Attenuation of Eimeria mivati (= mitis) by selection for precocious development.

By selection for early development of oocysts during serial passage through chickens the pre-patent period of the Houghton (H) strain of Eimeria mivati (= mitis) was reduced by over 20 h. The precocious parasite is less pathogenic than the H strain and has a reduced reproductive potential. Chicks inoculated with the precocious parasite were protected against challenge with the H strain. The pathogenicity, immunogenicity and reproduction of precocious E. mivati and an attenuated egg-adapted line of E. mivati were compared. Although the precocious parasite produced more oocysts than the egg-adapted parasite in chickens, neither parasite induced loss of weight and both were immunogenic.