Abstract
In order to study the reliability of the larval paralysis test as an in vitro for the detection of resistance of nematodes to levamisole and morantel tartrate, the influence of different parameters was evaluated using resistant and susceptible Ostertagia ostertagi strains. The operator, the sample (10% of the larvae present in the suspension), the incubation time (24, 48 or 72 h), the incubation temperature (20 or 25°C) and the observation period of the larvae (5 or 15 s) had no statistically significant influence on the test results. Statistical differences were obtained only when L 3 larvae of different ages (1 or 2 months) were used. Reversibility of the paralysis did not occur when concentrations of levamisole of ⩽200 μg ml −1 or morantel tartrate of <2000 μg ml −1 were used. The reproducibility of the test was fairly good, with a mean standard deviation of 21.3% for the LC 50 values. Morantel resistance in a strain of O. ostertagi was not confirmed as such by the larval paralysis test. The resistance factor was ⩽1, in spite of an efficacy of morantel of 77.4% as shown in vivo.
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