Abstract
Precocious lines of Eimeria praecox were obtained both from the Houghton laboratory strain and the progeny of a single sporocyst (derived from the Houghton strain) by selection for early development of oocysts in the mature host. The prepatent times of the parasites were reduced from 84 to less than 64 hours. The evolution of the trait for precocious development appeared to be a step-wise process culminating in the depletion of, probably, the fourth generation of schizont. Studies on the precocious line selected from the Houghton strain showed that it had a multiplication rate of less than 10% of its parent strain, yet chickens given small numbers of oocysts were almost completely immune to challenge with either the Houghton strain or any one of five field strains. The precocious line was also slightly less pathogenic. Examination of a sub-line subjected to 10 generations of relaxed passage showed that there was no marked loss of the traits for early prepatent time or for reduced fecundity.
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