The effect of daily infection with Ostertagia circumcincta larvae on the performance of pregnant and lactating sheep

The effect of the abomasal parasite Ostertagia airaumcinata on the productive performance of Poll Dorset ewes was studied. Two groups (C and I) of 20 sheep were formed during the last 6 weeks of pregnancy, and group I infected with 4000 larvae of 0. aircumainata/day for 6 weeks. At parturition all sheep were dosed with anthelmintic. During lactation·a cross-over treatment was imposed in which the pregnancy treatment groups were subdivided and half from each further infected at the same rate for the first 6 weeks of lactation and the rest remaining as controls, thus providing four groups (CC, CI, IC, II), not infected in pregnancy or lactation, infected in one period or in both. Establishment of infection was monitored by faecal egg counts and plasma pepsinogen levels. A pelleted food containing 18.03 MJ and 145g crude protein per kg DM was offered on the basis of the energy requirement for late pregnancy and lactation. Faecal egg counts showed that the peri-parturient rise in egg count and depression of the acquired resistance of the ewes occurred only during lactation. On the other hand plasma pepsinogen levels increased progressively during both pregnancy and lactation. This suggested that both groups of infected ewes suffered continuous and considerable pathological damage as a result of larval challenge. During pregnancy the only significant effect of parasitism was a low post-partum bodyweight of ewes which produced twin lambs. During lactation, parasitism reduced food intake over the whole trial by 16% and bodyweight loss was 2.6 times greater in infected than control ewes. Milk and wool production were only significantly reduced during the last 3 weeks of the trial, by 17% and 21% respectively. Over the whole trial this reduction was 8% and 17% respectively. Staple strength was reduced by 31%. Calculation of the energy balances showed some evidence for an effect of parasitism on efficiency of utilization of metabolizable energy. These results suggest that overwintered larvae on pasture may seriously impair performance of lactating ewes