PROTECTIVE EFFICACY OF DORAMECTIN AND IVERMECTIN AGAINST COCHLIOMYIA-HOMINIVORAX

Two studies were conducted in Brazil using induced infestations of the New World screwworm, Cochliomyia hominivorax, to investigate: a) the comparative prophylactic efficacy of doramectin and ivermectin at thei r recommended use levels (200 mu g kg(-1) SC), and b) the duration of protection of a single injection of doramectin. In the comparative eff icacy study, two groups of six animals each were treated with ivermect in or doramectin. Two hours after treatment, four incisions were made. Each incision was infested with 30 first instar C. hominivorax larvae and their status evaluated daily for 7 days post-treatment (p.t.). Do ramectin treatment was 100% effective in prevention of C. hominivorax infestations whereas ivermectin efficacy was incomplete. First instar larvae were eliminated in doramectin-treated calves by 48 h p.t., whil e in the ivermectin group, C. hominivorax developed in over 29% of the incisions. Healing began in wounds of doramectin-treated animals at 2 4 h p.t. and was in progress in 100% of all wounds at 2 days p.t., whi le 50% of ivermectin-treated calves showed incisions with active lesio ns. In the duration of protection study, 24 calves were allocated to s ix groups (T1-T6) of four animals each. Three groups (T1, T3 and T5) w ere treated with saline and three groups (T2, T4 and T6) with doramect in. Animals were infested as described previously according to the fol lowing schedule: T1 and T2 at day 14, T3 and T4 at day 21, and T5 and T6 at day 28 p.t. Incisions were evaluated daily for 8 days post-infes tation. Screwworm infestations and viable third-instar larvae develope d of all incisions of saline-treated calves, while doramectin was 100% effective preventing development of C. hominivorax for 21 days p.t. a nd showed partial activity at 28 days p.t. (C) 1997 Elsevier Science B .V.