| Infectious bronchitis (IB) caused by infectious bronchitis virus (IBV) is an acute and highly contagious disease, it has been the major infectious diseases of poultry. IBV exists numerous serotypes,but little or no cross protection exists between different serotypes,moreover,IB vaccines used in China cannot provide an effective protection against the pandemic strains,IB continues spread in immunized chicken.Therefore,the IBV strains used for developing vaccines should be best from local IBV serotype,it is important for the prevention and control of IB.An LX4 type IBV strain Sczy3 was isolated from our laboratory in 2009,The virus was attenuated by passaging 100times(designed as P100)through non-immune chicken embryonated eggs, and its pathogenicity was evaluated in chickens.The experiment results showed that the virus titer rose gradually,the pathogenicity of the virus to chicken was weaken.Because no clinical symptoms was observed when the Sczy3 P100 were inoculated.the morbidity and mortality were 0%.So the Sczy3 P100 strain was attenuated and apathogenic to chickens.In order to investigate the epidemiology of IBV in Sichuan in recent years,ten strains IBV were isolated from in part areas of Sichuan between 2012 and 2013.the S1 gene of the ten isolates were cloned and sequenced.and the sequences were compared with thirty reference strains which published in GenBank.the results showed that the nucleotide and deduced amino acid sequences homology of ten isolates with reference were 71.0%-99.8%,the ten isolates were 74.5%-98.7%.Through the genetic evolutionary tree,the forty strains were separated into six distinct genotypes and the Sichuan isolated were belonged to three different groups, seven isolates were belonged to LX4 genotype which is the predominant strain in China in recent years, two isolates were belonged to the TW genotype which were the special genotypes in Taiwan,most of the isolates has the distantly genetic relationships with the current vaccine strains.The results showed that:In recent years,LX4-type IBV is still the major epidemic strains in sichuan,but the TW-type also began to dissemination and popular.In order to evaluate the immune protective efficacy of Sczy3 P100 to homologous strain and current pandemic strains.selected two latest IBV isolates names CK/CH/SCEM/12I and ScTW,Sequence analysis of the S1 protein gene demonstrted that the two virus were representative of IBV in recent years in Sichuan.The Sczy3 P100 strain was given to 7-day-old non-immune chickens,challenged with homologous strain and field strains after two weeks.The efficacy of Sczy3 P1 00 strain was tested by vaccination-challenge test,serum antibody detection, organ protection in this study and the preparation of histopathological setions.The results showed that:All the immunized chickens showed positive serum antibody response.whereas,the non-immune control group showed negative serum antibody response.Sczy3 P100 could supply enough chmplete protection to homologous strain Sczy3 P3 and the same genotype strain CK/CH/SCEM/12I;Although Sczy3 P100 could supply enough clinical protection to the different genotype strain ScTW,high percentages of virus shedding were detected from trachea and kidney of challenged chickens. Now there are a lot of IBV genotypes circulating in china,so it’s urgent to develop new vaccines. |
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