Porcine reproductive and respiratory syndrome in China

Virus Research 154 (2010) 31–37

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Review

Porcine reproductive and respiratory syndrome in China Lei Zhou, Hanchun Yang ∗ Key Laboratory of Zoonosis of Ministry of Agriculture, College of Veterinary Medicine and State Key Laboratory of Agrobiotechnology, China Agricultural University, No. 2 Yuanmingyuan West Road, Haidian District, Beijing 100193, PR China

a r t i c l e

i n f o

Article history: Available online 24 July 2010 Keywords: Porcine reproductive and respiratory syndrome virus (PRRSV) Atypical PRRS Highly pathogenic PRRSV Vaccine Control China

a b s t r a c t Porcine reproductive and respiratory syndrome (PRRS) is an economically important viral disease for the pig industry worldwide. This disease has brought great losses to the Chinese pig production in recent years, particularly following the emergence of the highly pathogenic PRRS virus (PRRSV), and has become an intractable problem for the development of pig industry in China. This paper will review the history of PRRS, the epidemic of atypical PRRS caused by the highly pathogenic virus, and the molecular characteristics of the Chinese highly pathogenic PRRSV, and the development of vaccines against PRRS in China, as well as current control status and perspective of PRRS in China. © 2010 Published by Elsevier B.V.

Contents 1. 2. 3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . History of PRRS in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Epidemic of atypical PRRS in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Epidemiological status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Clinical signs and gross lesions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Etiology of PRRS in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Molecular characteristics of PRRSV in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Molecular characteristics of the highly pathogenic PRRSV in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vaccine development of PRRS in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control and future perspective of PRRS in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Porcine reproductive and respiratory syndrome (PRRS) is characterized by reproductive failure in sows, and respiratory disease in pigs (Albina, 1997; Hopper et al., 1992; Pejsak et al., 1997; Rossow, 1998), and causes economically great losses to the swine industry worldwide (Neumann et al., 2005; Pejsak et al., 1997). The disease was first recognized in 1987 in the United States of America (Keffaber, 1989), and subsequently became a pandemic disease in North America, Europe, and Asia within the succeed-

∗ Corresponding author. Tel.: +86 10 6273 1296; fax: +86 10 6273 1296. E-mail address: [email protected] (H. Yang). 0168-1702/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.virusres.2010.07.016

31 32 32 32 32 33 33 33 35 35 35 35

ing years (Baron et al., 1992; Bilodeau et al., 1991; Benfield et al., 1992; Bøtner et al., 1994; Kuwahara et al., 1994; Wensvoort et al., 1991). By now, PRRS has become an endemic disease in the global swine production, except for several countries, including Sweden, Switzerland, New Zealand, and Australia claimed to be free of this disease (Cannon et al., 1998; Cho and Dee, 2006; Motha et al., 1997). China not only is the biggest country of pig and pork production but also is the largest consumption market of pork in the world. In China, the number of slaughtered pigs and penned pigs reached 609 millions and 462 millions heads, respectively, and the amount of pork production was 46.2 millions tonnes in 2008, occupying 46% of global pork production (http://www.stats.gov.cn); meanwhile, China imported 2.2 millions tonnes of pork mainly from USA, and exported 4.3 millions tonnes of pork mainly to Russia and other

