Determination Of The Doses And Immunization Times Of An Eimeria Necatrix Vaccine Strain And Its Cross-protection Against Different Eimeria Spp.

Coccidiosis is a serious protozoan disease in chickens worldwide.Eimeria necatrix parasitize the middle part of the small intestine and has high virulence,which can cause acute clinical symptoms,and lead to the defective absorption of nutrients,and result in the development of secondary necrotizing enteritis.In this study,the immune effects of different immune doses and immune times of a precocious strain of E.necatrix were compared,and the cross-protection against different Eimeria spp.was evaluated by animal test.The results could provide a powerful basis for the widespread application of a live vaccine for coccidiosis control.The first part of this study was to test the pathogenicity of six virulent strains of Eimeria spp.and to determine the best infective dosage of each species for the interspecific cross-protection test.The infection doses were as follows:1.5×10~4sporulated oocysts per bird for the Hezhou strain of E.necatrix(NHZ),5×10~4 sporulated oocysts per bird for the E.tenella Meizhou strain(TMZ),6×10~4 sporulated oocysts per bird for the E.acervulina Heyuan strain(AHY),1.0×10~5 sporulated oocysts per bird for the E.maxima Laiyang strain(MLY),1.2×10~5 sporulated oocysts per bird for the E.brunette Baoding strain(BBD),and 3.0×10~5 sporulated oocysts per bird for the E.mitis Baoding strain.The second part of this study was to orally inoculate chickens with the E.necatrix precocious vaccine strain(PNHZ)at different doses and for different immunization times,to evaluate the immune protection against a virulent strain of E.necatrix(NHZ),and to determine the best immune dose and immunization times.Challenged with 1.0×10~5sporulated oocysts of NHZ,the groups that were first immuned with 300 sporulated oocysts of PNHZ,and then received a second immunization with either 9000 or 15000sporulated oocysts of PNHZ,experienced an induced protective immune effect;however,the group that received a second immunization of 3000 sporulated oocysts of PNHZ did not.All groups that received a third immunization with 3000,9000 and 15000 sporulated oocysts of PNHZ were able to resist the attack of 1.3×10~5 sporulated oocysts of NHZ.These results revealed that two times of supplementary vaccination are bette r than 1supplementary immunization.A first immunization with 300 sporulated oocysts of PNHZ and a second immunization with 9000 sporulated oocysts of PNHZ could facilitate the popularization and application of this vaccination protocol in clinical practice.In the third part of this study,the chicken groups vaccinated with PNHZ were challenged with six different virulent strains of Eimeria spp.The results showed that the immunity against Eimeria was species-specific,and the chicken group immunized with E.necatrix only experienced an induced protective effect against the virulent strain of E.necatrix,but could not resist the virulent strains of other Eimeria spp.and no interspecific cross-protection for E.necatrix immunization was found.