Persistent activity of doramectin and ivermectin against Ascaris suum in experimentally infected pigs

A study was conducted to investigate the persistent nematocidal activity of two avermectins against experimentally-induced infections of Ascaris suum in swine. Seventy-two nematode-free cross-bred pigs of similar bodyweight w ere randomly allotted to nine treatment groups of eight pigs each. Eight of the groups were treated with injectable solutions containing 300 mu g of d oramectin/kg (IM) or 300 mu g of ivermectin/kg (SC) either 0 (same day), 7, 14, or 21 days prior to an oral challenge of 50000 embryonated A, suum egg s. The ninth group (control) was challenged in parallel without any avermec tin treatment. At 41 or 42 days after challenge, pigs were euthanatized and adult and larval stages of A, suum were collected from the gastrointestina l tract of each pig and counted. Both avermectins significantly (P < 0.0002 ) reduced nematode counts when given on the day of challenge (0 days prior) , and the efficacy was 100% and 97.5% for doramectin and ivermectin, respec tively. Doramectin given 7 days prior to challenge significantly (P < 0.000 1) reduced nematode counts, and the efficacy was 98.4%. For all other averm ectin-treatment groups, nematode counts were not significantly reduced comp ared to those in control pigs. These data indicated that anthelmintic activ ity of ivermectin against A. suurn persisted for less than 7 days and the a ctivity of doramectin persisted for more than 7, but less than 14 days. (C) 1999 Elsevier Science B.V. All rights reserved.