Complement-fixation (CF) tests with sonicated aqueous extracts of adult forms of five species of gastrointestinal nematodes as antigens have been made on serial biweekly bleedings from two groups of parasitized calves grazing on infected pastures during the 1963 or 1964 seasons. Three of these nematode species, Cooperia oncophora, Ostertagia ostertagi and Nematodirus helvetianus, were found in large numbers in most of these animals. The calves were negative serologically before being placed on pasture but within 2 to 4 weeks some had developed considerable CF activity with Cooperia and other nematode antigens. Ten calves died, however, before significant CF titres had been attained. Eight control calves in the 1963 or 1964 groups which were grazed on “clean” pastures, remained serologically negative during the summer and autumn months. Four of the six surviving exposed animals in the 1964 group showed a fall in CF activity during the winter months when they were stabled, whereas the six surviving controls developed low CF titres suggesting that they were becoming mildly parasitized. In the spring of 1965 CF titres began to rise in some of the previously exposed and control yearlings even before they were placed on infected pasture in June
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