الفهرس 
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Abstract
The antigenic diversity in the parasites results in diversity in the pathogenicity that by r One of the most important parasitic diseases in chickens is coccidiosis, which is caused by protozoa of the genus Eimeria. It characterized by intestinal lesions, blood loss, weight loss, poor feed conversion, increased susceptibility to other disease agents and mortality in sever cases, causing economic losses upward of $ 3 billion worldwide annually..ole may result in ineffectiveness of biological or anticoccidial agents. Therefore, in this study we suggested a hypothesis, which is the determination of the virulance of a strain of E. tenella through using PCR and sequencing. Four E. tenella isolates collected from 4 different governorates in flocks suffered from caecal coccidiosis, from which single oocyst isolation was performed to obtain pure strains of E. tenella. Also, another field isolate (Rasheid) didn’t undergo SOI, treated as the isolates in addition to Coccivac B vaccine as a source for E. tenella (wild strain) for help in comparison the virulence of these isolates. DNA was extracted from all of samples under study, then PCR was performed for detection of E. tenella by using SCAR markers for amplification of genomic DNA fragment with expected size 539 bp. Sequencing and phylogenetic tree of six strains of E. tenella were done. Phylogenetic tree showed that all six strains were related to each other by 100% except Gharbia strain 97.86%. By SNPs, it was obvious that there were 10 nucleotides of Gharbia strain had changes than other strains, that make this percent of identity. We have obtained two GenBank accession numbers for our nucleotide sequences GenBank accession number for Coccivac B vaccine strain (E. tenella): gb|KT985454| GenBank accession number for Gharbia strain (E. tenella): gb|KT985455| Determination of virulence of four strains of E. tenella in battery cages: the virulence of these strains were done by infection of chicks aged 14 days by doses 104 and 25x103 sporulated oocysts/chick for each strain. Evaluation was done by parameters like, body weight gain, feed consumption, FCR, mortality rate and parasitological parameters as lesion score, faecal score, oocyst count and the data recorded as the following: Alex strain of E. tenella was the most virulent strain as it make 60% mortality by dose 25x103 sp.oocysts and 33.33 % by dose 104 sp.oocysts and reduction of weight gain which differ significantly than other groups infected and non infected control birds. Behera strain arranged the 2nd then Gharbia and the last is Kafrelsheikh strain which is the lowest one in virulence However, the degrees of virulence appeared in batteries for four strains not translated by sequencing and phylogenetic tree for example, similarity percent between Alex strain (most virulant) and Kafrelsheikh strain (the lowest virulant) is 100%. Therefore, it may be indicated that this part of genomic DNA for E. tenella that amplified and sequenced by SCAR marker was related to species (E. tenella) and not intraspecific marker. Also, nucleotide changes occurs in Gharbia strain may be of low value especially similarity percent of this strain with others is 97.86 %.