Comparison of molecular and biological characteristics of a modified live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine (ingelvac PRRS MLV), the parent strain of the vaccine (ATCC VR2332), ATCC VR2385, and two recent field isolates of PRRSV

Abstract

The objectives of this study were to compare the molecular and biological characteristics of recent porcine reproductive and respiratory syndrome virus (PRRSV) field isolates to those of a modified live virus (MLV) PRRS vaccine and its parent strain. One hundred seventeen, 4-week-old pigs were randomly assigned to six groups. Group 1 (n = 20) served as sham-inoculated negative controls, group 2 (n = 19) was inoculated with Ingelvac PRRS MLV vaccine, group 3 (n = 20) was inoculated with the parent strain of the vaccine (ATCC VR2332), group 4 (n = 19) was inoculated with vaccine-like PRRSV field isolate 98-38803, group 5 (n = 19) was inoculated with PRRSV field isolate 98-37120, and group 6 (n = 20) was inoculated with known high-virulence PRRSV isolate ATCC VR2385. The levels of severity of gross lung lesions (0 to 100%) among the groups were significantly different at both 10 (P < 0.0001) and 28 days postinoculation (p.i.) (P = 0.002). At 10 days p.i., VR2332 (26.5% +/- 4.64%) and VR2385 (36.4% +/- 6.51%) induced gross lesions of significantly greater severity than 98-38803 (0.0% +/- 0.0%), 98-37120 (0.8% +/- 0.42%), Ingelvac PRRS MLV (0.9% +/- 0.46%), and negative controls (2.3% +/- 1.26%). At 28 days p.i., 98-37120 (17.2% +/- 6.51%) induced gross lesions of significantly greater severity than any of the other viruses. Analyses of the microscopic-interstitial-pneumonia-lesion scores (0 to 6) revealed that VR2332 (2.9 +/- 0.23) and VR2385 (3.1 +/- 0.35) induced significantly more severe lesions at 10 days p.i. At 28 days p.i., VR2385 (2.5 +/- 0.27), VR2332 (2.3 +/- 0.21), 98-38803 (2.6 +/- 0.29), and 98-37120 (3.0 +/- 0.41) induced significantly more severe lesions than Ingelvac PRRS MLV (0.7 +/- 0.17) and controls (0.7 +/- 0.15). The molecular analyses and biological characterizations suggest that the vaccine-like isolate 98-38803 (99.5% amino acid homology based on the ORF5 gene) induces microscopic pneumonia lesions similar in type to, but different in severity and time of onset from, those observed with virulent strains VR2385 and the parent strain of the vaccine. Our data strongly suggest that isolate 98-38803 is a derivative of Ingelvac PRRS MLV and that the isolate is pneumovirulent.

FIG. 1.

Comparison of PRRSV-induced microscopic lung lesions at 28 days p.i. Shown are sections of lungs from pigs in Ingelvac PRRS MLV (A), VR2332 (B), 98-38803 (C), 98-37120 (D), and VR2385 (E) groups. Lesion severity ranges from no lesions (A) to moderate (B, C, and E) to severe (D) interstitial pneumonia.

FIG. 2.

Comparison of the PRRSV ELISA means (S/P ratios) among the PRRSV-inoculated groups. DPI, days p.i.

FIG. 3.

Dendrogram of the ORF5 nucleotide sequences (positions 13696 to 14459 of the PRRSV gene; GenBank accession no. PRU87392) of PRRSV isolates recovered from pigs at 10 (pigs 9, 41, 61, and 81) and 28 days p.i. (pigs 11, 51, 71, and 99). 98-38803, pigs 9 and 11; 98-37120, pigs 22 and 31; VR2385, pigs 41 and 51; VR2332, pigs 61 and 71; Ingelvac PRRS MLV, pigs 81 and 99. The phylogenetic relationships were estimated by using the ClustalV method.