The epidemiology of bovine ostertagiasis in the north temperate regions of North America.

Ostertagia ostertagi is widely distributed and is the most pathogenic of the parasitic nematodes affecting cattle in this region. Clinical ostertagiasis is seen mainly in calves and yearlings but outbreaks tend to be sporadic; the subclinical disease is of greater importance. Studies on the population dynamics of the free-living stages have shown that infective larvae can survive on pasture over the relatively severe winter conditions encountered in this region but that such pasture contamination declines during the succeeding grazing season and is lost by midsummer. In the host there is gradual acquisition of worms from pasture over the summer period, with relatively high burdens accumulating in the autumn. A dramatic shift in the proportion of adult to immature worms occurs during the autumn (October). By the time calves are housed, the proportion of larvae (mainly L4) is greater than 80%. This relative distribution of adults to larvae continues through the winter months until early spring when there appears to be a shift to a higher proportion of adults, presumably due to resumption of development of the L4. These persistent L4 stages are considered to be undergoing hypobiosis. Spring infections of calves appear to be mainly acquired from larvae that have successfully overwintered on pasture, and availability of these larvae is drastically reduced by midsummer. There appears to be negligible development of the eggs that result from the spring infections until late summer and early autumn. At this time because of favorable climatic conditions there is rapid larval development which can result in heavy infections in susceptible calves in the autumn.(ABSTRACT TRUNCATED AT 250 WORDS)