| Infectious bronchitis(IB),caused by infectious bronchitis virus(IBV),is an acute,highly contagious disease.IBV is quite easy to mutate,there are many serotypes/genotypes and little or no cross-protection occurs between different serotypes of IBV.Existing vaccines often do not provide full protection against emerging genotypes of IBV.Until a vaccine against a new genotype of the IBV strain becomes available,a combination of existing vaccines can provide a broader spectrum of cross-immune protection,these are considered as "protectotypes".In 2009,TW I type IBV was firstly introduced into Chinese mainland and it has spread to other areas in China,and TW I type genotype strains that account for about 15%of the total isolates.In order to effectively prevent and control the epidemic of TW I type strain,the present study was done to screen a "protectotypes" vaccine combination,and develop an attenuated vaccine,in order to protection against challenge with TW I type IB V.1.Screening of "protectotypes" vaccine combinations for TW I type IBVFour commercial monovalent vaccine were used to evaluate the efficacy to against TW I type IBV strain in parallel in this study,containing QXL87、LDT3-A、4/91 and Ma5.1-day-old commercial chicks were divided into five groups(n=24),Four groups were immunization groups and chicks were inoculated with four kinds of commercial attenuated vaccine by intranasal and intraocular routes,and the dose of vaccinated was one dose per chicken,the other group was non-immune challenge group.14-day after vaccination,per chick was infected with TW I type AH 1103 strain by intranasal and intraocular routes,containing 106.0EID50 virus.The clinical symptoms of the chicks were observed daily after challenge.5-day post-challenge,the gross and histopathological lesions were observed.6-day post-challenge,tracheal cilia activity was evaluated.5 and 9 days after challenge,the virus shedding of the throat and cloaca was detected.The results showed that QXL87 attenuated vaccine only provide clinical protection to TW I type IBV,but high proportion of virus shedding was detected from trachea and kidney of challenged ckickens.LDT3-A,4/91 and Ma5 attenuated vaccines showed unsatisfactory protective effects.Severe clinical symptoms appeared,and showed serious virus shedding after challenged.The QXL87 vaccine was combined with LDT3-A,4/91 and Ma5 vaccines,respectively,to evaluate the protective effect of different vaccine combinations against TW I type IBV strain.1-day-old commercial chicks were divided into five groups(n=24),They were QXL87-LDT3-A group,QXL87-4/91 group,QXL87-Ma5 group,QXL87 group and non-immune challenge group.QXL87+LDT3-A,QXL87+4/91 and QXL87+Ma5 group were inoculated with 1 dose of per chick for each vaccine.QXL87 group inoculate 2 doses of per chicken.14 days post immunization,5 groups were inoculated with TW I type AH1103 strain(106.0EID50/chicken)by intranasal and intraocular routes.The clinical symptoms of the chicks were observed daily after challenge,gross lesions and ciliostasis score was evaluated on time,and the samples were collected for the histopathology and virus shedding detection.The results showed that the combination of QXL87 attenuated vaccine and other serotypes of vaccine could relieve the clinical symptoms of chickens,and reduce virus shedding.Considering the clinical symptoms,gross lesions,ciliostasis score,virus shedding and histopathology,the combination of QXL87 and 4/91 showed better cross-protection against TW I type IBV.Thus,the combination of QXL87 and 4/91 is "protectotypes" vaccine combination for TW I type IB V strain.2.Development and evaluation of attenuated vaccine strain for TW I type IBVThe TW I type AH1103 strains in SPF chicken embryo to 160 passages.Then,we selected 120,140 and 160 passages of strains inoculate 7-day-old SPF chicks,to evaluate the safety of the vaccine.The clinical symptoms,ciliostasis score,viral load in organs and histopathology were evaluated,we found that 5 passages strain can cause respiratory symptoms,such as tracheal congestion,renal urate deposition,the 120,140 and 160 passages had no obvious pathogenicity.Then,we evaluated the immunogenicity of the 120,140 and 160 passages strain respectively,it was found that the chickens in the immunization group had no clinical symptoms after challege TW I type IBV,the tracheal cilia had a high degree of protection,and the virus shedding of tracheal and cloaca was significantly lower than that the non-immune challenge group.Therefore,120,140,160 passages strains showed good immunogenicity could provide a immune protection,and it is expected to be good candidate strains of TW I type IBV. |
