Oocysts from faeces of bird infected with Eimeria tenella
These oocysts have been isolated by salt floatation from the faeces of a bird infected with E.tenella. Salt floatation is commonly used in the diagnosis of many faecal borne parasites and is based on the principle that parasite material eg cysts, oocysts and eggs have a lower specific gravity than the other material in the faeces. By manipulating the SG of the floatation fluid it is possible to arrange for the parasite material to float while the debris sinks.
Notice that these oocysts are unsporulated ie their content has not differentiated into sporocysts and sporozoites. These oocysts are, therefore, are not yet infective for other hosts - this only happens after 2-3 days on the ground.
As oocysts are the product of sexual reproduction their presence is associated with the chronic stages of infection in birds. Pathological consequences are associated with the asexual stages and, therefore, develop before oocysts can be found in the faeces.