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Asian regions in the same year (http://www.chinafeed.org.cn). The Chinese pig-producing regions are mainly located along the Yangtze River, in North China, and some major grain-producing provinces. The breeding pigs include Large White, Landrace, Duroc, Hampshire and Pietrain, and domestic breeding pigs, as well as cross-breeding pigs. Breeding pigs are imported from Europe or North America in China each year. 12,000 heads of breeding pigs were imported in 2008 (http://www.caaa.org.cn). China has large numbers of pig farms with diversity of size, different level of management and bio-security, this leads to the difficulty to completely prevent and control the infectious diseases. Since PRRS outbreaks were recognized in an intensive pig farms in North China at the end of 1995 (Guo et al., 1996a), the disease has been accompanying the Chinese pig production, and brought considerable cost to the swine industry in China each year although no exact amount was estimated. In particular, the epidemic of atypical PRRS caused by the emerging highly pathogenic PRRS virus (PRRSV) has resulted in huge economic losses to the Chinese pig industry since 2006 (Tian et al., 2007; Yang, 2007). 2. History of PRRS in China The initial outbreak of PRRS was documented in an intensive pig farm of North China at the end of 1995 (Guo et al., 1996a). Frequently, the disease spread to the majority of pig farms in North and Northeast of China, then to middle, east, south and southwest regions within the following 2–3 years (Dong et al., 2000; Lou and Lin, 1998). Majority of pig farms suffered an abortion catastrophe in pregnant sows, like the situation in North America and Europe. In the subsequent 10 years, the disease became a common condition for the Chinese pig production. Clinically, the disease was mainly characterized by reproductive failure including late abortion, elevated fetal losses (mummies, stillbirths and neonatal death), and reduced growth performance and elevated mortality in weaning and nursery pigs, as well as respiratory disorders along with secondary bacterial infections in pigs of all ages. Before the year 2006, PRRS exhibited the following characteristics in practice in China: (i) PRRSV was the major pathogen causing productive failure and respiratory disorder in many pig farms, and PRRSV infection in pregnant sows caused dead fetuses, irregular abortions and other reproductive problems including late-gestation abortions and early or delayed farrowings with dead and mummified fetuses, stillborn, and weak-born pigs; (ii) higher serologically positive rate of antibody to PRRSV existed in the affected pig farms (Lu et al., 2006); (iii) the co-infections of PRRSV with classical swine fever virus (CSFV), pseuodorabies virus (PRV), swine influenza virus (SIV) and porcine parvovirus (PPV), as well as Mycoplasma hyopneumoniae resulted in the more complicated situation of swine disease in practice (Dong et al., 2006; Jia et al., 2004; Meng et al., 2006; Zhao et al., 2005); (iv) the secondary infections of streptococcus suis type 2, S. choleraesuis and actinobacillus pleuropneumoniae, as well as haemophilus parasuis were very common in the pig herds infected by PRRSV (Tang et al., 2005); (v) the dual-infection of PRRSV and porcine circovirus type 2 (PCV2) was easily detected in the suffered pigs (Li et al., 2003), since the outbreak of postweaning multisystemic wasting syndrome (PMWS) caused by PCV2 infection in China (Wang et al., 2002). 3. Epidemic of atypical PRRS in China 3.1. Epidemiological status In 2006, a clinical outbreak characterized by high body temperature, rubefaction on the skin, respiratory disorder, and high mortality and morbidity in the affected pig herds attacked on the

pig-producing areas of China (Tian et al., 2007; Yang, 2007). Due to the unknown causative agent in the beginning, the outbreak was called as ‘swine high fever disease, SHFD’. In May of 2006, the outbreak occurred initially in Jiangxi province of China. Very soon, the identical clinical situation was recognized in neighbouring provinces, including Hunan, Hubei, Jiangsu, Zhejiang, Fujian and Anhui. In August, this disease spread to Henan, Shandong, Hebei, Beijing, Tianjin and Guangdong provinces. By the beginning of 2007, the epidemic was involved in Inner Mongolia, Liaoning, Jilin, Heilongjiang, Ningxia, Sichuan, Chongqing, Guangxi, Shanxi, Hainan, Yunnan and Guizhou provinces. This outbreak firstly happened in backyard-, small- and middle-size pig farms, and then spread to intensive pig farms. The epidemic of the outbreak resulted in a dramatic decline of pig amount and bankrupt of large amount of smalland middle-size pig farms, especially in severe epidemic areas, and great economic losses to the Chinese swine industry. This disease was the major reason for skyrocketing prices of pig and pork in 2007 in China.

