The phenomena of host resistance and the course of infection of Ostertagia ostertagi in calves

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Abstract
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.