Further studies on the pathogenicity and immunogenicity of an embryo-adapted strain of Eimeria tenella.

The pathogenicity, immunogenicity and reproduction of a strain of E. tenella, serially passed in embryos was studied. A strain which had completed 62 embryo passages no longer regained pathogenicity after nine passages in chickens. The reproduction of the strain in chickens, judged by oocyst production was poor, but increased after nine serial passages in chickens. The embryo passaged strain was immunogenic and a single dose of oocysts gave substantial protection against challenge with the pathogenic strain 14-42 days later. Histological studies confirm earlier studies that the loss of pathogenicity accompanying repeated embryo passage resulted in the gradual disappearance of large second generation schizonts. It is also possible that the large sub-epithelial second generation schizonts gradually change to a smaller type as they develop in epithelial, as opposed to sub-epithelial cells.