The effect of host immunity on the free-living stages of common trichostrongylid parasites was studied in a series of experiments, involving both artificially infected housed animals and naturally infected animals in the field. In Perendale ewes, bred for either enhanced or lowered resistance to nematodes, reduced developmental success of eggs to infective larvae was found in the resistant animals at some times of the year (p<0.01). This was consistent with the hypothesis of an adverse effect of increased host immunity on the development of the free-living stages of gastrointestinal nematodes. In lambs, this effect had been demonstrated previously and again in 1998, whereas results from 1997 were inconsistent. In fleece-weight selected and control lines of Romney lambs, exposed to the same level of pasture larval challenge, developmental success decreased with time (p<0.001), although the two lines did not differ. This was consistent with an increasing level of host immunity in both lines and provided strong support for the hypothesis of host immunity having an adverse effect on larval development. Nematode eggs from lambs in the field treated orally with either ivermectin or albendazole, did not differ in developmental success, providing no evidence that host immunity was influenced by the type of anthelmintic used. A lower developmental success of O. circumcincta in an LDA (p<0.001) was found in animals relatively immune to this parasite compared to control animals. In faecal cultures a significant difference was not demonstrated, but group sizes were very small. An effect of host immunity on the developmental success and infectivity of larvae of T. colubriformis, could not be demonstrated in trickle-infected groups of lambs that differed in their immunity to this parasite, one group being immunosuppressed with cortico-steroids. An adverse effect of small intestinal mucus and contents on larval development was demonstrated. This was more potent in intestinal contents than mucus. Although source animals differed greatly in their immunity to T. colubriformis, differences between immune and immunosuppressed animals in the magnitude of the effect of intestinal mucus and contents on larval development were not found. The results suggested that the effect of intestinal mucus and contents was not immunological but rather caused by some physical and non-specific properties. Overall, the results reported in this thesis further support the hypothesis of host immunity having an adverse effect on the development of the free-living stages of gastrointestinal nematodes of sheep. This was most evident in animals with naturally acquired infections and in housed animals infected with O. circumcincta. It is suggested that the failure to demonstrate this in experimental infections with T. colubriformis may have been due to the use of cortico-steroids to suppress immune responses.
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Jørgensen, L. T., Leathwick, D. M., Charleston, W. A. G., Godfrey, P. L., Vlassoff, A., & Sutherland, I. A. (1998). Variation between hosts in the developmental success of the free-living stages of trichostrongyle infections of sheep. International Journal for Parasitology, 28(9), 1347-1352.