3.2. Clinical signs and gross lesions The outbreak exhibited following unique characteristics in comparison with typical PRRS: (i) it widespread quickly in major pig-producing areas of China and involved in almost country in 1 year; (ii) it had obvious epidemic and transmission features when the outbreak occurred in one pig farm. Once the affected cases were observed, the whole pig herd was infected within 3–5 days, and in the following 1–2 weeks the disease spread the whole pig farm and transmitted to nearby areas; (iii) the body temperature of affected pigs reached 41–42 ◦ C, and the disease course was usually 1–3 weeks. In one farm with the outbreak, the pig death could be found within 5–7-day and the mortality gradually reduced within 3 weeks; (iv) the disease could affect pigs of all ages. Meanwhile, the affected pig herd displayed high morbidity and mortality. The morbidity was usually 50–100%, and the mortality ranged from 20% to 100%, usually 100% in suckling piglets, 70% in nursery pigs, and 20% in finishing pigs. Pregnant sows in different stages could be affected. More than 40% of infected pregnant sows suffered abortion, and the mortality in pregnant sows was usually 10%, and was 20–30% in some pig farms; (v) in most pig farms, the disease emerged initially in pregnant sows or finishing pigs, and then transmitted to nursery pigs. The affected pigs exhibited the clinical symptoms including depression, anorexia, lethargy, and rubefaction on the skin and in the ears. Most of the diseased pigs showed obvious respiratory distress, i.e. sneezing, coughing, dyspnea, increased eye secretion, conjunctivitis; constipation or diarrhea, and neural signs were seen in some cases. In the long-term course, the diseased pigs became pale, emaciation, rough hair coats. Some affected pigs could survive and recovered gradually. At autopsy, the common lesions included dermatorrhagic, severe pulmonary edema and consolidation, and edema of lymph node. Some cases presented pulmonary interstitial hyperplasia and congestion, bleeding in larynx and trachea, and mucosal congestion, hydropsia and ulcers in gastro-intestinal, even edema and congestion in brain. Among these gross lesions, severe lesions in skin, lung, gastro-intestinal and brain were unique for atypical PRRS. With co-infection of other pathogens or secondary bacterial infection, focal hemorrhagic spots, abscessation, and hemorrhagic pneumonia in lung, pericarditis, fibrinous pleuropneumonia and peritonitis could be observed. The outbreak showed no obvious seasonal difference. In addition, due to no strict control measures at the beginning and no effective measures to restrict pig trade and movement among the different areas in China, the outbreak finally became epidemic all over the country.

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4. Etiology of PRRS in China 4.1. Molecular characteristics of PRRSV in China

Table 1 Amino acid similarity of nonstructural and structural proteins of Chinese PRRSV strains (BJ-4, CH-1a, HB-1(sh)/2002 and HB-2(sh)/2002) with VR-2332 (%). ORF

