A study of the changes in the worm burden in calves receiving daily doses of infective larvae of Ostertagia ostertagi indicates that the following processes occur:(1) A constant loss of adult worms at a rate depending on the number present.(2) An inhibition of development in the early fourth stage which depends partly on the presence of adult worms. This inhibition is not irreversible; larvae resume their development, the number which do so depending on the number present.(3)A stunting of adult worms, those developing late in the infection failing to grow as large as those which developed earlier.(4)An inhibition of ovulation, distinct from the stunting, which prevents worms developing later from becoming as prolific as those which developed earlier.(5)A resistance to the establishment of newly acquired worms, which begins to operate after prolonged exposure to infection.The loss of adult worms and their replacement by the further development of fourth stage larvae leads to a frequent exchange of populations of adult worms.The effect of these phenomena in determining the course of the infection is discussed.

The course of the worm burdens of calves infected daily with larvae ofOstertagia ostertagiat five different rates was studied by making post-mortem worm counts on groups of calves killed after experiencing infection for different periods of time.The relationship between worm numbers and infection rates together with evidence that a turnover of worms occurred, led to the conclusion that burdens of adult worms were regulated primarily by a loss of worms at a rate proportional to the number present.Further, estimates based on the number of developing fourth-stage larvae indicated that the proportion of the larvae administered which became established decreased gradually as the experiment progressed.At very low infection rates, the rate of turnover of the worms was slower than at high infection rates but in all groups it remained constant through the 6 months duration of the experiment.The development of a small proportion of the larvae administered was arrested at the early fourth stage. At the highest infection rate used, the proportion of worms that became inhibited was somewhat greater.

Just as human helminth infections are vitally influenced by the way in which people live, so the helminthiases of domestic animals depend on how these are managed.Veterinary helminthology is a branch of agriculture. The agricultural industry changes quickly, more quickly than the ideas of scientists. Ideas, in their turn change more rapidly than their practical implications are understood. A discussion of anthelmintic use must therefore be partly agricultural, partly historical.

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