Development of simultaneous resistance in Ostertagia circumcincta to thiabendazole, morantel tartrate and levamisole

Abstract

Le Jambre L.F., Southcott W.H. and Dash K.M. 1978. Development of simultaneous resistance in Ostertagia circumcincta to thíabendazole, morentel tartrate and levamisole. International Journal for Parasitology 8: 443–447. A field strain of Ostertagia circumcincta was divided into five strains based on anthelmintic selection in the laboratory. The first strain was selected with 50 mg/kg thiabendazole, the second with 4 mg/kg morantel tartrate, the third with 3.2 mg/kg levamisole, the fourth was not selected and the fifth strain was selected with all three anthelmintics in each generation. The present paper reports the dose response of the eighth generation of the multi-selected strain to thiabendazole, morantel tartrate and levamisole and compares these results with those from the eighth generation of the single selected and unselected strains. In adult O. circumcincta the LD 95 for thiabendazole, morantel tartrate and levamisole was 172.0, 9.2 and 8.4 mg/kg respectively in the multiselected strain, compared with corresponding values of > 200, 6.1 and 6.9 for the single selected strains and 14.5, 2.8 and < 1.6 in the unselected parent strain. Multiple selection with three anthelmintics was associated with an increase in O. trifurcata from less than 0.1 % in the unselected strain to 16% in the multi-selected strain. An increase in inhibition was a feature of both multiple selection and selection by levamisole alone. The parent strain had less than 0.1 % inhibition but the incidence increased to 16% in the levamisole selected strain and to 2% in the multi-selected strain. Approximately 8% of the inhibited larvae in the levamisole selected strain were resistant to dose levels of levamisole from 1.6 to 8.0 mg/kg. Resistance in inhibited larvae was further enhanced in the multi-selected strain and after eight generations 100% of larvae in this strain were resistant to all dose levels up to 100 mg/kg thia-bendazole, 20 mg/kg morantel tartrate and 8 mg/kg levamisole. Apparently when selected with anthelmintics that are less effective against larvae than adults, O. circumcincta responds by increasing the percentage and resistance of inhibited larvae.