title: Suspected resistance of Ostertagia ostertagi in cattle to levamisole.
abstract: Observations on a beef cattle farm in Flanders led to the suspicion of resistance to levamisole in a strain of Ostertagia ostertagi. After treating a group of six animals with levamisole (5 mg kg-1 L.W., i.m.) the reduction in the number of trichostrongylid eggs per gram of faeces varied between 0 and 66.6%, whereas a similar group treated with fenbendazole (7.5 mg kg-1 L.W., p.o.) showed a reduction in worm burdens of 100%. Coproculture showed that the remaining eggs in the first treatment group were all Ostertagia sp. The suspected field strain was compared with a reference strain of O. ostertagi by means of the in vitro larval paralysis test. This test showed LC95 values of 9.12 micrograms ml-1 and of 99.04 micrograms ml-1 for the reference and the field strain respectively, which indicates a resistance factor for the latter of 10.9. These results were not unequivocally confirmed by the post mortem findings on a tracer calf necropsied 4 days after treatment with levamisole.
mesh_terms: D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000188:drug therapy* / Q000469:parasitology; D004351:Drug Resistance; D005243:Feces / Q000469:parasitology; D005273:Fenbendazole / Q000494:pharmacology / Q000627:therapeutic use; D007814:Larva; D007978:Levamisole / Q000494:pharmacology*; D010028:Ostertagia / Q000187:drug effects*; D010029:Ostertagiasis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*; D010270:Parasite Egg Count; D014252:Trichostrongyloidiasis / Q000662:veterinary*
chemical_list: D007978:Levamisole; D005273:Fenbendazole