Rickard L G, Zimmerman G L, Hoberg E P, Wallace D H
College of Veterinary Medicine, Oregon State University, Corvallis 97331.
Vet Parasitol. 1991 Aug;39(3-4):267-77. doi: 10.1016/0304-4017(91)90043-u.
The effect of ivermectin delivered from a sustained-release bolus (I-SRB) on the weight gain of beef cattle through a grazing season was evaluated using 20 yearling beef steer calves randomly divided into two groups of ten animals each. Calves in the control group each received a placebo bolus, while those in the treatment group each received an I-SRB designed to release 12 mg ivermectin day-1 for approximately 90 days. All animals were weighted and samples of feces were collected from the rectum at monthly intervals, beginning on Day 0, until trial termination (Day 148). Pasture larval counts were also conducted on herbage collected on each sampling date. On Day 119, two control and two treated calves were removed from pasture, housed in isolation for 3 weeks, then necropsied for recovery of gastrointestinal nematodes. Three sets of parasite-naive tracer calves were utilized to evaluate the initial, interim and final levels of pasture contamination by nematode larvae. A fourth set was used to evaluate the level of pasture contamination the following spring. The use of the I-SRB resulted in a greater than 99% reduction in fecal egg counts of trichostrongyles and numbers of gastrointestinal nematodes in the treated principals, as well as an average daily gain advantage of 0.114 kg over the 148 day period. A 67-98% reduction in pasture larval nematode contamination occurred on pastures grazed by the treated animals, as indicated by the parasite burden in tracer calves and pasture larval counts. The treatment effect was eventually lost by the following spring since tracer calves on the treated pasture had only 33% fewer nematodes than those on the control pasture.
通过放牧季节，使用20头一岁的肉用公牛犊进行评估，这些牛犊随机分为两组，每组10头，以研究缓释丸剂（I-SRB）形式的伊维菌素对肉牛体重增加的影响。对照组的牛犊每头接受一个安慰剂丸剂，而治疗组的牛犊每头接受一个设计为每天释放12毫克伊维菌素、持续约90天的I-SRB。从第0天开始，每月对所有动物进行称重，并从直肠采集粪便样本，直至试验结束（第148天）。在每个采样日期采集的牧草上也进行牧场幼虫计数。在第119天，从牧场中取出2头对照牛犊和2头治疗牛犊，单独饲养3周，然后进行剖检以回收胃肠线虫。使用三组未感染寄生虫的示踪牛犊来评估线虫幼虫对牧场的初始、中期和最终污染水平。第四组用于评估次年春季牧场的污染水平。使用I-SRB使治疗组的主要牛只的毛圆线虫粪便虫卵计数和胃肠线虫数量减少了99%以上，并且在148天期间平均日增重优势为0.114千克。从示踪牛犊的寄生虫负荷和牧场幼虫计数可以看出，在治疗组动物放牧的牧场上，牧场幼虫线虫污染减少了67%-98%。到次年春季，治疗效果最终消失，因为在治疗过的牧场上的示踪牛犊比对照牧场上的示踪牛犊的线虫仅少33%。
