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One hundred and seventy, 5-year old Coopworth ewes were synchronised and mated. Pregnant twin bearing ewes and non-pregnant ewes were then allocated to four periods of treatment, infection timed relative to the post-parturient changes in host susceptibility to nematode infection. These commenced either 4 weeks prior to lambing (period 1), at lambing (period 2), 6 weeks after lambing (period 3) or 12 weeks after lambing (period 4). Within each period of infection, pregnant ewes were further allocated to four groups (n=6) viz initial slaughter (group A); the remaining animals were treated with anthelmintic and given either a single infection with 20,000 larvae of Ostertagia circumcincta (group B) followed by slaughter after 21 days, 4000/larvae per day for 50 days (group C) or no infection (group D). Groups C and D were again dosed with anthelmintic and give a single infection with 20,000 larvae on day 57 and slaughtered 21 days later. Non-breeding ewes (group BA -n=6) were challenged on day one of each period with 20,000 larvae and slaughtered 21 days later. All ewes were housed indoors and offered a pelleted diet. Faecal egg counts were determined weekly, and ewe liveweight, food intake, milk production and serum pepsinogen measured weekly. Total Worm counts and numbers of eggs in utero were determined from samples obtained at slaughter. A relaxation in the immune response to infection was apparent during late pregnancy (period 1) and early lactation (period 2) as judged by faecal egg counts, but appeared to be greatest around parturition when maximum worm burdens occurred (period 2). Significantly higher worm burdens were found in response to challenge in breeding ewes compared to non-breeding ewes at all times, except period 4. By 12 weeks after lambing worm burdens in breeding ewes were similar to those found in non-breeding ewes throughout the experiment. From mid-lactation (periods 2.3), increasing signs of host resistance to infection were evident Host suppression of faecal egg output was seen during periods 2 and 3, with egg output being reduced by 50% and 95% respectively, compared to that seen during period 1. Numbers of eggs in utero per adult worm were 25, 45 and 37 eggs/worm in period 1, 2 and 3 respectively. Faecal egg counts (period 1) and worn burdens (period 1 and 2) of previously infected ewes were lower than their initially non-exposed counterparts. Indicating that the effect of larval stimulus during late pregnancy and lactation (periods 1 & 2), evoked a substantial immune response in lactating ewes after rechallenge at day 57. Parasitism reduced milk production by 10-59% and wool staple strength by 44-29%. During lactation food intake, liveweight and serum pepsinogens were affected by parasitic infection but little effect was seen prior to parturition.