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 were randomly allotted to nine treatment groups of eight pigs each. Eight of the groups were treated with
injectable solutions containing 300 microg of
doramectin/kg (IM) or 300 microg of
ivermectin/kg (SC) either 0 (same day), 7, 14, or 21 days prior to an oral challenge of 50000 embryonated A. suum eggs. The ninth group (control) was challenged in parallel without any
avermectin treatment. At 41 or 42 days after challenge, pigs were euthanatized and adult and larval stages of A. suum were collected from the gastrointestinal 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, respectively.
Doramectin given 7 days prior to challenge significantly (P < 0.0001) reduced nematode counts, and the efficacy was 98.4%. For all other
avermectin-treatment groups, nematode counts were not significantly reduced compared to those in control pigs. These data indicated that
anthelmintic activity of
ivermectin against A. suum persisted for less than 7 days and the activity of
doramectin persisted for more than 7, but less than 14 days.