Importance of diagnostic aspects in ostertagism.

Diagnostic methods which reveal only the presence or absence of Ostertagia in grazing animals are of little importance since all will acquire some degree of infection when grazed in the temperate regions of the world. Ostertagia infections may be (1) light--no disease or performance losses occur; (2) moderate--disease may or may not be overtly visible but performance losses occur or (3) heavy--disease is overt with or without death loss. The differentiation of these levels of infection is a quantitative matter insofar as the number of parasites correlates with the degree of pathology and is much less certain than is the presence or absence of infection. Historically, the quantitative parasite egg count in feces (eggs per gram (e.p.g.)), combined with the usual subjective and objective information obtained in clinical examinations has been the procedure used to obtain a quantitative estimate of the level of infection. More recently, the use of plasma pepsinogen levels as an indication of gastric pathology has been adopted by some laboratories. Neither of these methods have met with the desired level of accuracy, but they do have considerable value when their limitations are recognized. In the present era of emphasis on epidemiologically based control programs we are faced with yet another aspect of diagnosis, namely that of a quantitative estimate of numbers of infective stages (L3) on herbage. Methods to assess the number of L3 on forage are not satisfactory for routine clinical use. This paper evaluates the primary objectives of diagnosis as applied to the herd and flock. It briefly evaluates current methodology insofar as the diagnosis of parasitic gastroenteritis is concerned. The use of e.p.g. counts in clinical practice is discussed. Examples applicable to herds of weaner and stocker cattle are presented.