Distribution of Some Parasitic Nematodes in the Alimentary Tract of Sheep, Cattle, and Rabbits

Abstract

The linear distribution of nematodes along the small intestine of sheep, cattle, and rabbits has been examined. In all three hosts the order of succession of species along the intestine appears to be the same. The shape of the distribution usually approximates a normal frequency curve even when the parasite is in an unusual host. In the ovine abomasum, thirdand early fourth-stage larvae of Haemonchus contortus tend to aggregate in the fundus, but larvae of Trichostrongylus axei occur in aggregations which appear to be scattered at random over the mucosa. It is well known that each of the various species of helminth parasites of the alimentary tract tends to be restricted to a particular location, but there are few detailed studies of distribution, and these are mostly concerned with a single species (Nishi, 1936; Bull, 1953b; Sommerville, 1954; Krupp, 1961). Experimental studies of the distributions of two or more species in concurrent infections (Holmes, 1961, 1962a, b) are rare. This is surprising, for a study of distributions might be expected to make a significant contribution to an understanding of relationships between different species of helminths in the same part of the host's gut. Possibly the most extensive studies of the distribution of helminths in natural infections have been made on the numerous species which parasitize the small intestine of the sheep (Tetley, 1935, 1937; Andrews, 1937; Davey, 1938). Each species tends to be located about the region of greatest abundance as a normal frequency distribution, and there is a regular order in the succession of species along the intestine although the individual distributions overlap. But certain species, which are more usually parasites of cattle, were found to have erratic distributions along the sheep intestine (Tetley, 1937), a phenomenon which was believed to reflect the incomplete adaptation of these species to sheep. The following data on nematodes in the abomasum and small intestine of sheep, cattle, Received for publication 20 November 1962. and rabbits were collected both to confirm and extend the findings of earlier workers and to provide a basis from which to study the role of interspecific relationships in the regulation of the distribution and abundance of helminth