Seasonal variation in anthelmintic response by cattle to dermally applied levamisole.

Four experiments in 1978-79, and 2 in 1982 designed to define the optimum dose rate of dermally applied formulations of levamisole are described. These experiments showed that the absorption of levamisole with resultant blood levels and anthelmintic activity is strongly influenced by temperature. In warm to hot conditions percutaneous absorption is rapid and high blood levels with high anthelmintic activity against Haemonchus placei, Ostertagi sp, Trichostrongylus sp, Cooperia sp, Oesophagostomum radiatum, O. venulosum and Dictyocaulus viviparus result from dose rates of 10 mg/kg or more. In cold weather the high efficacy against H. placei, Cooperia sp, and Oesophagostomum sp is unchanged, but efficacy against Ostertagia sp, T. axei and D. viviparus decreases to the extent that a mean dose rate of 20 mg/kg (range 15 to 25 mg/kg) is necessary if the anthelmintic activity of dermally applied levamisole is to match that of either parenterally or orally administered material. These anthelmintic data are supported by the levamisole blood profile which in winter months, peaks at a figure of one quarter or less of that obtained from a similar dose rate in warm conditions. The implications of this variability in action are discussed.