PERSISTENT ANTHELMINTIC ACTIVITY OF IVERMECTIN AGAINST GASTROINTESTINAL NEMATODES OF CATTLE

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 mu g/kg of body weight and pour-on (PO) formulation at a dosage of 500 mu g/kg were compared in 5 groups of calves (n = 6/group). Calves were treated at 14 and 7 days prior t o experimentally induced infection. Experimental groups were: 1-untrea ted, 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 degre e of efficacy of IVM PO product (93.8%) for all nematodes up to 14 day s, 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 plac ei. In the second experiment, persistent efficacy of IVM PO in prevent ing establishment of O ostertagi in hibited larvae was observed. In 3 trials, groups of 4 treated and 4 untreated control calves were expose d 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 fr om pasture. After the day 0 to day 7 infection exposure, NM PO efficac y was 100% for all stages of O ostertagi, 3 adult male Cooperia spp, a nd Oesophagostomum radiatum; 94.9% for H placei; and 98.2% for Cooperi a 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 osterta gi stages, 84.9% for H placei, and 100% for Oes radiatum. However, ove rall values for Cooperia spp decreased to 69.7%. Efficacy values for t he 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 aga inst Cooperia spp was zero.