Efficacy of doramectin for treatment of experimentally induced infection with gastrointestinal nematodes in calves.

Objective: To determine the efficacy of injectable doramectin against experimentally induced infections with adult and larval (L4) stages of gastrointestinal nematodes in calves.

Procedures: In each experiment, between 20 (experiments 1 and 2) and 30 (experiment 3) parasite-free yearling calves were selected and allotted to groups of 10/group. All calves received oral inoculation of the following infective larval stages of these nematode species: Cooperia onchophora, C punctata, C zurnabada, Cooperia spp, Haemonchus placei, Nematodirus helvetianus, Oesophagostomum radiatum, Ostertagia ostertagi, Trichostrongylus axei, and T colubriformis. Infections or treatments were timed so that, by day of treatment, the population of each parasite species had matured to the stage to be tested (adult or L4). On day of treatment, calves received doramectin (200 micrograms/kg of body weight, SC), and infected nonmedicated control calves received saline solution SC at the same time. All calves were slaughtered at least 14 days after treatment.

Results: Efficacy against the following adult and L4 species was > or = 99%: C onchophora, C punctata, C zurnabada, Cooperia spp, H placei, Oes radiatum, O ostertagi, and T colubriformis. Doramectin was also 100% efficacious against the L4 stage of T axei. Efficacy against the adult and L4 stages of N helvetianus was 9.6 and 77.4 to 83.3%, respectively.

Conclusions: Doramectin, at a dosage of 200 micrograms/kg, is effective in controlling the prevalent gastrointestinal nematodes (adult and L4 stages) found in naturally infected calves.

Clinical Relevance: Doramectin, a new anthelmintic, was effective against recent field strains of the most prevalent gastrointestinal parasites found in cattle in the United States, and could provide another alternative for control of these parasites.