The effect of host immunity on the development and survival of the free-living stages of common trichostrongylid parasites of sheep : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University, Palmerston North, New Zealand

Content removed due to coyright restriction 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.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