The effect of selenium supplementation on immunity, and the establishment of an experimental Haemonchus contortus infection, in weaner Merino sheep fed a low selenium diet

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Abstract
SUMMARY: Immunity in 12 weaner Merino sheep fed a low selenium (Se) diet (low Se sheep) was compared with that in 10 matching sheep fed the same diet but each given an intraruminal Se pellet (high Se sheep), while the sheep were housed in individual, sheltered pens. All sheep were challenged with killed Brucella abortus cells (days 0 and 28), rabbit red blood cells (days 0, 7 and 28) and corynebaclerium pseudotuberculosis toxoid (days 0 and 28), and serum antibody titres were measured weekly for 8 weeks from day 0. The sheep were then experimentally infected with Haemonchus conforfus, and slaughtered 8 weeks later.The mean antibody titre to B. abortus, measured by 4 different tests, was significantly higher in the high Se sheep on occasions during the primary immune response phase (Rose Bengal test ‐ day 21 (p < 0.05), day 28 (p < 0.025); complement fixation ‐ day 7 (p < 0.05); enzyme‐llnked immunosorbent assay ‐ day 14 (p < 0.01); serum agglutination ‐ no differences), but not during the secondary phase. The mean antibody titre to rabbit red blood cells, measured by haemagglutination test, was marginally higher in the high Se sheep on day 49 (p = 0.049). The mean antibody titre to C. pseudotuberculois, measured by enzyme‐linked immunosorbent assay, was not significantly different between the groups at any time during the trial. In addition, the mean invitro responsiveness of peripheral blood lymphocytes to stimulation with phytohaemagglutinin in the high Se sheep was significantly greater than that in 10 sheep from the low Se group on day 22 (p < 0.01), but not day 50. However, there were no significant differences in the mean number of sheep in which the infection with H. contortus established, time to first shedding of eggs in faeces, abomasal worm burdens at necropsy, or inflammatory response in the abomasal mucosa in the sheep in each group. The results showed that the low Se sheep produced strong overall immune responses that were largely comparable to those in the high Se sheep.