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Two experiments are described in each of which three groups of calves received infective larvae of Ostertagia ostertagi daily, each group at a different rate. Changes in worm burdens were followed by the periodic slaughter of calves.Numbers of adult worms quickly rose to and remained at levels which bore a positive relation to the infection rate. There was evidence that a turnover of worms was occurring and it appeared that burdens of adult worms were regulated by a loss of worms which depended on the number present. As the host's experience of infection increased, the adult worms present were smaller and an increasing proportion of females lacked a fully developed vulval flap. Evidence is presented which suggests that these were effects of resistance and that they depended on separate mechanisms.Faecal egg counts of groups carrying different numbers of worms rose to the same peak and then declined according to the same logarithmic curve. This finding, together with data concerning the number of eggs in the uteri of the worms, suggested that the egg output of the entire population was restricted by the host to a limit which depended on previous egg-laying.Infection did not have an adverse effect on the growth of the calves unless the infection rate exceeded a critical value. The resistance of calves to infection was impaired by clinical ostertagiasis.

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The faecal egg counts of seven groups of calves exposed to infection with O. ostertagi on pastures were found to follow a similar course to those of calves experimentally infected on one occasion or of calves infected daily. They declined from an early peak according to an apparently logarithmic curve. It was concluded that faecal egg counts were largely independent of worm numbers and tended to follow a stereotyped pattern. Acute clinical ostertagiasis was as liable to lead to high egg counts as was poor thriving due to the other causes. The relevance of these findings to the epidemiology of ostertagiasis and to the use of faecal egg counts in the diagnosis of parasitic gastro-enteritis are discussed.The author thanks Miss C. N. Herbert for the calculations and curve fitting in this paper.

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