The causative agent of PRRS, the PRRS virus (PRRSV), was identified in Europe and the United States in 1991 and 1992 (Collins et al., 1992; Wensvoort et al., 1991). The virus is classified as a member of the order Nidovirales, family Arteriviridae, and genus Arterivirus (www.ICTVdb.org/ICTV/index.htm). PRRSV is an enveloped, singlestranded positive-sense RNA virus with approximately 50–65 nm in diameter. PRRSV has two major prototypes: the European isolate (Lelystad virus, LV) and the North American isolate (VR-2332), representing two genotypes with antigenic differences—type 1 and type 2, respectively (Nelson et al., 1993). There are great differences in genomic nucleotide sequences and amino acid sequences of the open reading frame (ORF) regions between LV and VR2332 (Murtaugh et al., 1995; Nelsen et al., 1999). The genome of PRRSV is approximately 15 kb in length (Meulenberg et al., 1993). PRRSV is characteristic of genetically extensive variation with the genetic/antigenic diverse strains in North American and European isolates (Kapur et al., 1996; Meng, 2000; Nelsen et al., 1999; Stadejek et al., 2002). Within the genome of PRRSV, the nonstructural protein 2 (Nsp2)-coding region is a variable gene which has been recognized to undergo the most remarkable genetic variation associated with natural point mutations and deletions (Gao et al., 2004; Fang et al., 2004; Han et al., 2006); another variable gene of PRRSV is ORF5 encoding the viral envelope protein (GP5) (Pirzadeh et al., 1998). Therefore, Nsp2-coding region and ORF5 are considered to be important targets for analyzing the genetic variation and molecular epidemiology of PRRSV. In 1996, the first strain (CH-1a) of Chinese PRRSV was isolated from the aborted fetus in one pig farm where PRRS had outbreak (Guo et al., 1996a). Meanwhile, the serological survey by using immunofluorescence assay indicated that the positive rate of PRRSV antibody in pig farms of different areas reached about 58.6% (Guo et al., 1996b). The existence of PRRS and PRRSV infection was confirmed from both serological and etiological evidence in China. Several strains of PRRSV were subsequently isolated and identified, including BJ-4, HB-1(sh)/2002 and HB-2(sh)/2002 (Gao et al., 2004; Yang et al., 1997, 1998), and their full-length genomes were sequenced in the following years (Gao et al., 2004; Yang et al., 2001). The whole genomic alignment of these Chinese PRRSV strains exhibit the identity of above 89% with North American viruses, and the identity of lower than 60% with European viruses. Therefore, all Chinese PRRSV isolates are classified into genotype 2, and the Nsp2, Nsp1␤ and ORF3-5 are variable regions within their genomes (Table 1). The research in molecular epidemiology and genetic variation of PRRSV is emphasized in China. Genetic variation analyses based on the amino acid sequence of ORF5 indicate that the Chinese strains present a great genetic diversity, and are divided into two or three subgroups (Chen et al., 2006; An et al., 2007; Zhou et al., 2009b). All the strains in subgroup 1 were found to be highly variable, while the strains in subgroup 2 share a high similarity with MLV vaccine and its parental virus VR-2332. Among the Chinese isolates, some strains might be derived directly from the vaccine virus (An et al., 2007). In addition, the PRRSV with 12-aa deletion within Nsp2coding region was firstly reported in China (Gao et al., 2004). 4.2. Molecular characteristics of the highly pathogenic PRRSV in China To date, current evidences indicate that the unparalleled largescale outbreak of atypical PRRS occurred in China in 2006 is caused by a highly pathogenic PRRSV with 90 nucleotides deletion, namely 30-amino-acid (30-aa in 481 and 533–561) deletion in its Nsp2-

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1a

Cleavage products

NSP1␣ NSP1␤ NSP2 NSP3 NSP4 NSP5 NSP6 NSP7 NSP8 1b NSP9 NSP10 NSP11 NSP12 Structural proteins GP2a ORF2b GP3 GP4 GP5 M N

BJ-4

HB-1 (sh)/2002

HB-2 (sh)/2002

100 98.8 99.1 86.7 99.2 81.9 99.6 95.5 100 96.6 99.4 92.9 100 93.8 98.8 91.9 100 100 99.7 97.7 99.3 95.7 98.2 94.6 100 96.8

