Protective efficacy of a candidate live attenuated vaccine derived from the SD-R strain of lineage 1 porcine reproductive and respiratory syndrome virus against a lethal challenge with HP-PRRSV HuN4 in piglets.

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen endangering the swine industry worldwide. Our previous study confirmed that the live attenuated vaccine SD-R strain could provide broad cross-protection against homologous and heterologous NADC30-like viruses. However, it is unclear whether SD-R can provide protection for piglets against HP-PRRSV. In the present study, 4-week-old PRRSV-free piglets were vaccinated intramuscularly with 10 6.2 TCID 50 of SD-R and then challenged intramuscularly (2 mL) and intranasally (2 mL) with HP-PRRSV HuN4 (1 × 10 5.0 TCID 50 /mL). The results showed that two of five piglets died in the HuN4 challenge group and five of five immunized piglets survived in the immune challenge group. Five piglets in the HuN4 challenge group had a fever for 6-12 days and showed obvious clinical symptoms, but 3 of 5 piglets had a fever for only 1 day at 5 days post inoculation, and only 1 of the 5 piglets in the immune challenge group had a cough. Compared to challenged control piglets, vaccinated piglets had significantly improved daily weight gain and obviously alleviated pathological lesions. Furthermore, piglets immunized with SD-R had reduced thymus atrophy caused by HP-PRRSV HuN4. These results demonstrate that the lineage 1 PRRSV attenuated vaccine SD-R strain provides protection against lethal HP-PRRSV HuN4 challenge. As such, it is a promising vaccine candidate for use against PRRSV infection. IMPORTANCE Both highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) and NADC30-like PRRSV have caused tremendous economic losses to the Chinese pig industry. In this study, a good challenge model was established to evaluate the protection afforded by the candidate SD-R vaccine against infection with a representative HP-PRRSV strain (HuN4). The control piglets in the challenge experiment displayed obvious clinical symptoms of PRRSV infection, with a mortality rate up to 40%. In contrast, all the piglets in the vaccinated challenged group survived, and only some pigs had transient fever. The daily gain of SD-R immunized group piglets was significantly increased, and the pathological changes were significantly reduced. In addition, the viral replication levels in the serum of the immunized group were significantly lower than those of the challenged control group. The live attenuated vaccine SD-R strain can provide protection against HP-PRRSV challenge, indicating that the SD-R strain is a promising vaccine candidate for use in the swine industry.