Changes in the resistance of the ewe to infection with Teladorsagia circumcincta during late pregnancy and lactation

During each of four periods commencing either 4 weeks before, at, and at 6 and at 12 weeks after parturition four groups of reproductive sheep and one group of non-reproductive sheep (no. = 6 in all cases) were brought indoors with their lambs from pasture. One group of reproductive sheep (PA) was slaughtered to determine worm burdens carried at pasture. At the same time non-reproductive sheep (B) and a group of reproductive sheep (PB) were treated with anthelmintic (day -7) and on day 0 given 20 000 larvae of T. circumcincta and slaughtered on day 21 for the measurement of worm burdens. The remaining two groups of reproductive sheep were maintained indoors: one group (PC) was given 4000 larvae of T. circumcincta per day for 30 days and the final group (PD) uninfected. Both groups were treated with anthelmintic after 50 days and on day 57 given 20 000 larvae of T. circumcincta and slaughtered on day 78 for worm counts. Production data were obtained for sheep in groups PC and PD. Chronic infection of ewes (PC) was associated with a 0.15 reduction in growth rates of their lambs and an increase in wool tenderness (0.3, P < 0.01). Worm burdens and eggs per g in reproductive sheep outdoors were highest during mid to late lactation which was confirmed by the pattern of change in resistance to challenge infection on removal from pasture. This contrasted with the situation indoors where resistance was least around parturition but was restored within 3 weeks of parturition and was especially evident in the group exposed to larvae during lactation (group PC). It is argued that the prolonged period of susceptibility of the sheep at pasture could have a nutritional basis, reflecting their much greater body-weight loss during early lactation than in the sheep indoors. The implications of the findings for the management of sheep flocks with reduced chemical inputs are discussed.