Investigation Of MDV Infection And Screening And Analysis Of Wild Virus Strains In Some Areas Of Aksu,Xinjiang

Marek’s disease virus(MDV)is a kind of avian herpesvirus that can cause tumor disease in chickens.There are three serotypes of MDV.The only serotype that can infect chickens and cause disease is blood type I,also named Gallid herpesvirus 2(Gahv-2).In 1909,since the discovery of the disease,the treatment of the disease no specific medicine,only rely on vaccine immunization to prevent the disease.After the appearance of the CVI988 vaccine,the prevention and control of CVI988 were effective,but the protection rate of this vaccine could not reach 100%.MD vaccine was non-clearance immunity,which could not prevent wild strain infection.Therefore,the surveillance of MDV in poultry farms should not be relaxed.According to the characteristics of different MDV genes,the MEQ,PP38,and 132 bp repetitive sequences of MDV were detected,and the virulence of Marek’s disease virus(MDV)wild strain was monitored at all times.To understand the prevalence of MD infection in chickens in some areas of Aksu,455 samples(feather pulp,tissue,and organ)from 16 broiler and layer farms were examined during 2021-2022,and a total of365 positive samples were screened out by using chicken Marek fluorescence quantitative nucleic acid detection kit.According to the national standard MD diagnostic technology(GB/T 18643-2021),MEQ and132 bp repeat primers were synthesized and used for the differential diagnosis of 365 positive samples,a total of 297 vaccine strains and 2 wild strains were obtained.The results showed that 297(65.27%)of the455 samples were vaccine viruses,2(0.44%)were wild viruses,156(0.44%)were not carrying MDV,and34.29% of the total(156/455).A total of 453 out of 455 samples tested were free of MDV and vaccine virus,indicating that the MD vaccine was effective and not prevalent in this region.In order to study the effect of MD on the internal organs of chickens,liver samples from two chickens that were positive for the wild strain of MD virus were prepared for pathological examination.Microscopic examination showed slight to medium size lymphocyte infiltration in the liver tissues;there are a lot of pleomorphic lymphocytes around the liver cells,which are typical of MD.In order to investigate the virulence characteristics of Aksu Prefecture MD wild strains,the two wild strains were named XJAKS2022-1 and XJASU2022-2,and the MEQ,PP38,and 132 BP repetitive sequences were amplified by using MEQ,PP38,and 132 BP specific primers.The MEQ,PP38,and 132 BP repetitive sequences were 1020 bp,820bp,and 314 BP,respectively.Meq and pp38 were cloned and sequenced.The nucleotide homology between PP38 and nine strains was analyzed by MEGALIGN software.The results showed that the homology of MEQ and PP38 was 100% and 99.99%.Then the nucleotide and amino acids of two MEQ strains were compared with 15 domestic and foreign strains.The phylogenetic tree was constructed based on the analysis of gene loci and homology.The homology between MEQ and Chinese strain was 99.7%-99.1%,and that between Meq and American strain was 98.6%-98.5%.The homology of amino acids and Chinese isolates was 95.95%-93.8%,and that of American isolates was93.3%-92.6%.XJAKS2022-1 and XJASU2022-2 isolates had higher homology with Chinese isolates and lower homology with American isolates,x JAKS2022-1 strain showed mutation at position E,80(D-Y),115(V-A),139(T-A),176(P-R)and 217(P-A),and XJASU2022-2 strain showed mutation at position E-D at position 77,the mutation site was not found in the previous epidemic strains in China,and the effect of the mutation site on the virulence of MD is still to be studied.The other mutation sites of the XJASU2022-2strain were consistent with that of the XJAKS2022-1 strain.There was four consecutive proline(PPPP)at positions 152-155,191-194 and 232-235 in the two strains,which were in accordance with the characteristics of three PPPP in China,two strains were obtained from the phylogenetic tree,which was closely related to the prevalent strains in China and far from the United States,and the genetic loci of XJAKS2022-1 and XJASU2022-2 strains and vaccine viruses were analyzed,there was a deletion of 3bases(CAC)at position 575-577,and mutations of PRPP and Papp were found at positions 176 and 217.The extra three bases(CAC)and mutations at positions 176 and 217 were the hallmarks of virulent MDV strains.The results showed that XJAKS2022-1 and XJASU2022-2 strains isolated from Xinjiang Aksu Prefecture were highly prevalent in China.