Persistent anthelmintic activity of ivermectin against gastrointestinal nematodes of cattle

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Abstract
Two experiments were conducted to determine persistence of ivermectin (ivm) anthelmintic activity. In the first experiment, the injectable (inj) formulation at a dosage of 200 μg/kg of body weight and pour-on (po) formulation at a dosage of 500 μg/kg were compared in 5 groups of calves (n = 6/group). Calves were treated at 14 and 7 days prior to experimentally induced infection. Experimental groups were: 1-untreated, infected controls; 2-ivm po, day -14; 3-ivm po, day -7; 4-ivm inj, day -14; and 5-ivm inj, day -7. Calves were necropsied on days 28 and 29 after infection. Results of this experiment indicated a high degree of efficacy of ivm po product (93.8%) for all nematodes up to 14 days, in contrast to poor activity for ivm inj (26.3%) for all nematodes. Seven-day persistence was excellent for both ivm formulations against Ostertagia ostertagi and Cooperia spp, but a lower degree of efficacy (77.0% for PO and 88.5% for inj) was observed against Haemonchus placei. In the second experiment, persistent efficacy of ivm po in preventing establishment of O ostertagi inhibited larvae was observed. In 3 trials, groups of 4 treated and 4 untreated control calves were exposed to natural pasture infection at 0 to 7, 10 to 17, and 20 to 27 days after treatment. Calves were necropsied 15 to 16 days after removal from pasture. After the day 0 to day 7 infection exposure, ivm po efficacy was 100% for all stages of O ostertagi, 3 adult male Cooperia spp, and Oesophagostomum radiatum; 94.9% for H placei; and 98.2% for Cooperia spp adult females. Efficacy values remained high through the day 10 to day 17 infection exposure, ranging from 93.6 to 95.6% for O ostertagi stages, 84.9% for H placei, and 100% for Oes radiatum. However, overall values for Cooperia spp decreased to 69.7%. Efficacy values for the day 20 to day 27 exposure period were 84.5% for O ostertagi adults and 80.7% for H placei. Efficacy against O ostertagi inhibited larvae was 66.4%. With the exception of C pectinata adult males, efficacy against Cooperia spp was zero. Two experiments were conducted to determine persistence of ivermectin (ivm) anthelmintic activity. In the first experiment, the injectable (inj) formulation at a dosage of 200 μg/kg of body weight and pour-on (po) formulation at a dosage of 500 μg/kg were compared in 5 groups of calves (n = 6/group). Calves were treated at 14 and 7 days prior to experimentally induced infection. Experimental groups were: 1-untreated, infected controls; 2-ivm po, day -14; 3-ivm po, day -7; 4-ivm inj, day -14; and 5-ivm inj, day -7. Calves were necropsied on days 28 and 29 after infection. Results of this experiment indicated a high degree of efficacy of ivm po product (93.8%) for all nematodes up to 14 days, in contrast to poor activity for ivm inj (26.3%) for all nematodes. Seven-day persistence was excellent for both ivm formulations against Ostertagia ostertagi and Cooperia spp, but a lower degree of efficacy (77.0% for PO and 88.5% for inj) was observed against Haemonchus placei. In the second experiment, persistent efficacy of ivm po in preventing establishment of O ostertagi inhibited larvae was observed. In 3 trials, groups of 4 treated and 4 untreated control calves were exposed to natural pasture infection at 0 to 7, 10 to 17, and 20 to 27 days after treatment. Calves were necropsied 15 to 16 days after removal from pasture. After the day 0 to day 7 infection exposure, ivm po efficacy was 100% for all stages of O ostertagi, 3 adult male Cooperia spp, and Oesophagostomum radiatum; 94.9% for H placei; and 98.2% for Cooperia spp adult females. Efficacy values remained high through the day 10 to day 17 infection exposure, ranging from 93.6 to 95.6% for O ostertagi stages, 84.9% for H placei, and 100% for Oes radiatum. However, overall values for Cooperia spp decreased to 69.7%. Efficacy values for the day 20 to day 27 exposure period were 84.5% for O ostertagi adults and 80.7% for H placei. Efficacy against O ostertagi inhibited larvae was 66.4%. With the exception of C pectinata adult males, efficacy against Cooperia spp was zero.

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