CH-1a

93.9 86.7 77.9 93.7 95.1 91.8 93.8 91.1 97.8 98.0 95.7 94.2 95.5

97.0 85.3 78.6 95.3 96.1 92.9 93.8 90.3 97.8 97.5 95.9 94.2 94.8

99.2 98.6 98.8 98.9 98.0 98.9 100

94.1 90.4 88.6 90.4 88.5 97.7 96.7

91.8 93.2 88.5 88.8 88.5 97.7 95.1

95.3 90.4 88.6 90.4 91.5 97.1 96.7

coding region (Li et al., 2007; Tian et al., 2007; Zhou et al., 2009b; Zhou et al., 2008) (Fig. 1). Several laboratories in China have isolated the same PRRSV strain in the epidemic pig farms of the outbreak, and completed the whole genomic sequencing of the viruses. Also, by using reverse genetics, the viruses were rescued from infectious clones of the isolates. Both the rescued viruses and wild-type isolates showed fatality to pigs under experimental inoculation (Lv et al., 2008; Zhou et al., 2009a). All these evidences point to the same conclusion: the virus is highly pathogenic and is the causative agent of the outbreak in China. Therefore, this outbreak should be called atypical PRRS instead of ‘SHFD’. The origin of the virus is an interesting issue. The emerging Chinese highly pathogenic PRRSV in 2006 displays higher homology with previous strains within each ORF of the genome (Table 2). Evolution analyses based on complete genomic sequence of Chinese PRRSV isolates indicate that the Chinese highly pathogenic PRRSV shares 94.9–95.4% of nucleotide identity with CH-1a (isolated in 1996), 96.7–97.2% with HB-1(sh)/2002 (isolated in 2002), and 97.6–98.2% with NB/04 (isolated in 2004), and 97.1–97.2% with BJ0706 (isolated in 2007, GenBank accession no.: GQ351601), respectively (Fig. 2). NB/04 and BJ0706 only have 1 amino acid deletion at 481 residue of Nsp2-coding region. It is postulated that NB/04 and BJ0706 might be an intermediate virus during the evolution of the Chinese highly pathogenic PRRSV. Thus, it could be speculated that the emerging of the highly pathogenic PRRSV in China experienced a gradual variation and accumulation progress of genome changes from the Chinese domestic viruses (Zhou et al., 2009b). The whole-genome-based phylogenetic analyses of Chinese PRRSV strains and representing strains suggest that the newly emerging Chinese highly pathogenic PRRSV originates from CH-1a-like virus (An et al., 2010). Additionally, complete Nsp2 genes of PRRSV were amplified from clinical specimens collected during 2006–2008 and sequenced in our recent work, and the data indicate that the diversity of PRRSV strain exists in the field, and the highly pathogenic PRRSV with the 30-aa deletion in Nsp2 is the dominating virus in China in recent years (Zhou et al., 2009b). Since all highly pathogenic PRRSV isolates during the outbreak of atypical PRRS in China have the same characteristics of 30-aa deletion in their Nsp2-coding regions, this deletion is proposed as a genetic hallmark of highly pathogenic PRRSV. To answer the question whether the 30-aa deletion is related with the increased pathogenicity, we applied the reverse genetic manipulation to generate two cloned

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L. Zhou, H. Yang / Virus Research 154 (2010) 31–37

Fig. 1. Alignment of amino acid in Nsp2 deletion region of Chinese highly pathogenic PRRSV with North American VR-2332, and other previous Chinese strains. The highly pathogenic isolates include JXA1 (GenBank accession no. EF112445), JX143 (GenBank accession no. EU708726), HUN4 (GenBank accession no. EF635006), SY0608 (GenBank accession no. EU144079), JXwn06 (GenBank accession no. EF641008), BJsy06 (GenBank accession no. EU097707) and NX06 (GenBank accession no. EU097706). Other previous Chinese strains include CH-1a (GenBank accession no. AY032626), HB-1(sh)/2002 (GenBank accession no. AY150312), HB-1/3.9 (GenBank accession no. EU360130), Em2007(GenBank accession no. EU262603), NB-04 (GenBank accession no. FJ536165) and BJ0706 (GenBank accession no. GQ351601). Dots indicate conserved residues; dashes indicate deleted amino acids at positions 481 or 533–561. The deleted amino acid positions were determined based on the genome of VR-2332.

viruses: one in which the Nsp2 region of highly pathogenic PRRSV containing the 30-amino-acid deletion was replaced by the corresponding region of the low-virulence PRRSV strain and another in which the same deletion in Nsp2-coding region of the lowvirulence PRRSV strain as highly pathogenic PRRSV was performed. Finally, the pathogenicity analyses showed that the two viruses maintained their own virulence as their parental viruses, respec-

tively. The study reveals that the 30-aa deletion in Nsp2-coding region of the highly virulent PRRSV emerging in China is not related to its virulence (Zhou et al., 2009a). Therefore, the 30-aa deletion can be as a marker to distinguish the Chinese highly pathogenic virus from North American strains but cannot be utilized as a definition of highly pathogenic strain of PRRSV in general. It is speculated that the virulence of the Chinese highly pathogenic virus may be

