An egg-adapted (attenuated) line of Eimeria necatrix: further studies on its reproduction, pathogenicity and immunogenicity.

An egg-adapted line of Eimeria necatrix has now been passaged 40 times in the chorio-allantoic membranes of embryonated eggs. Between the 21st and 40th passage in eggs the parasite was subjected to a selection for precocious development, and its pre-patent period in this host is now 123-125 h. The parasite continued to adapt to the chorio-allantoic membrane throughout the period of the passages and its development in this tissue caused the growth of the host embryo to be markedly stunted. In chickens, both the reproduction and the pathogenicity of the egg-adapted line decreased with increasing numbers of passages in eggs. A comparison of the immunogenicity of the egg-adapted line and its parent strain in chickens kept on litter, showed that substantially more oocysts of the egg-adapted line had to be given to induce complete protection.