Observations on the resumption of development of inhibited Ostertagia, Cooperia and Nematodirus infections in calves stabled overwinter.

Two groups of three month old, parasite-free calves grazed a permanently infected pasture for 14 days, Group A during the first two weeks of September and Group B during early November. Half of each group was killed 14 days after removal from the pasture and the remainder stabled overwinter before slaughter and parasitological examination. Marked inhibition of development occurred for Cooperia oncophora with a variable lower level of inhibition for Ostertagia ostertagi and practically none for Nematodirus helvetianus in those calves grazing late in the fall. Under the conditions of this study, inhibited Cooperia larvae resumed development in several calves soon after they were stabled while small numbers of Ostertagia resumed development regularly during the winter and spring with a considerable number of Ostertagia still present when the calves were slaughtered at the end of the stabling period. On the other hand, Nematodirus and practically all Cooperia worms were lost during the stabling period. In three of seven calves grazing late fall pastures, large Cooperia infections were either not established or failed to become patent.