Epidemiological Investigation Of Porcine Reproductive And Respiratory Syndrome In Shandong Province From 2020 To 2022

Porcine reproductive and respiratory syndrome(PRRS),also known as "pig blue ear disease," is an immunosuppressive disease caused by the Porcine reproductive and respiratory syndrome virus(PRRSV)in pigs.The classic strain mainly causes reproductive disorders in pregnant sows and respiratory diseases in piglets.In the early 1990 s,the Lelystad(LV)and VR-2332 strains caused an outbreak of pig blue ear disease in Europe and America,followed by an outbreak in Asia.In 1995,China experienced a classic outbreak of PRRS,and in 2006,highly pathogenic PRRSV broke out in China.Since 2012,China has successively introduced recombinant strains and NADC30 like strains,causing huge losses to China’s pig industry.At present,new mutant strains of PRRSV are constantly emerging,making PRRS one of the most important diseases in modern pig farming.In order to understand the prevalence of PRRS in Shandong Province in recent years,this study tested suspected PRRS clinical case samples collected from pig farms of different scales in Shandong Province from 2020 to 2022,and analyzed the epidemic characteristics of the disease in Shandong Province.This study aims to further understand the epidemiological characteristics of PRRS in Shandong Province by observing clinical symptoms,dissecting lesions,RT-PCR detection,and PRRSV antibody ELISA analysis of suspected cases,as well as analyzing the genetic evolution of the GP5 gene.The survey results show that pigs infected with PRRSV at different stages exhibit respiratory symptoms and reproductive disorders in pregnant sows.The pathological changes of severe pigs infected with PRRSV during autopsy mainly manifest as acute interstitial pneumonia in the lungs and hemorrhagic inflammation of lung related lymph nodes.We collected a total of 3880 pig samples from 16 cities in Shandong Province.Through RT PCR detection,794 samples were found to be positive for PRRSV,with a positive rate of 20.46%.Among the mixed infection cases,the mixed infection rate of PRRSV and PCV2 was the highest;A survey on the incidence of PRRS in pig farms of different sizes in Shandong Province found that autumn and winter were the most severe,and the positive rate of PRRSV infection showed an upward trend from 2020 to 2022;5160 serum samples were tested for PRRSV N protein antibody ELISA,and the total positive rate of the antibody was 66.4%(3426/5160),indicating a high trend of PRRSV antibody positivity in different scale pig farms in Shandong Province.In order to further study the genetic and molecular characteristics of PRRSV epidemic strains,positive samples were sequenced and the GP5 gene of 76 PRRSV strains and the NSP2 gene of 58 PRRSV strains were screened.Through genetic evolution analysis,it was found that the PRRSV prevalent in large-scale pig farms in Shandong Province are all American strains,mainly distributed on four branches,indicating that the PRRSV epidemic strains in Shandong Province are complex and diverse,with the highest proportion of American strain genotype 1(NADC30/34-Like),close to 70%.Among them,28 GP5 sequencing samples and NSP2 sequencing samples are the same.Analysis of these 28 strains found that about 57% of the sequencing results belong to the same class,and 43% of the classification is inconsistent.Therefore,we can speculate that there is a possibility of recombination of PRRSV strains.Currently,there are significant differences between vaccines on the market and pandemic strains,which cannot effectively protect immunized pigs from infection with the American strain 1.In summary,at present,there is a high pressure of PRRSV infection in pig farms in Shandong Province.However,due to the complex and diverse types of strains,the immune effect of the vaccine is not good,and many immune pig farms still have cases of PRRSV infection,causing serious economic losses.Therefore,strengthening the epidemiological investigation of PRRSV has important practical significance.For pig farms,a sound biosafety system,strengthened production management,and comprehensive prevention and control measures are the scientific approaches to address PRRS.This study further enriched the epidemiological data of PRRS in Shandong Province,laying a theoretical foundation for vaccine development and immune prevention and control of the disease.