Eimeria media: selection and characterization of a precocious line.

A precocious line of Eimeria media was obtained by selection for early development of oocysts in rabbits. The prepatent period was reduced from 108 to 72 h. The precocious line was less pathogenic than the original strain, and its multiplication rate was lower. Rabbits given oocytes of the precocious line were totally immune to challenge with the original strain as assessed by change in weight gain but were partially protected as assessed by oocyst output. Selection for precocious development was accompanied by morphological changes in the sporulated oocytes; each sporocyst contained only one large refractile body instead of the two smaller bodies seen in the original strain.