 There is a broad range of current literature reporting developing resistance of "O. ostertagi" to all major classes of anthelmintic worldwide including the United States, New Zealand, Brazil, Argentina, and the UK.As recently as January 2016, "O. ostertagi" anthelmintic resistance was reported to all three major drug classes on 20 dairy farms in Southern Australia
The two most widely accepted anthelmintic resistance diagnosis methods for "O. ostertagi" are "in vivo" methods