Table 2 Amino acid similarity of nonstructural and structural proteins of Chinese highly pathogenic PRRSV strains (JXwn06, BJsy06, NX06. JXA1, HUN4 and SY0608) with Chinese previous strains [CH-1a and HB-1(sh)/2002] (%). ORF

Cleavage products

JXwn06

BJsy06

NX06

JXA1

HUN4

SY0608

1a

NSP1␣

97.0a 98.2b 88.5 96.3 88.3 94.4 98.4 98.0 96.6 99.0 95.3 98.8 100 100 96.5 96.9 100 97.8 99.1 99.4 98.0 98.9 97.8 97.8 97.4 97.4

96.4 97.6 88.5 97.2 88.1 94.3 97.8 97.3 96.1 98.5 94.7 98.2 100 100 96.9 97.3 100 97.8 98.8 99.1 98.0 98.9 98.2 98.2 96.8 96.8

96.4 97.6 88.1 96.8 87.9 94.1 98.2 97.8 96.1 98.5 94.7 98.2 100 100 96.9 97.3 100 97.8 98.6 98.9 97.7 98.6 98.2 98.2 97.4 97.4

97.0 98.2 88.5 96.3 87.9 94.1 98.0 97.5 96.6 99.0 95.3 98.8 100 100 96.9 97.3 100 97.8 98.8 99.1 97.3 98.2 97.8 97.8 97.4 97.4

97.0 98.2 89.0 96.8 88.1 94.1 98.0 97.5 96.6 99.0 95.3 98.8 100 100 96.9 97.3 100 97.8 98.6 98.9 98.0 98.9 97.8 97.8 97.4 97.4

97.0 98.2 87.6 96.3 87.9 94.3 98.2 97.8 96.6 99.0 94.7 98.2 100 100 96.9 97.3 100 97.8 98.8 99.1 97.7 98.6 97.8 97.8 97.4 97.4

96.5 96.5 94.5 94.5 93.3 93.3 97.2 97.2 93.5 94.5 97.7 99.4 91.1 92.7

96.1 96.1 94.5 94.5 93.3 93.3 97.2 97.2 93.5 94.5 97.1 98.9 95.1 96.8

96.1 96.1 94.5 94.5 93.3 93.3 97.2 97.2 93.5 94.5 97.7 99.4 95.1 96.8

96.5 96.5 94.5 94.5 92.9 92.9 94.9 94.9 93.0 94.0 97.7 99.4 95.1 96.7

96.1 96.1 94.5 94.5 93.3 93.3 97.2 97.2 93.5 94.5 97.7 99.4 95.1 96.7

96.5 96.5 94.5 94.5 93.3 93.3 98.3 98.3 93.0 94.0 97.7 99.4 94.3 96.0

NSP1␤ NSP2 NSP3 NSP4 NSP5 NSP6 NSP7 NSP8 1b

NSP9 NSP10 NSP11 NSP12

Structural proteins GP2a ORF2b GP3 GP4 GP5 M N a b

Alignment with CH-1a. Alignment with HB-1/(sh)2002.

