Ostertagia ostertagi, commonly known as the medium or brown stomach worm, is an important parasitic nematode (round worm) of cattle. O. ostertagi can also be found to a lesser extent in sheep, goats, wild ruminants and horses. The species causes ostertagiosis, which is potentially fatal in cattle. It is found worldwide and is economically important to cattle industries, particularly those found in temperate climates.The abomasal nematode O. ostertagi is a Clade V nematode of the order Strongylida, the family Trichostrongylidae and genus Ostertagia. Ransom first described the genus Ostertagia in 1907, which currently contains approximately 15 species. All species of the genus Ostertagia infect domestic or wild ruminants. These species form a large and complex group, the taxonomy of which ha