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A strain of Haemonchus contortus, which had become partially resistant to cambendazole after 4 successive exposures to the drug in experimentally infected lambs (Kates, Colglazier, and Enzie, 1973), was subjected to additional drug exposures at increasing dose levels up to 60 mg/kg. After the 10th drug exposure, data from definitive controlled anthelmintic tests showed that the magnitude of resistance had increased substantially. In these terminal therapeutic trials, 2-fold (40 mg/kg) and 3-fold (60 mg/kg) increments in the ordinarily effective therapeutic dose level (20 mg/kg) of cambendazole resulted in efficacies of only 45 and 72%, respectively; whereas, in concomitant trials, both dose levels were almost 100% effective against the original cambendazole-sensitive strain of Haemonchus. The resistant strain showed no reversion in sensitivity after 2 passages in nonmedicated lambs between the 5th and 6th drug exposure trials. We reported the experimental development of cambendazole resistance in an ovine strain of H. contortus that had been highly sensitive to the drug (Kates et al., 1973). The known literature at that time on resistance to anthelmintics by various nematode species was also summarized. There have been no additional relevant publications in the interim. The purpose of this report is to present data on the increased level of cambendazole resistance that developed in this strain of H. contortus after additional exposures to the drug in lambs. MATERIALS AND METHODS The general procedures were similar to those described previously (loc. cit.). In those trials, the experimental H. contortus strain was exposed in 5 successive groups of lambs to drug levels of 5, 5, 10, 20, and 20 mg/kg. After the 5th drug exposure, feces were collected and cultured from the lambs to obtain infective larvae for the drug exposure trials reported here. However, it was necessary to passage this H. contortus strain twice through nonmedicated lambs until enough parasitefree animals became available from the next lamb crop for continuing the drug exposure trials. The resistant strain was then subjected to additional exposures during successive passages through 5 groups of lambs (Table I). Groups of 3 medicated and 3 nonmedicated control lambs were used for each of the 6th through 10th drug exposure trials. After each drug exposure test, fecal cultures were prepared from the lambs to provide infective larvae for inoculating the next test group. Thus, the experimental H. contortus strain was subjected, in sequence, to the following levels of cambendaReceived for publication 20 November 1973. * Animal Parasitology Institute, ARS, Beltsville, Maryland 20705. zole during successive passages through lambs: 5, 5, 10, 20, 20, 0, 0, 20, 40, 40, 60, and 60 mg/kg (Kates et al., 1973, and Table III). After the 10th drug exposure trial, definitive co trolled anthelmintic tests were conducted concomitantly with the cambendazole-exposed and cambendazole-sensitive parasite strains. In these trials, 3 groups of 8 lambs each were used for each strain; 2 groups were given 40 or 60 mg/kg cambendazole, respectively, and a comparable group was used as nonmedicated controls (Table II). Each lamb was given a standard dose of 5,000 infective larvae (+? |
