Oakley G A
Res Vet Sci. 1982 Mar;32(2):163-9.
A five year ley pasture was used as a source of natural infection with Dictyocaulus viviparus for cattle in anthelmintic trials. Pasture larval counts, faecal larval counts of permanently grazing calves and lungworm burdens harboured by tracer calves were monitored in three grazing seasons to assess the pattern of infection. Carrier calves were introduced at the beginning of the grazing season in the first two years of the study but not in the third. In the fourth year the pasture was subdivided into two paddocks where overwintered infection with and without carrier infection were compared. A control paddock exposed to carrier infection but no overwintered infection was also monitored. Pasture larvae survived the winter but carrier infection appeared to make a larger contribution to pasture larval counts and the onset of parasitic bronchitis in susceptible calves. In the absence of grazing cattle at the end of the grazing season the concentration of D viviparus larvae on the herbage fell rapidly to undetectable levels. Discrepancies between contamination of herbage by infective D viviparus larvae and infectivity of pasture for susceptible cattle occurred in all years but were particularly marked on the third year when natural immunity appeared to influence the number of lungworms accumulating in tracer calves. Failure to recover lung worms from tracer calves cannot be regarded as an accurate indication of lungworm free pasture. In the first three years the proportion of the lungworm population which was inhibited in tracer calves was higher early and late in the grazing season and negligible in mid season. This suggests that a predisposition to inhibition in larvae which have overwintered on pasture may influence the time of onset of parasitic bronchitis in the next grazing season, but results from the fourth year did not support this hypothesis.
在驱虫试验中，一片五年生的多年生黑麦草草场被用作牛感染胎生网尾线虫的自然感染源。在三个放牧季节中，监测了草场幼虫数量、永久放牧犊牛的粪便幼虫数量以及追踪犊牛体内的肺线虫负荷，以评估感染模式。在研究的前两年放牧季节开始时引入了带虫犊牛，但第三年没有。第四年，草场被分成两个围场，比较了有和没有带虫感染的越冬感染情况。还监测了一个暴露于带虫感染但没有越冬感染的对照围场。草场幼虫在冬季存活下来，但带虫感染似乎对草场幼虫数量和易感犊牛寄生性支气管炎的发病起到了更大的作用。在放牧季节结束时，如果没有放牧的牛，牧草上胎生网尾线虫幼虫的浓度会迅速下降到检测不到的水平。在所有年份中，感染性胎生网尾线虫幼虫对牧草的污染与草场对易感牛的感染性之间都存在差异，但在第三年尤为明显，当时自然免疫力似乎影响了追踪犊牛体内积累的肺线虫数量。未能从追踪犊牛体内回收肺线虫不能被视为草场无肺线虫的准确指标。在前三年中，追踪犊牛体内受抑制的肺线虫种群比例在放牧季节的早期和晚期较高，而在中期可以忽略不计。这表明，在草场上越冬的幼虫受抑制的倾向可能会影响下一个放牧季节寄生性支气管炎的发病时间，但第四年的结果并不支持这一假设。
