and host defence Gastrointestinal nematodes may elicit a variety of host immune responses depending on the initial immune status of the host, parasite species, and environmental conditions. The body has several physical defense mechanisms against parasites including the continual sloughing of the gut epithelium to prevent parasite attachment. However, once an infection has occurred, the host's immune system attempts to limit the damage caused by the worm. Apart from the importance of the extrinsic factors of weather, climate and grazing management, the immune status of cattle is perhaps the most significant of all host factors influencing infection with O. ostertagi. Unlike other common gastrointestinal nematodes of cattle, who are subject to a quick host immune response after relatively short periods of exposure and immune system memory, a protective host immune response against O. ostertagi requires far longer periods of exposure and is not always permanent. The failure to respond quickly to Ostertagia may be a result of the suggested immunosuppression or impairment of antibody and cellular responses. O. ostertagi has been shown to induce cytokines and T-cells in the adaptive immune response in cattle, and recent advances have been made to produce suitable vaccines targeting adult stage Ostertagia. 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. Diagnosis 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. The direct counting of nematode eggs in feces is the method of choice for yearling animals, whereas in adult animals it may yield high variability due to a cow's immunity and low egg output. Other biochemical methods have been developed to help more accurately diagnose O. ostertagi parasitism. These include the determination of specific anti-parasite antibodies in milk. Enzyme-linked immunosorbent assays (ELISAs) have been used as a diagnostic tool to quantify the impact of gastrointestinal nematodes in dairy cattle by measuring antibodies in milk. Higher levels of antibodies measured by ELISA methods, referred to as optical density ratios (ODRs), are associated with decreased milk production in dairy cattle. Similar results have been established in the dairy industry between