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 micrograms/kg of body weight and pour-on (PO) formulation at a dosage of 500 micrograms/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.(ABSTRACT TRUNCATED AT 250 WORDS)