Coccidiosis refers to the disease condition caused by infection with one or more of the many species of coccidia, a subdivision of the phylum Protozoa. The genus Eimeria contains the species of major economic importance in domestic birds. While coccidiosis occurs in practically all kinds of birds, the parasites are host specific and each species occurs in a single or in a limited group of related hosts. On the other hand, avian hosts are known to harbor more than one species of coccidia. The two most important species from the aspect of economic loss are E. tenella and E. acervulina. Additional important species in chickens include E. maxima, E. necatrix, E. mivati and E. brunetti with E. mitis, E. praecox and E. hagani causing infections of lesser importance.
Other types of livestock, e.g., cattle, sheep, goats and pigs also can suffer severely from coccidiosis with resultant loss of productivity.
Among domesticated birds, chickens are the most susceptible to significant economic losses from coccidiosis, although losses can also occur within turkeys, geese, ducks, and guinea fowl. Coccidiosis has also produced serious losses in pheasants and quail raised in captivity. The effects of a coccidiosis infection can take the highly visible form of devastating flock mortality, but another undesirable effect is morbidity and/or weight loss which results from infection.
When chickens recover from infection with coccidia they generally are immune. However, all prior efforts to vaccinate poultry against coccidiosis using non-living extracts of Eimeria have met with failure.