006 have a sister-like relationship with those of pandemic (H1N1) 2009 virus (Technical Appendix [ 1,054 KB, 11 pages], panels E–J). The PB1 gene of swine/Shandong/275/2008 was an exception, grouping with Eurasian avian subtype H9N2 virus (Technical Appendix [ 1,054 KB, 11 pages], panel F). All 6 internal genes of the 3 Shandong isolates were located in the European avian-like lineages (Technical Appendix [ 1,054 KB, 11 pages], panel E–J). All 8 genes of the 6 subtype H1N1 Guangdong isolates formed 1 cluster and belonged to classical swine lineages (Technical Appendix [ 1,054 KB, 11 pages], panels C–J), indicating that none of these viruses were recent reassortants (Table).

Conclusions
Influenza A subtypes H1N1, H1N2, and H3N2 viruses co-circulate in China. Genetic analysis showed that the single subtype H1N2 virus and all subtype H3N2 viruses examined were either double- or triple-reassortant viruses, which have been rarely documented in China. Finding a gene fragment ostensibly of highly pathogenic avian influenza (H5N1) virus in a subtype H3N2 virus implies that subtype H5N1 viruses may be able to contribute genes to virus pathogenic processes in pigs. Moreover, European avian-like swine (H1N1) virus undergoes reassortment with avian (H9N2) viruses.

Some researchers have hypothesized that pigs may serve as hosts for genetic reassortment between human and avian influenza viruses (4). Our results show that subtypes H3N2 and H1N2 and 1 European avian-like swine (H1N1) virus were all derived from relatively recent reassortment events. The gene fragments of the subtype H3N2 viruses comprised those of human subtype H3N2 (A/Victoria/75-like and A/Moscow/99-like) and the strains H1N1 classical swine, Eurasian H5N1, and H9N2 avian. Infection of pigs with avian H5N1 and H9N2 viruses in China has been reported, and swine H1 and H3 viruses appear widely established in the pig population in China and elsewhere in Southeast Asia (5–9). These findings raise more questions about the generation of novel viruses, which may have zoonotic potential, in pigs.

Pandemic (H1N1) 2009 virus probably resulted from reassortment of recent North American influenza subtypes H3N2 and/or H1N2 swine viruses with Eurasian avian-like swine viruses (2). The current situation, therefore, presents continued risk for further reassortment of swine influenza virus in pig populations and continued spread of pandemic (H1N1) 2009 virus to pigs worldwide. Systematic influenza virus surveillance in pigs is needed in China.

Acknowledgments
We thank Shu Yuelong for providing the inactivated antigen and serum.

The study was supported by the National Natural Scientific Foundation (30950002, 30901072), National Key Technologies R&D Program (2006BAD06A01, 2010BAD04B01), National Basic Research Program (973) (2005CB523003), 863 program (2006AA10A205), and the Program for Cheung Kong Scholars and Innovative Research Team in University of China (No. IRT0866). J.H.L. was also funded by the Taishan Scholar Foundation.

Dr Bi is a PhD student at the College of Veterinary Medicine, China Agricultural University, and has research interests in the epidemiology and pathogenic mechanism of influenza virus and paramyxovirus.

References
1.Garten RJ, Davis CT, Russell CA, Shu B, Lindstrom S, Balish A, et al. Antigenic and genetic characteristics of swine-origin 2009 A(H1N1) influenza viruses circulating in humans. Science. 2009;325:197–201. PubMed DOI
2.Smith GJ, Vijaykrishna D, Bahl J, Lycett SJ, Worobey M, Pybus OG, et al. Origins and evolutionary genomics of the 2009 swine-origin H1N1 influenza A epidemic. Nature. 2009;459:1122–5. PubMed DOI
3.Liu J, Bi Y, Qin K, Fu G, Yang J, Peng J, et al. Emergence of European avian influenza virus–like H1N1 swine influenza A viruses in China. J Clin Microbiol. 2009;47:2643–6. PubMed DOI
4.Ito T, Couceiro JN, Kelm S, Baum LG, Krauss S, Castrucci MR, et al. Molecular basis for the generation in pigs of influenza A viruses with pandemic potential. J Virol. 1998;72:7367–73.
5.Wen NX, Wu DM, Song YH, Zhou MH, Huang XQ, Liang HH. Isolation and identification of type A swine influenza virus. Progress in Veterinary Medicine. 2008;29:53–5.
6.Yu H, Zhang GH, Hua RH, Zhang Q, Liu TQ, Liao M, et al. Isolation and genetic analysis of human origin H1N1 and H3N2 influenza viruses from pigs in China. Biochem Biophys Res Commun. 2007;356:91–6. PubMed DOI
7.Chen YJ, Li HY, Shen ZY, Chen HL, Yu KZ, Bi YZ, et al. Molecular evolution of hemagglutinin gene of H1N1 subtype swine influenza viruses isolated from the mainland of China. Chin J Prev Vet Med. 2005;27:13–7.
8.Guo YJ, Webster RG, Zhuge YH. Swine (H1N1) viruses isolated from pigs in China and studies on the origin of isolates. Chinese Journal of Experimental and Clinical Virology. 1992;6:347–52.
9.Yu H, Hua RH, Zhang Q, Liu TQ, Liu HL, Li GX, et al. Genetic evolution of swine influenza A (H3N2) viruses in China from 1970 to 2006. J Clin Microbiol. 2008;46:1067–75. PubMed DOI
Tables
Table. Genetic origin of swine influenza viruses
Appendix Table 1. Antigenic characterization of influenza (H3N2) viruses isolated from pigs
Appendix Table 2. Antigenic characterization of influenza subtype H1 viruses isolated from pigs

Suggested Citation for this Article
Bi Y, Fu G, Chen J, Peng J, Sun Y, Wang J, et al. Novel swine influenza virus reassortants in pigs, China. Emerg Infect Dis [serial on the Internet]. 2010 Jul [date cited]. http://www.cdc.gov/EID/content/16/7/1162.htm

DOI: 10.3201/eid1607.091881

open here to see the full-text:
Swine Influenza Virus Reassortants in Pigs | CDC EID