Authors: Williams, R. B.; Marshall, R. N.; Pagès, M.; Dardi, M.; del Cacho, E.

The apicomplexan genus Eimeria comprises at least 1160 species that parasitize vertebrate animals (Levine, 1988). Those found in farm animals cause serious financial losses worldwide, with poultry perhaps being the worst affected. In chickens, Eimeria acervulina, Eimeria brunetti, Eimeria maxima, Eimeria mitis, Eimeria necatrix, Eimeria praecox and Eimeria tenella each causes a separately recognizable disease resulting from the fundamental differences among the pathogenic effects characteristic of each species.

Despite a common but mistaken notion that E. praecox is practically innocuous, its effect is clinically similar to that of E. acervulina, which is generally accepted to be damaging to commercial chickens. The reported variability in the effects of either species may often result from differences in the numbers of oocysts administered or other exogenous factors, but may also reflect differences in strain virulence.