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Fig. 2. Phylogenetic tree based on the complete genomic sequences of Chinese representative strains, together with VR-2332, 16244B, NVSL 97-7985 IA 1-4-2, P129, 16244B, RespPRRSMLV and MN184A from GenBank. The numbers on scale bar indicate nucleotide substitution (100×). The dotted lines mean no enough length of line to show the actual nucleotide substitution.

associated with the ORF1b-coding region based on our unpublished data. However, it is required to further explore the exact mechanisms related to its virulence from the view of virology. 5. Vaccine development of PRRS in China In order to control the PRRS in China, an inactivated vaccine was developed by using the first isolate CH-1a, and was used in field after registering the license by Chinese Ministry of Agriculture in 2000. And also an attenuated live vaccine derived from the isolate CH-1a was approved to be registered in 2007 in China. However, the efficacy of both the two vaccines for controlling PRRS was not satisfactory as expected; clinical cases associated with productive failure in pregnant sows and respiratory disorder in nursing piglets occurred frequently in the vaccinated pig farms. In a short time after the outbreak of the atypical PRRS, China developed an inactivated vaccine using the isolated virus (JXA1). However, the vaccine could not provide effective clinical immune protection as expected. Laboratory trials also indicated that the inactivated vaccine was not capable of protecting the pigs against the highly pathogenic PRRSV challenge. Moreover, Chinese investigators tried to develop novel generation vaccine against PPRS, including recombinant live pseudorabies virus or adenovirus expressing GP5, M and other proteins of PRRSV, recombinant plasmid DNA vaccine expressing GP5 of PRRSV (Jiang et al., 2006a,b, 2007). Although these trials indicated that these vaccines could induce effective immune protection to PRRSV infection in pigs or antibody response in mouse model, none of them was practically applied in field.

conducted in a fewer farms. The means conducted in North America and Europe, including whole herd depopulation/repopulation, test and removal and herd closure has been shown effective in eliminating PRRSV from positive pig herds (Dee et al., 1994; Dee and Molitor, 1998; Torremorell et al., 2002), almost no pig producers applied them due to their large economic costs in China. Currently, the epidemic of atypical PRRS is not completely controlled yet in China. A number of small- and middle-size pig farms still suffer the highly pathogenic PRRSV frequently, and the high mortality and morbidity are still observed in pig herds. The production performance is unstable in pig farms with the virus infection, especially in suckling and nursery pigs. Thus, the highly pathogenic PRRSV will continue to exert impact on the Chinese swine industry for a long time. Although an attenuated vaccine derived from the highly pathogenic virus by passage on MARC-145 cell can provide effective protection for pigs (Tian et al., 2009), and has been approved in China, its safety should be considered, and much attention should be paid to its reversion to virulent virus. The continued spread of the highly pathogenic PRRSV and currently extensive vaccination of live vaccine in Chinese pig farms will enhance the genetic diversity of PRRSV and further aggravate the complicated situation of PRRS. Therefore, PRRS will be a more complicated swine disease in China. A lot of works for completely controlling PRRS need to be done in China, particularly in the prevention of PRRSV spread among different areas, implement of properly regional control and eradication programs for PRRS. Meanwhile, the great genetic diversity of Chinese PRRSV should be taken into consideration for developing effective and safe vaccine and choosing the vaccine for the prevention control of PRRS in China.

6. Control and future perspective of PRRS in China Acknowledgments The control and eradication of PRRSV is a global problem for swine production. Considering the difference in production mode between China and other developed countries, as well as the lower level of management in pig farms, this problem is undoubtedly more difficult to Chinese pig farms. Thus, vaccination is the primary chooses for majority of pig producers for preventing and controlling PRRS in China. In recent years, the mostly used commercialized vaccine is Ingelvac® MLV that was registered in China in 2005, although this vaccine could only provide partial immune protection to the Chinese highly pathogenic PRRSV. In addition, the measure to effectively control secondary bacterial infection in the affected pig herds using antibiotics is implemented universally in pig farms. Also in China, means of exposure including serum inoculation, contact with infected animals with clinical signs was

This work was supported by National Natural Science Funds for Distinguished Young Scholar (30825031) from National Natural Science Foundation of China, and National Key Basic Research Plan Grant (#2005CB523204) andNational Key Technology R&D Program of China (Grant No. 2006BAD06A03) from the Chinese Ministry of Science and Technology, and the Program for Cheung Kong Scholars and Innovative Research Team in University of China (No. IRT0866).

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