Influence of ivermectin on cellular and humoral immune responses of lambs

In view of the extensive use of anthelmintics in sheep and the fact that their activity may in part depend upon the immune system, we were interested to determine if ivermectin had any influence on aspects of the sheep immune response. Ten parasite-free 6-month-old lambs were drenched with ivermectin and 1 day later were given intravenously human erythrocytes and subcutaneously ovalbumin. Ten other lambs with injected antigens were not drenched and served as controls. Both groups were bled at intervals for cells and serum. The procedure was repeated on day 28. Lymphocytes from the drenched lambs, cultured in vitro in RPMI plus 50% autologous serum collected up to 7 and 14 days after the first and second antigen injections respectively, had decreased blastogenic activity compared with lymphocytes from control lambs. Similar results were obtained with lymphocytes cultured in RPMI 1640 supplemented with 50% autologous serum plus concanavalin A (Con A) or phytohaemagglutinin (PHA). When washed, lymphocytes were cultured in RPMI 1640 supplemented with 5% foetal calf serum (FCS) or 5% FCS plus Con A or PHA, decreased blastogenesis was observed but blastogenesis depression was not as marked as that observed with autologous serum. Similar antibody responses were seen for the drenched and control groups in response to the two injections of both antigens except that after the second injection, there was a significant reduction in antibody response to ovalbumin in the ivermectin-treated lambs. There were no differences in serum complement or serum nitric oxide levels between the two groups at any stage, but insulin-like growth factor-1 levels were significantly reduced in serum of the ivermectin-treated group, 4 days after each drench. Growth hormone levels were consistently significantly higher 22 days after both drenchings. There was no difference in mean body weight increase between the groups during the experiment.