Life-trait evolution of a parasite strongyle nematode in response to host resistance: an experimental approach using Haemonchus contortus in black belly lambs

Abstract

The selection of resistant sheep in farms could increase the selective pressure during the parasitic phase and this might affect life-traits of the parasite nematodes. Before selecting for host resistance, it is crucial to estimate the potential adaptation of these nematodes to resistant hosts. The objective of this study was to determine the ability of the nematode parasite, Haemonchus contortus, to adapt its life-traits to different resistance states of sheep. The isolates of the nematode Haemonchus contortus were obtained from five locations in Guadeloupe (French West Indies) and a synthetic strain was prepared, which was representative of these isolates on the basis of isoenzymes and morphometric traits. This synthetic strain was reproduced during 10 generations in hosts of marked different resistance status. Primary infected 6-to 8-months old Black Belly ewe lambs, a wellknown to be highly resistant breed to nematode parasites, were used as resistant host. The “susceptible” lambs were obtained by treating with long-acting corticoids to depress the resistance of lambs. The establishment rate of worms and their egg-laying capacities were highly reduced in resistant lambs compared with susceptible ones. It can be assumed that the experimental design of infection apparently mimicked well the differences observed between genotypically resistant and susceptible sheep. To assess the adaptation, resistant, susceptible and naive lambs were infected with the two H. contortus lines, previously reproduced in resistant or susceptible sheep. The hosts were genetically related lambs (monozygotic twin lambs, within-litter brothers) allocated in an experimental plan designed to enhanced the power of the experiment. The use of monozygotic twins did not prove as useful as expected for parasitologi-cal data. There was no significant difference between the life-traits of the two lines. H. contortus did not seem to adapt to the resistance status of the host. Therefore, it was considered that selection for resistant sheep was possible with a limited risk of adaptation of the nematode parasites, although long-term monitoring of nematode adaptation should be scheduled if host selection is planned.