RESUMEN
Guinea pigs infected with Trichostrongylus colubriformis were used to develop an assay for anthelmintic resistance by determination of worm burdens following treatment with test anthelmintics. To achieve comparable efficacy with the recommended dose of thiabendazole and levamisole in sheep, dose rates in guinea pigs had to be increased two to four fold. For example, thiabendazole at 100 mg/kg in guinea pigs was 96.6 per cent effective against a thiabendazole susceptible (GS) strain of T colubriformis, but had no effect against a thiabendazole-resistant (VRSG) strain. In sheep 50 mg/kg of thiabendazole would have a similar efficacy against each strain respectively. Morantel tartrate at 10 mg/kg in guinea pigs was 99 to 100 per cent effective against the GS strain but only 54 per cent effective against a morantel resistant (PF4) strain. A slope ratio assay was used to calculate the relative potency of anthelmintics by comparing efficacies against resistant strains with efficacy against the GS strain. Resistance of the VRSG strain to thiabendazole was confirmed with a relative potency for this drug of 0.047 in guinea pigs. The PF4 strain was resistant to both thiabendazole and levamisole which had relative potencies of 0.168 and 0.255 respectively. The advantages of this statistical treatment together with the cost and time savings of the guinea pig model over a conventional critical anthelmintic assay in sheep are discussed.