| Coccidiosis is recognized as the parasitic protozoan disease that has the greastest economic impact on poultry production. The annual worldwise cost is estimated at about $ 2 billion. This debilitating disease is characterized by poor weight gain and inefficient feed conversion. Currently, coccidiosis is mainly controlled by ionophores or synthetic drugs and they have played a major role in the growth of poultry industry, but some degree of resistance to all anticoccidial drugs has development and the reports of drug-residue in food have heightened the consumer's concerns. Therefore, those facts has stimulated the development of immune preventing to coccidiosis.Jeffers (1975) was the first researcher to describe a precocious strain of chicken Eimeria species and obtained a precocious strain of E.tenalla. Compared with the parental strain, the precocious strain had the following characteristics: the prepatent period was shortered, the reproductive capacity and the pathogenicity was decreased, but remains much of the immunogenicity of the parental strain. The characteristics of the precocious strain are genetically stable and cann't reinstate by relaxing the selection pressures. Thereafter, precocious strain have been developed from the other acknowledged chicken Eimeria species: E.acervulina (McDonald et al., 1982), E.mitis (McDonald and Shirley, 1984), E.praecox (Shirley et al.. 1984), E.necatrix (Shirley and Bellatti, 1984), E.maxima (McDonald et al., 1986 and Shirley and Bellati, 1988), E.brunetti Shirley et al., 1986), E.tenella (McDonald et al., 1986). Some scholars on the basis of the characteristics of precocious strain, and in-depth understanding of chicken coccidiosis protective immune mechanisms developed the anti-chicken coccidiosis precocious attenuation vaccine. Live vaccine,consisting of either virulent coccidial strains or live attenuated parasites of several species, is proved to be effective in preventing coccidial infection. At present, Paracox? (England) and Livacox? (Czech) attenuation vaccine have been commercialize. The precocious strains could have wider application in the development of anticoccidial vaccines for sustainable control of coccidiosis.A precocious strain of E.tenella was selected by serial 26 passage through chickens apply to a method as described by Jeffers (1975), the prepatent period of precocious strain was reduced from 141 h of parent strain to 118 h. We have further studied on the sporulated oocysts size,reproductive potential,pathogenicity and immunogenicity compared with the parental strain. The results indicated that the size of sporulated oocysts of precocious strain is remarkably smaller than that of the parent strain (P<0.05), reproductive potential was correspondent with that of parent strain (P>0.05); in the pathogenicity aspect, average weight gains of 4 different doses of the precocious strain hasn't significant difference compared with the parent strain (P>0.05), whereas average weight gains of the parent strain was decrease markedly than contral group (P<0.05 or P<0.01); the mortality of 4 different doses of the precocious strain is zero, whereas that of the parent strain is 0,25%,25%,31.3% respectively; the more dose infected, the more difference in leision score between the precocious and parent strain, suggesting that the precocious strain of E.tenella less pathogenic than parent strain. With same dose immunize, analysis the average weight gains,mortality,leision score and so on, the results indicated that the immunogenicity was correspondent with the that of parental strain. In addition to, immune groups have better inhibition to oocyst production. With same species same strain oocyst challenge, oocyst inhibittion ratio was better than same species different strain in per immune group. After two times immunity the immunological protection of both strains was both high, and the occyst inhibition ratio was all above 95%.The purpose of the genetic stablility, the precocious strain was passaged 3 generations by relaxed selection, the investigation indicated: the prepatent period and the ratio of daily oocysts production to the total oocysts were stable, suggesting that the precocious strain of E.tenella was a stable strain.The further investigation to the precocious strain of E.tenella on gene degree, in this paper, according to communional primers of the eukaryotic organism by Barta previded, 18S rRNA genes from sporozoited oocysts of the precocious strain and parent strain of E.tenella were amplified by PCR. The PCR products were checked by agrose gel electrophoresis and purified by agrose gel fraction method. The purified products of 18S rRNA were ligated to pGEM-T-easy vector successfully. The ligated products were transferred into content cell DH5α. The specific recombinant plasmid was identified by PCR and sequencing and sequence analysis. The results indicated that the size of 18S rRNA genes of the precocious strain was 1 796 bp in length and the parent strain that was 1 799 bp, the precocious strain shared a high of 99% identity with the parent strain; thedifference of their secondary structure was significantly different, the gene login codes of the precocious strain and parent strain were EF210326 and EF210325 respectively. 18S rRNA gene of precocious strain has been first time logined in GeneBank. |
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