Porcine Reproductive and Respiratory Syndrome

The genome of Lelystad virus (LV), the causative agent of porcine epidemic abortion and respiratory syndrome (previously known as mystery swine disease), was shown to be a polyadenylated RNA molecule. The nucleotide sequence of the LV genome was determined from a set of overlapping cDNA clones. A consecutive sequence of 15,088 nucleotides was obtained. Eight open reading frames (ORFs) that might encode virus-specific proteins were identified. ORF1a and ORF1b are predicted to encode the vital RNA polymerase because the amino acid sequence contains sequence elements that are conserved in RNA polymerases of the torovirus Berne virus (BEV), equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), the coronaviruses, and other positive-strand RNA viruses. A heptanucleotide slippery sequence (UUUAAAC) and a putative pseudoknot structure, which are both required for efficient ribosomal frameshifting during translation of the RNA polymerase ORF 1b of BEV, EAV, and the coronaviruses, were identified in the overlapping region of ORF1a and ORF1b of LV. ORFs 2 to 6 probably encode viral membrane-associated proteins, whereas ORF7 is predicted to encode the nucleocapsid protein. Comparison of the amino acid sequences of the ORFs identified in the genome of LV, LDV, and EAV indicated that LV and LDV are more closely related than LV and EAV. A 3′ nested set of six subgenomic RNAs was detected in LV-infected cells. These subgenomic RNAs contain a common leader sequence that is derived from the 5′ end of the genomic RNA and that is joined to the 3′ terminal body sequence. Our results indicate that LV is closely related evolutionarily to LDV and EAV, both members of a recently proposed family of positive-strand RNA viruses, the Arteriviridae.

The characterization of an isolate of swine infertility and respiratory syndrome (SIRS) virus (ATCC VR-2332) is reported. A commercial cell line (CL2621) was used for the propagation of the virus for all assays. Laboratory studies indicate that this isolate is a fastidious, nonhemagglutinating, enveloped RNA virus. Cesium chloride-purified virions visualized by electron microscopy were spherical particles with an average diameter of 62 nm (range: 48-83 nm) and a 25-30 nm core surrounded by an envelope. Virus replication was restricted to the cytoplasm, as demonstrated by immunofluorescence. The virus did not react serologically with antisera to several common porcine viruses or with antisera to known viruses in the alphavirus, rubivirus, pestivirus, and ungrouped lactic dehydrogenase virus genera of the Togaviridae. However, convalescent sow sera and rabbit hyperimmune sera neutralized the SIRS virus at titers of 1:256 and 1:512, respectively. The virus was stable at 4 and -70 C, but was labile at 37 and 56 C. The properties of this isolate of SIRS virus resemble those of the family Togaviridae but do not match the described genera.

The Lelystad virus or one of two US isolates (VR2385, VR2431) of porcine reproductive and respiratory syndrome virus were given intranasally to 25 4-week-old cesarian-derived colostrum-deprived pigs. Pigs from these groups were necropsied at 1, 2, 3, 5, 7, 10, 15, 21, or 28 days postinoculation. The Lelystad virus and VR2431 induced mild transient pyrexia, dyspnea, and tachypnea. VR2385 induced labored and rapid abdominal respiration, pyrexia, lethargy, anorexia, and patchy dermal cyanosis. All three isolates induced multifocal tan-mottled consolidation involving 6.8% (n = 9; SEM = 3.4) of the lung for Lelystad, 9.7% (n = 9, SEM = 2.7) of the lung for VR2431, and 54.2% (n = 9, SEM = 4.4) of the lung for VR2385 at 10 days postinoculation. Characteristic microscopic lung lesions consisted of type 2 pneumocyte hypertrophy and hyperplasia, necrotic debris and increased mixed inflammatory cells in alveolar spaces, and alveolar septal infiltration with mononuclear cells. Lymphadenopathy with follicular hypertrophy, hyperplasia, and necrosis was consistently seen. Similar follicular lesions were also seen in Peyer's patches and tonsils. Lymphohistiocytic myocarditis and encephalitis were reproduced with all three isolates. Clinical respiratory disease and gross and microscopic lung lesion scores were considerably and significantly more severe in the VR2385-inoculated pigs. All three viruses were readily isolated from sera, lungs, and tonsils throughout the 28 days of the study. The lymphoid and respiratory systems have the most remarkable lesions and appear to be the major site of replication of these viruses. This work demonstrated a marked difference in pathogenicity of porcine reproductive and respiratory syndrome isolates.