The significance of the worm burden in the diagnosis of parasitic gastroenteritis is examined. It is emphasised that the worm population is in a dynamic state with the calf constantly losing part of its worm burden while picking up new infective larvae. The numbers and proportion of late fourth stage Ostertagia ostertagi larvae gives an indication of the success of the animal in controlling its worm burden. During an examination of the worm population it is also necessary to look at the condition and degree of development of adult female worms to assess the efficiency of the calf's acquired resistance to infection.
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