Immune Status Study Of Laying Hens Against H9N2 Subtype Avian Influenza(AI)in Northern Shanxi Province And Isolation And Identification Of AIV Epidemic Strains

H9N2 subtype avian influenza virus(AIV)is one of the most important pathogens that seriously harm large-scale laying hens breeding in northern areas of Shanxi Province.In order to investigate the prevalence of H9N2 AI in the region,a large number of laying hens' serum samples were collected from 9 different cities between 2018 and 2019 to monitor the level of antibody against H9N2.Among them,clinical comprehensive diagnosis,virus isolation culture,PCR detection,and genetic evolution were carried out to analysis the suspected 27 samples,which further revealed the molecular epidemiological characteristics of H9N2 in those regions.Over the past two year,5122 of laying hens serum samples with different age were taken to analysis the level of antibody by hemagglutination inhibition(HI).The results showed that the overall hens immunity against H9N2 was generally better.The average antibody level was 8.48 log2 to 10.98 log2 with relatively high anti-infection coefficient.Compared with those in 2018,the antibody against H9N2 indexes were preferable in 2019.In each year,the level of antibody in spring and winter is higher than that in summer and autumn,respectively.The antibody mean could reach 1 log2 at least,with the excellent ratio of antibody more than 10%.The antibody titers of different age groups were ideal and increased gradually with the increasing age.Attention should be paid to the low mean and excellent rate of antibody in chickens(60 ? age ? 120),which might be at the risk of infection.At the same time,after laboratory testing of 27 suspected cases,11 positive H9N2 subtype avian influenza cases were detected,indicating that the detective ratio of this disease accounted for 40.74%.By combining clinical diagnosis with laboratory diagnosis,it was fully confirmed that H9N2 subtype AIV was one of the main pathogens causing early respiratory diseases and "bronchial embolism" symptoms in chickens of all ages.At last,three endemic strains were successfully isolated from the 11 positive samples and termed as SXJB1,SXJB2,and SXJB3,respectively.Among the isolates,the nucleotide sequence homology of HA genes was 96.4%~97.3%,and the amino acid homology was 97.2%~97.7%.Compared with 12 reference strains,the isolated nucleotide identity was 88.4%~98.0%.The isolated nucleotides homology with vaccine F and SS strains was less than90%,while the homology with WJ57 vaccine strains reached 95.5%~96.2%,indicating that the isolates had a good antigenicity.According to the results of phylogenetic analysis,the isolates belong to the BJ/94-like Eurasian branch,and are related to Fujian's,Jiangxi's,Anhui's and Hebei's strains.This study provides a reliable reference for the future clinical diagnosis and comprehensive prevention and control of the disease.