Antiparasitic activity of several compounds was evaluated over a long period (about 25 years) in the same flock of sheep. Haemonchus contortus was of special interest, including its relation to drug resistance, especially to
thiabendazole and other
benzimidazoles, in addition to
phenothiazine. Eleven compounds were evaluated in 15 controlled tests, done between 1966 and 1989 in naturally infected lambs (n = 145) born and raised on the same pasture. Sheep were first placed on the pasture in 1962, and a few more were added thereafter. Internal parasites in these sheep were classified in 3 general categories: indeterminate exposure to
parasiticides; H contortus, resistant to
thiabendazole; and H contortus, resistant to
phenothiazine. The
parasitic infections probably became more homogeneous after several years because of few introductions of outside sheep after initial establishment of the flock. Activity against naturally acquired internal helminths was evaluated for
cambendazole (CBZ: dosage, 20 mg/kg of
body weight),
fenbendazole (FBZ: 5 or 7.5 mg/kg),
mebendazole (MBZ: 10 mg/kg);
oxfendazole (OFZ: 3.5 or 10 mg/kg),
oxibendazole (OBZ: 10 mg/kg);
parbendazole (PBZ: 15 mg/kg),
phenothiazine (PTZ: 550 mg/kg);
pyrantel pamoate (PRT: 25 mg base/kg), tetramizole (TET: 15 mg/kg);
thiabendazole (TBZ: 30 or 44 mg/kg), and
trichlorfon (TCF: 100 mg/kg).
Thiabendazole was used more often (9 tests) than the other compounds.
Thiabendazole was more active against mature H contortus in later years than when first used in 1966, although it was never 100% effective. Efficacy against immature H contortus for TBZ did not exceed 86%. Activity against immature and mature stages of this parasite was good overall for the other
benzimidazoles.(ABSTRACT TRUNCATED AT 250 WORDS)