INFLUENCE OF ARTIFICIAL REARING ON BEHAVIORAL AND IMMUNE-RESPONSE OF LAMBS

The influence of artificial rearing at different ages was evaluated in Comisana lambs. Animals were assigned to three test groups (2d, 15d and 28d) on the basis of age (2, 15 or 28 days, respectively) at the time of their separation from the ewe, and three corresponding control non-separated groups (2c, 15c and 28c). On the day of separation from their dam, animals were injected i.m. with 250 mu g of keyhole limper hemocyanin in complete Freund's adjuvant. Antibody titers and surface leukocyte antigens were determined at weekly intervals. Group 2d showed a lower antibody response than the corresponding control group (F=8.0, P <0.01), whereas groups 15d and 28d did not differ from the control. No significant difference between test and control groups was found in the proportion of cells bearing different surface antigens. Furthermore, lambs were subjected to a behavioral test (isolation from tactile and visual contact with other animals). Younger animals showed reduced duration of movement (162.8 vs. 272.1 and 221.1 s for 2d, 15d and 28d, respectively; U=21.0, P<0.01 and U=30.5, P < 0.05, respectively; Mann-Whitney U test) and enhanced latency time (15.8 vs. 3.4 and 2.3 s for 2d, 15d and 28d, respectively; U=28.0, P<0.01 and U=14.5, P<0.001, respectively; Mann-Whitney U test) in comparison with older animals. Cortisol concentration was evaluated immediately before the separation from the mother and 15, 45, 90, 150, and 225 min after. An increased cortisol level was observed in group 2d 15 min after removal from the dam (LSD: P <0.001), whereas the other two groups did not show any significant increase. In addition, during the first 28 days of age all the animals were weighed at birth and at weekly intervals. When the animals were removed from the mother either at 2 or 15 days of age, the daily weight gain decreased significantly in comparison with the corresponding control groups (107.1 vs. 157.1 g for 2d and 2c, respectively, F=4.8, P<0.05; 37.3 vs. 126.8 g for 15d and 15c, respectively, F=5.8, P< 0.05). In conclusion, early separation from the ewe affects humoral immune response and post-separation performance of lambs as a possible consequence of a reduced ability of young animals to cope with emotional and nutritional stresses.