DURATION OF ANTHELMINTIC EFFICACY OF DORAMECTIN AND IVERMECTIN INJECTABLE SOLUTIONS AGAINST NATURALLY ACQUIRED NEMATODE INFECTIONS OF CATTLE

A comparison of persistent efficacy of doramectin injectable (D) and i vermectin injectable (I) was investigated under field conditions with treated permanent principal (PP) and interval-grazed principal (IGP) c alves. The experiment was initiated on October 13, 1992 (day 0). Cattl e used were crossbred beef heifers of 185 kg average weight and 8 to 1 0 months old. By random allotment, 66 calves were divided into two gro ups of 15 PP-D and PP-I calves for each treatment and two groups of 15 IGP-D and IGP-I calves for each treatment. Three extra or replacement calves were allotted for each group. Permanent principal calves in th ree replicates of five cattle per treatment grazed continuously on nem atode-contaminated replicate pastures from day 0 to day 70. At 2-week intervals, i.e., days 0 to 14, 14 to 28, 28 to 42, 42 to 56 and 56 to 70, one IGP-D and one IGP-I calf was grazed with each of the respectiv e PP-D and PP-I calf replicates and necropsied 21 days after removal f rom pasture. All respective PP calves and IGP calves were treated with doramectin at 200 mu g kg(-1) or ivermectin at 200 mu g kg(-1) by SC injection on day 0. After the day 0-14 interval, all IGP-D calves had zero egg counts. From the day 14-28 interval through the next three gr azing intervals, the arithmetic mean egg counts of IGP-D calves were 1 8, 90, 281 and 31; those of IGP-I calves were 30, 226, 74 and 185. Thi s suggested a persistence effect of approximately 2 to 4 weeks. In PP- D calves, egg counts reached a mean maximum at day 56 of only five egg s per gram, while counts of PP-I calves reached a peak of 40 on day 42 . From the day 14-28 interval and through all subsequent intervals, ar ithmetic mean total worm counts from IGP-I calves were 58 to 73% great er than those in IGP-D tracers. A maximal total worm count of 4159 was observed in IGP-D calves of the day 42-56 interval; total worm counts in IGP-I calves from the day 14-28 interval through the day 42-56 int erval were: 5420, 6739 and 9979, respectively. Haemonchus and Cooperia were higher in prevalence than Ostertagia in both treatments. Results of PP-D egg counts and total worm burdens in IGP-I calves indicated a high level of doramectin persistent activity for approximately 4 to 5 weeks and an advantage over persistence activity of ivermectin. (C) 1 997 Elsevier Science B.V.