A comparative study of two lines of Eimeria tenella attenuated either by selection for precocious development in the chicken or by growth in chicken embryos.

A precocious line (HP) selected from the virulent Houghton (H) strain of Eimeria tenella was found to be attenuated and to have a shorter prepatent time than the parent strain. The second generation schizonts of the HP line were smaller than those of the H strain and contained fewer merozoites. A sub-line of HP, derived from a single oocyst, showed no sign of reversion to virulence after 19 passages without selection for precocity. Similarly, clones (the progeny of single sporozoites) of an attenuated embryo- adapted line (TA) remained avirulent after up to 26 passages in the chicken. In contrast, an uncloned TA line reverted to virulence after 20 passages in chickens. The reproduction in chickens of the HP line was greater than that of the TA line. Chickens kept in wire-floored cages and given small numbers of oocysts of HP or TA were not substantially immune to challenge with the virulent H strain. However, when chickens kept in litter pens were given the HP line they developed a strong immunity to a large challenge with either the H, Weybridge or each of four field strains. In the same conditions, prior inoculation with TA led to only partial resistance against challenge.