Aspects of the biology of Ostertagia ostertagi in relation to the genesis of ostertagiasis.

Adaptations such as the capacity of free-living stages to survive environmental stress, inhibition and density-dependent effects on population size and fecundity show O. ostertagi to be a consummate parasite of cattle in temperate environments. Knowledge of these adaptations, within an animal management context, provides the key to understanding the occurrence of disease and a basis for control of parasite numbers. Substantial infections do arise from low egg contamination rates of pastures. Discontinuities in rates of infection are caused by poorly predicted seasonal events such as the effects on eggs of oxygen-deficient environments, fluctuating temperatures and the absence of sufficient moisture for migration of infective larvae from faeces to herbage. Time delays of several months between pasture contamination and availability of infection are therefore common. Ingestion of large numbers of larvae, over a short period early in the grazing season, gives rise to Type I disease or subclinical infections which decrease liveweight gains. In some ecotypes, environmentally induced inhibition leads to the accumulation of large populations of early 4th stage parasites within the host. These populations, under poorly defined conditions, can mature synchronously to produce severe diarrhoea, debility and even death in a proportion of mature cattle--the Type II disease.