Comparative Activity of Pyrantel Tartrate, Parbendazole, and Levamisole at Two Dose Levels against Naturally Acquired Helminth Infections in Sheep

Abstract

Forty-nine ram lambs similarly exposed on pasture to common gastrointestinal helminth infections were assigned to 7 groups of 7 lambs each in a controlled anthelmintic trial involving 2 dose levels of each of 3 anthelmintics. All lambs were necropsied 5 to 7 days posttreatment for residual worm counts. Both mature and immature stages of some nematode parasites were recovered from the lambs. The most numerous species in the unmedicated controls were Ostertagia spp. (mainly 0. circumcincta), Haemonchus contortus, and Trichostrongylus spp. (mainly T. vitrinus). The drugs and dosages were: pyrantel tartrate 25 and 50 mg/kg; parbendazole 15 and 20 mg/kg; levamisole 8 and 12 mg/kg; unmedicated controls. The calculated efficacies at the 2 dose levels were, respectively, H. contortus: pyrantel tartrate 99 and 100%, parbendazole 54 and 78%, levamisole 99 and 99%8; Ostertagia spp.: pyrantel tartrate 86 and 96%, parbendazole 88 and 87%, levamisole 82 and 88%; Trichostrongylus spp.: pyrantel tartrate 98 and 99%, parbendazole 99 and 100%, levamisole 99 and 98%; Nematodirus spathiger: pyrantel tartrate 100 and 100%, parbendazole 85 and 91%, levamisole 91 and 96%; Oesophagostomum venulosum: pyrantel tartrate 96 and 100%, parbendazole 100 and 100%, levamisole 100 and 100%. In most instances, the higher dosages of the 3 anthelmintics appeared to be more efficacious than the lower dosages, but in most cases these differences were not statistically significant. Both 4thand 5th-stage H. contortus were somewhat resistant to parbendazole at the 2 dose levels used, but they were highly susceptible to pyrantel tartrate and levamisole. Three of the newer broad-spectrum anthelmintics for use in sheep are pyrantel tartrate (Austin et al., 1966), parbendazole (Actor et al., 1967), and levamisole, the active isomer of dl-tetramisole (Thienpont et al., 1966; Bullock et al., 1968). Although the accumulated data indicate that all three drugs have a similar spectrum of activity in sheep, each has been somewhat ineffective occasionally against certain species, stages, or strains when administered as a drench at the recommended therapeutic dose. The purpose of this trial was to compare the efficacy of two dose levels of the three drugs against naturally acquired multiple species infections in sheep. Most of the reports on the activity of parbendazole and levamisole against gastrointestinal nematodes of sheep were summarized recently by Kates et al. (1971). Parbendazole at 15 mg/kg of body weight does not compare favorably with other available anthelmintics against certain strains of H. contortus and Received for publication 6 April 1971. * National Animal Parasite Laboratory, Veterinary Sciences Research Division, ARS, Beltsville, Maryland 20705. tAnimal Science Research Division, ARS, Beltsville, Maryland 20705. against Nematodirus spp., whereas levamisole at 8 mg/kg of body weight is not reliably effective against Ostertagia spp. Austin et al. (1966) first published on the anthelmintic activity of pyrantel tartrate against sheep nematodes. Several investigators have confirmed the initial findings that a dosage of 25 mg/kg of body weight was highly effective against many nematode species (Corwell, 1966; Corwell et al., 1966a, b, 1967a, b; Cornwell and Jones, 1969b; Gibson and Parfitt, 1968; Colglazier et al., 1971; Reinecke and Anderson, 1967). Also, low-level continuous administration was effective prophylactically (Cornwell and Jones, 1969a, b), and in certain limited studies dosages of 45 or 50 mg/kg tended to enhance efficacy slightly against some species (Comwell, 1966). MATERIALS AND METHODS Origin and infection of experimental lambs These aspects were similar to those reported previously (Kates et al., 1971). Fifty-three lambs (black-faced, crossbred) were purchased from a dealer in the western United States and shipped by truck to Beltsville, Maryland, in mid-May 1969. Upon arrival the lambs averaged 31 kg in weight, and initial fecal examinations indicated that they had minimal infections of a few nematode species,