Induction of protective immunity to Dictyocaulus viviparus in calves whileunder treatment with endectocides

Three groups of five parasite-naive calves were used. The treatments were: (a) Group 1 calves were weighed on Day 0 and injected with doramectin at 20 0 mu g/kg. From Day 1 to 19 they were dosed orally with 2000 infective larv ae of Dictyocaulus viviparus. On Day 28 they were again injected with doram ectin, and infected with D. viviparus larvae from Days 33 to 41. They were then left untreated until Day 81 when they were infected with 20 infective larvae of D. viviparus per kg body weight. They were killed on Day 110 and lungworms were counted; (b) Group 2 calves were immunised with oral lungwor m vaccine on Days 0 and 28, and infected and slaughtered as Group 1 on Days 81 and 110, respectively; (c) Group 3 calves acted as infection controls. Blood samples were taken at Days 0, 21, 49, 77 and 110 for antibody tests t o D. viviparus. At autopsy there were no significant differences between th e number of lungworms from Groups 1 and 2 (Means 17.4 and 31.3, respectivel y); Group 1 had significantly less value than Group 3 (Mean 228) (p < 0.05) . Increased antibody titres to the larval sheath of the infective larvae We re observed from Groups 1 and 2, showing that the larvae in Group 1 had pen etrated the intestine before being killed by the circulating anthelmintic. This experiment shows that if calves are exposed to infective larvae while under systemic endectocide cover, an immune reaction is stimulated. (C) 200 0 Elsevier Science B.V. All rights reserved.