Evaluation of immunoprotection of SD-R against homologous and heterologous strains
Twenty-six 28-day-old PRRSV-free piglets were obtained from two PRRS-free farms in Harbin. Thirteen PRRSV-free piglets used to assess homologous protection were randomly divided into three groups (A1-C1). Other PRRSV-free piglets used to test heterologous protection were randomly divided into three groups (A2-C2). Five piglets for each group were used for immunization and inoculation (A1, B1, A2 and B2), except for three piglets in negative control groups C1 and C2. Piglets in groups B1 and B2 were inoculated intramuscularly with 105.0TCID50 SD-R. After 28 days post-vaccination (dpv), piglets in groups A1, B1 and A2, B2 were infected with 5th-passage SD (4×105.0TCID50 per pig) and 5th-passage HLJWK108-1711 (4×105.0TCID50 per pig) intramuscularly (2 ml) and intranasally (2 ml), respectively. The animals were maintained in individual biosafety rooms. Clinical signs and rectal temperatures were recorded daily. The body weights of the piglets were measured weekly. Blood samples were periodically collected from individual piglets and tested for viremia. All of the piglets were euthanized at 21 dpi. Ten tissue samples were obtained from the hearts, livers, spleens, lungs, kidneys, lymph nodes, tonsils, small intestines, bladders, and stomachs for viral detection by TaqMan®-based real-time fluorescence quantitative RT–PCR (65).