Different strains of a species may vary in pathogenicity. Environmental factors affecting the survival of the oocysts. Site of development within the host. Coccidia that develop superficially are less pathogenic than those that develop deeper. Age of the bird.
Young birds are generally more susceptible than older ones.
Nutritional status of the host. Poorly fed birds are more susceptible. Coccidiosis in chickens is generally classified as either intestinal or cecal. Most serious cases of intestinal coccidiosis are caused by E. necatrix . Cecal coccidiosis is due to E. tenella .