In March 2007, the suspected PRRS-like disease was initially found in Hai Duong Province, the northern province of Vietnam. The disease later spread to nearly the entire country and affected at least 65,000 pigs. Meanwhile, the recurrence of PRRSV infections in China was officially announced in May 2007 by the Chinese Ministry of Agriculture.

Figure 1

Figure 1. Pathologically dissected specimens from pigs infected by porcine respiratory and reproductive syndrome viruses. Blood spots in kidneys and lung hemorrhages are indicated by white arrows.

The affected pigs exhibit the clinical features of high fever, depression, and shivering. Viscera (e.g., brain, kidney, lung, heart, liver, and spleen) were sampled from those dead pigs from different provinces in Vietnam and China. Pathologic examination showed severe lesions viscera, for example, blood spots in the kidney and hemorrhages in the lung (Figure 1, panels A, B), findings similar to those observed in the 2006 PRRS outbreaks in China (8,11). The inocula (from each representative specimen) were propagated in Marc-145 cells (QIAGEN, Hilden, Germany) for the viral genomic RNA isolation. Based on our previous observation of the 2006 PRRS epidemics in China, we designed 2 pairs of specific primers (Nsp2-F: 5′-AAA GAC CAG ATG GAG GAG GA-3′ and Nsp2-R: 5′-GAG CTG AGT ATT TTG GGC GTG-3′; orf5-F: 5′-ATG TTG GGG AAG TGC TTG ACC-3′ and orf5-R: 5′-CTA GAG ACG ACC CCA TTG TTC CGC-3′). They correspond to the DNA fragment covering a putative discontinuous deletion of 30 aa in Nsp2 (666 bp), and ORF5 in full length, respectively. PCR-based detection showed that all the samples collected so far, are positive for PRRSV, but negative for African swine fever, classical swine fever, and foot-and-mouth disease viruses.

To test the virulence of Vietnam PRRSV isolates, 6 specific pathogen–free piglets were challenged with a representative strain (termed 07QN, which had been isolated from a dead pig in Quang Nam Province, Vietnam). The piglets were monitored for clinical signs every 6 hours. All procedures were conducted in a facility qualified with Biosafety Level 3 and approved by Vietnamese Committee for Approval of Drugs and Cosmetics ethics committee. As we expected, the piglets reproduced the serious symptoms similar to those observed in our field investigation, which indicated that the viral agent is highly pathogenic.

Figure 2

Figure 2. Whole genome–based phylogenetic relationship of porcine respiratory and reproductive syndrome viruses (PRRSVs) LV4.2.1, the prototype of type I (European PRRSV) (shown in green), and VR2332, the standard strain of type II...

To better understand the genetic relationship of Vietnamese PRRSV isolates to the 2006 and 2007 Chinese isolates, 5 strains of PRRSVs (1 isolate from Vietnam, 07QN, and 4 newly isolated isolates from China, 07HEBTJ, 07BJ, 07HEN, and 07NM) were subjected to whole genome sequencing as described (8). The viral genomes were found to be >15,300 bp (GenBank accession nos. are available from the authors). Bioinformatics analysis further revealed that the 2007 Vietnamese isolate (07QN) and 2007 viruses (e.g., 07BJ) have 99% identity to 2006 PRRS isolates in China at the level of nucleic acid sequences. Whole genome–based phylogenetic relationship also showed that all these viral isolates are grouped into the same subclade in the type II genotype (Figure 2). All of the 2007 strains, together with those Chinese viruses collected in 2006, were found to be similar to 3 Chinese strains reported previously (HB-1, HB-2, and CH-1a) (3,12) and 1 NA-type virulent strain, P129 (8,9).

In addition, >30 sequences of orf5, a highly variant gene, were sequenced, and ORF5-based genotyping also supported the classification of these 2007 isolates (from Vietnam and China) into a subgroup of type II, while far from other subclades with VR-2332, a prototype of type II (not shown). Moreover, the multiple alignments of Nsp2 demonstrated that the 2007 PRRSV isolates from Vietnam (51 total), together with 2007 Chinese strains (6 total) are nearly identical and share a discontinuous deletion of 30 aa at the position of 482 aa and 534–562 aa, which is consistent with deletions in the strains from the 2006 epidemic in China (8).

PRRS has become a serious challenge to the global pig industry, causing serious economic losses (2). In particular, the unparalleled 2006 PRRS outbreaks engulfed China, and adult pigs were not exempt, which indicates that these variants may have evolved to exhibit new characteristics of pathogenicity (8). These outbreaks caused extensive concern worldwide (13). Even though the deletion of 30 aa in Nsp2 has been proposed as a potential virulence factor (8), the molecular mechanisms underlying its high virulence are yet to be elucidated.

Origin of these lethal variants in Vietnam is still obscure, although Kamakawa et al. (14) suggested that PRRSV may have been present in Vietnam before 1999. Convergent, but separate, evolution of PRRSVs in Vietnam and China may explain in part the emergence of the nearly identical PRRSV variants in these 2 neighboring countries, although this hypothesis does not rule out the possibility that 2006 Chinese PRRSV variants were transmitted into Vietnam and then circulated rapidly. Additionally, intraprovincial and interprovincial transportation of live pigs and the similar climate in Vietnam and China may have contributed to these outbreaks (15).

In summary, our findings provided robust evidence that nearly identical variants of NA-type PRRSVs are the causative pathogens that triggered PRRS epidemics in Vietnam and China in 2007. This finding highlights the importance of prevention and control of this highly transmissible infectious agent.