A genus of coccidia (family Eimeriidae) in which the oocyst contains eight naked sporozoites. Important species are Tyzzeria anseris, a relatively nonpathogenic species found in the small intestine of domestic and wild geese, whistling swans, and certain wild ducks, and Tyzzeria perniciosa, which occurs in the small intestine of the domestic duck in North America and Europe, and is pathogenic in ducklings.