Distribution of Oesophagostomum columbianum larvae along the alimentary tract of the sheep

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Abstract
Linear distribution of the third-stage larvae of O. columbianum along the alimentary tract, and ectopic distribution within the organs of the body cavity, were investigated in sheep which were given first, second, and third infestations with 5000 larvae. Two types of nodule were recognized: (1) small gritty lesions, which predominated in the small intestine; (2) caseous nodules, mostly found in the large intestines. Linear distribution of the larvae was characterized by two peaks in numbers along the small intestine, the first immediately posterior to the common bile duct and the second at the ileocaecal valve. The origin of these peaks is discussed in relation to the exsheathment and subsequent behaviour of third-stage larvae and the flow of ingesta along the gut. Three peaks in larvae numbers were recognized in the large intestine; these are discussed in relation to the movements of ingesta. After second and third infestations the numbers of caseous nodules increased and those of ectopic migrations decreased. Also a third peak in the numbers of larvae invading the wall of the small intestine occurred at the beginning of the ileum. This is discussed in relation to the immunity reactions of the host. Of O. columbianum larvae, 75% were found to exsheath in the rumen after 12 hr; pH alone had little effect on exsheathment, except at a value of 6.5, which is equivalent to the pH of the ruminal contents when 10% of the larvae exsheathed after 8 hr. The effects of fresh bile on larval activity was also studied. These results are discussed in relation to the linear distribution of the larvae. It is suggested that after larvae have exsheathed, they respond specifically to environmental change by penetrating the gut wall. When the larvae penetrate into the body cavity, environmental changes are absent and the resultant larval migrations are completely random. It is also suggested that the final area in which the adult worm will settle depends on the part of the gut in which the larvae exsheathed and the area of the gut wall which the activated larvae penetrated. The distribution of caseous nodules is discussed in relation to the host specificity of O. columbianum.

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