Identification And Pathogenicity Of Porcine Reproductive And Respiratory Syndrome Virus In Pigs With Healthy Clinical Characterization From Shandong Province

Porcine reproductive and respiratory syndrome （PRRS） is one respiratory tract disease whichis harmful to pig farms, with characters as abortion, death of embryo, mummified fetuses, andrespiratory problems in piglet. Pathogen of the disease was considered as porcinereproductive and respiratory syndrome virus （PRRSV）. In recent years, some susceptible pigswhich infected with mutational PRRSV strains showed healthy clinical symptoms, and thesestrains were homological to HP-PRRSVstrains. In order to explore the reason why the pigsinfected with PRRSV show healthy clinical symptoms, we studied the biologicalcharacteristics of the mutational PRRSV strains isolated from these farms. In this study, fouraspects were included:1. Etiological investigation of PRRSV in pigs with healthy clinical sign during2010²011was carried out by RT-PCR. The results showed that mutational PRRSV strains wereprevalent in large-scale pig farms in Shandong, and perfect protective effects to pigs couldn’tbe provided by inmmuned with attenuated PRRS vaccine in the clinical practice. Themutational PRRSV strains and vaccine virus were discovered in the same swine herd.2. Ten PRRSV strains were isolated from the RT-PCR-positive samples, and two strains （JNand JN62） were isolated from serums of non-vaccinated pigs with healthy clinical sign, whileeight strains were isolated from pigs with PRRS clinical symptoms. There was no significantdifference among the cytopathic effect （CPE） in Marc-145cell of the PRRSV strains.3. According to the infection model developed by Wei Tian-chao, animal experiments werecarried out. The results demonstrated that PRRSV JN and JN62strains were less pathogenicthan the highly pathogenic PRRSV strains. According to the experiment data, JN and JN62strains had different pathogenicity, JN and JN62strains were not the highly pathogenicPRRSV strains..4. According to genetic variation, PRRSV JN and JN62strains belonged to the Americanstrains, and there was high homology between the two strains and HP-PRRSV for exampleJXA1strain. Compared with traditional strains and variant strains, the presumable result isthat JN and JN62are recombination strains with traditional strain and variant strain. the ORF4S⁶⁶、ORF5D³⁴of two strains were closed relation between the virulence of the epidemic PRRSV strains and the gene mutations.