Recombination in vaccine and circulating strains of porcine reproductive and respiratory syndrome viruses

Abstract

Em2007, a porcine reproductive and respiratory syndrome virus (PRRSV) variant with a unique 68 aa deletion in Nsp2, was recently isolated in China. Phylogenetic and molecular evolutionary analyses indicated that Em2007 is a natural recombinant between a vaccine strain of PRRSV and circulating virus. We also tested its pathogenicity in piglets.

Figure 1

Recombination event analyses of the Em2007 strain of porcine reproductive and respiratory syndrome virus (PRRSV). A) Similarity plot analysis using Em2007 as query sequence. Analysis made use of a sliding window of 200 bases and a step size of 20 bases. The y-axis shows the percentage similarity between the selected PRRSV sequences and the query sequence. The other comparisons are not shown for clarity. B) Bootscan analysis using Em2007 as the query sequence. JXA1 is used as the outgroup to determine the breakpoints. The y-axis shows the percentage of permutated trees using a sliding window of 600 bases and a step size of 20 bases. Red vertical lines and numbers indicate the recombination breakpoints identified by the Genetic Algorithm for Recombination Detection (GARD). Pink numbers indicate the maximal χ² value of each breakpoint. Numbers corresponding to CH-1R, WUH1, and JXA1 indicate the quantity of informative sites in 7 zones defined by 6 recombination breakpoints, respectively.

Figure 2

Pathogenicity comparison among the Em2007, CH-1a, and WUH1 strains of porcine reproductive and respiratory syndrome virus (PRRSV). Forty-day-old piglets (9 piglets in each group) free of PRRSV were inoculated intramuscularly with 10^5.0 mean tissue culture infectious doses/2 mL of Em2007, CH-1a, WUH1, respectively. Two piglets from each group were euthanized and necropsied at 7 and 10 days postinoculation (dpi) for viral load analyses and histopathologic examinations. The remaining 5 piglets in each group were used to evaluate rate of death. Mean rectal temperature (A) and survival rate (B) of each group were recorded for 21 dpi.