Trichostrongylus colubriformis and Ostertagia sp resistant to levamisole, morantel tartrate and thiabendazole: isolation into pure strain and anthelmintic titration.

Strains of Trichostrongylus colubriformis (designated PF4) and Ostertagia sp (O circumcincta/O trifurcata, designated PF5), with varying degrees of resistance to levamisole, morantel tartrate and thiabendazole were isolated into pure culture. Detailed dose response studies showed that both T colubriformis and Ostertagia sp were highly resistant to levamisole and morantel with low level resistance to thiabendazole. The effective dose required to remove 80 per cent worm burdens (ED80) was calculated for each anthelmintic. For T colubriformis the ED80 for levamisole and thiabendazole were 12.6 and 40.1 mg/kg respectively. For Ostertagia sp, the ED80 for levamisole, thiabendazole and morantel were 20.4, 45.2 and 35.8 mg/kg respectively. The implications of these results are discussed with reference to alternative means of chemical control.