A preliminary study of the nature of infection and immunity in chickens given an attenuated line of Eimeria acervulina.

The Houghton (H) strain of Eimeria acervulina was attenuated by serial passage through chickens of the first oocysts produced during infection. This selection pressure resulted in a reduction in the pre-patent period of the parasite, shown to be due to the selection of a line predominantly with only 3 instead of 4 generations of schizonts. The precocious line had a reproductive potential much lower than that of the parent strain and it was significantly less pathogenic. Chickens given oocysts of the precocious line were almost completely immune to challenge with the Houghton strain.