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Forty-five ram lambs uniformly exposed to natural infections of various helminths were assigned to 5 groups of 9 lambs each and used to conduct controlled anthelmintic trials. One group of lambs served as unmedicated controls, and 4 groups were treated as follows: (1) thiabendazole 50 mg/ kg; (2) thiabendazole 100 mg/kg; (3) levamisole 8 mg/kg; (4) parbendazole 15 mg/kg. All lambs were necropsied 4 to 6 days posttreatment for residualworm counts. Both mature and immature stages of several parasite species were recovered from the unmedicated control lambs; Haemonchus contortus was the most numerous. For the medicated groups 1, 2, 3, and 4 above, the calculated efficacies of the treatments against all stages of parasites were, respectively, H. contortus 48, 88, 99, and 79%; Ostertagia spp. (mainly 0. circumcincta) 94, 99, 76, and 99%; Trichostrongylus spp. (T. axei, T. vitrinus, and T. colubriformis) 99, 100, 99, and 100%; Nematodirus spathiger 90, 100, 96, and 83%; Oesophagostomum venulosum 98, 100, 100, and 99%. The data indicate the H. contortus strain in these lambs was somewhat resistant to thiabendazole and parbendazole. Fourth-stage larvae were recovered only of H. contortus, Ostertagia spp., and N. spathiger, and generally were not removed as effectively as 5th-stage parasites. However, levamisole was highly effective against 4th-stage H. contortus and N. spathiger, and 100 mg/kg of thiabendazole removed all 4th-stage N. spathiger. The 3 anthelmintics significantly reduced strobilar volume of Moniezia expansa, but had no activity against the scoleces. During the last decade, three new broadspectrum anthelmintics, viz., thiabendazole, tetramisole, and parbendazole, have shown considerable promise against helminths, particularly gastrointestinal nematodes of sheep and other domestic animals. The effective action of thiabendazole [2-(4'thiazolyl)-benzimidazole] against a wide variety of nematode parasites of various animals was first reported by Brown et al. (1961). Since that time a substantial literature (see Gibson, 1965, 1969; Levine, 1968) has accumulated supporting the original findings. Recently, however, Theodorides et al. (1970) and Colglazier et al. (1970a) summarized a number of published reports pertaining to strains of Haemonchus contortus that were resistant to the drug. Thienpont et al. (1966) first reported on the anthelmintic activity of dl-tetramisole [2,3,5,6tetrahydro-6-phenyl-imidazo (2,1-b) thiazole hydrochloride] in man and animals. Subsequently, Bullock et al. (1968) separated the dextro and levo isomers of the drug, and showed that the anthelmintic action was attribReceived for publication 3 September 1970. * National Animal Parasite Laboratory, Veterinary Science Research Division. t Animal Science Research Division, ARS, USDA, Beltsville, Maryland 20705. utable primarily to the levo form. Considerable published data have now accumulated which attest to the broad-spectrum activity of this chemical, and to the fact that the levo isomer (levamisole) is about twice as active as the dl compound (Walley, 1966; Fitzsimmons, 1966; Forsyth, 1966, 1968; Gibson, 1966; Gibson et al., 1968a, b; Hart and Curr, 1968; Hart et al., 1969; Lyons et al., 1968; Pankhurst and Sutton, 1966; Pretorius, 1967; Ross, 1966; Shone and Philip, 1967; Colglazier et al., 1969, 1970b). In 1967, Actor et al. published the first report on another promising livestock anthelmintic, parbendazole [5(6)-butyl-2-benzimidazole carbamate], a benzimidazole compound related chemically to thiabendazole. Parbendazole had an anthelmintic spectrum similar to that of thiabendazole, but it was active at lower dose level. Several studies on the activity of parbendazole against gastrointestinal nematodes of sheep and goats have been reported which essentially confirm and extend the earlier findings, viz., Bennett (1968), Danek et al. (1969), Johns and Mendel (1969), Lammler et al. (1969), Luque Forero et al. (1969), Ross (1968), Theodorides et al. (1968, 1969). Theodorides et al. (1970) also reported that a number of thiabendazole-resistant strains of H. contortus were also cross-resistant to parbendazole. |
