COMPARATIVE PERSISTENT EFFICACY OF DORAMECTIN, IVERMECTIN AND FENBENDAZOLE AGAINST NATURAL NEMATODE INFECTIONS IN CATTLE

A study was conducted in Argentina, to investigate the period of prote ction of a single injection of doramectin administered subcutaneously (SC) at 200 mu g kg-l (1 ml/50 kg) compared with single treatments of ivermectin (200 mu g kg(-1) SC) and fenbendazole (5 mg kg(-1) PO), aga inst field infections of gastrointestinal parasites of cattle. Eighty- three animals were selected and ranked on the basis of serial fecal eg g counts (e.p.g.'s). From this group, three animals were slaughtered b efore treatment and their lungs, abomasum, small and large intestines, were processed for parasite counts and identification. The remaining 80 animals were allocated in ranked groups of four to a control or one of three treated groups. Animals of the four groups were sated togeth er in the same pasture for the duration of the study. Treatments were administered on Day 0. Individual fecal samples were collected at week ly intervals for the first 49 days post-treatment and twice a week fro m Day 52 to Day 84 (end of study). At each collection day fecal sample s were pooled for coprocultures. On Day 28 and 56, two animals from ea ch group, previously identified on Day 0, were killed and their parasi te burdens determined. The duration of protection of a single injectio n of doramectin was longer than ivermectin or fenbendazole treatment. On Day 56, the total number of parasites found in doramectin-treated a nimals was significantly (P < 0.05) lower than parasite burdens found in either ivermectin-or fenbendazole-treated animals. The longer persi stent activity of doramectin was expressed by the lower number of adul ts and L-4 stages of Ostertagia ostertagi. Data from this experiment d emonstrated the limitations of using fecal egg counts to evaluate the persistent efficacy of anthelmintics. The duration of activity of dora mectin was demonstrated more accurately by parasite counts in cattle f rom each group since decreasing e.p.g.'s were seen in non-medicated an imals without changes in total parasite burdens. (C) 1997 Elsevier Sci ence B.V.