 The major limitations to reducing parasitic load using vaccines is the complex and dynamic host-parasite interaction that is unique to each species of host and parasite, which is often influenced by several environmental factors.The presence of "O. ostertagi" within a host may be inferred by several methods.Faecal worm egg counts (FECs) in particular (preferably with speciation by way of larval culture and differentiation), and total worm counts are the tests most commonly employed in the diagnosis of helminth infections in ruminants.