Challenges to the development of new tests for diagnosis of infection and prediction of resistance of sheep to gastrointestinal nematodes.

Strategies for control of gastrointestinal nematode (GIN) infections in sheep require information on the severity of infection and species (or genus) of parasite present. Tests for diagnosis of GIN fall into 3 classes. Current tests measure either: 1) the presence of eggs, worm antigens or worms themselves; 2) components of host immunity (e.g. antibodies, eosinophils, other immune mediators); or 3) components of host pathology (e.g. wool growth, body growth, appetite, blood loss, digestive enzymes, anaemia, hypoproteinaemia, odours). To offer advantages over current diagnostic methods, new tests need to be more informative, more accurate, more timely, cheaper, technically easier, or suitable for use in the field. To improve sheep management, a new test needs to determine severity or predict the onset and severity of infection. This is a technically more difficult challenge than qualitative diagnosis of the presence of infection and creates a substantial obstacle to the development of new diagnostic methods. Estimation of the performance characteristics of a test including its sensitivity, specificity and predictive value is important before widespread adoption. This paper reviews current diagnostic tests for GIN, and opportunities for new tests that aid management of infections or that inform the estimated breeding value of animals for use in programs that breed sheep for resistance to GIN. Gene marker and biomarker tests for resistance to GIN infection or disease will require validation in the population in which they are to be used and may require revalidation as the genetic background of the population changes over time. Estimation of the specificity, sensitivity, and predictive value of gene markers and biomarkers for GIN infection may help inform the value of these markers as selection criteria for use in breeding programs.