Stromberg B E
Department of Veterinary PathoBiology, College of Veterinary Medicine, University of Minnesota, St. Paul 55108, USA.
Vet Parasitol. 1997 Nov;72(3-4):247-56; discussion 257-64. doi: 10.1016/s0304-4017(97)00100-3.
The free-living stages of gastro-intestinal nematodes on pasture (i.e. eggs, developing and infective larvae) are often overlooked and not considered by producers. The factors that influence the development, survival, distribution, or migratory behavior of the free-living larvae seen on pasture are primarily weather related. The deposition of helminth eggs in feces is the starting point for the contamination of the pasture and the subsequent development of the parasites. The cumulative numbers of eggs can be considerable by the end of the grazing season. Environmental factors will influence both development and survival of the larvae on pasture, as well as their distribution onto the herbage. Eggs deposited on the pasture will hatch and develop to the infective L3 stage. Under optimal conditions of moisture and temperature, larvae of Ostertagia ostertagi will reach the infective stage in approximately 5-6 days. Developmental time varies from one geographic region to another depending on prevailing weather. Eggs hatch and develop more slowly at lower temperatures. The rate of development increases to a maximum at higher temperatures after which development will be adversely affected and death of the larvae occurs. It is difficult to evaluate the effects of temperature without considering moisture. Moisture must be present to prevent desiccation and death of the developing larvae. Moisture has an effect on the movement and motility of the larvae. As the fecal pat dries out the larvae may desiccate and die. If the environment is dry, movement onto surrounding herbage would probably not be possible, thus forcing movement or migration into the soil beneath the fecal pat. Too much moisture (rain) may disrupt the pat, moving the larvae onto the surrounding herbage or carrying them away in the runoff. When moisture is adequate the movement of larvae on pasture may be affected by temperature. Temperature in the optimal range allows for larval activity and thus motility. Moisture provides a medium in which the larvae can move (there is probably little movement on dry surfaces). The effect of rainfall on dispersal of the larvae is important, a drop may transport larvae as far as 90 cm from the dung pat. However, optimal larval recovery is only approximately 5 cm (2-3 inch) from the edge of the fecal pat, with decreasing numbers out to approximately 25 cm. Several authors report larval migration into the soil as deep as 15 cm and as far as 40 cm from the center of the fecal pat. Soil type may have a major effect of the ability of larvae to migrate. The predilication of larvae to remain relatively close to the fecal pat may have substantial impact on transmission, as cattle do not graze close to fecal pats until forage is very limited. Therefore, intensive rotational grazing may enhance transmission by forcing animals to graze the highly contaminated areas. At the same time, intensive rotational grazing causes increased disruption of fecal pats, thus enhancing desiccation. Many investigators have utilized tracer calves grazing a pasture for a prescribed period of time to provide an indication of both species and numbers of worms present. Some investigators have used pasture larval counts, reporting number of larvae per kg of dry herbage and the genera present.
胃肠道线虫在牧场的自由生活阶段（即虫卵、发育中和感染性幼虫）常常被生产者忽视且未加以考虑。影响牧场上自由生活幼虫发育、存活、分布或迁移行为的因素主要与天气相关。蠕虫虫卵在粪便中的沉积是牧场污染以及寄生虫后续发育的起点。到放牧季节结束时，虫卵的累积数量可能相当可观。环境因素会影响牧场幼虫的发育和存活，以及它们在牧草上的分布。沉积在牧场上的虫卵会孵化并发育到感染性L3阶段。在湿度和温度的最佳条件下，奥斯特他线虫的幼虫大约在5 - 6天内会达到感染阶段。发育时间因地理区域而异，取决于当地的天气情况。在较低温度下，虫卵孵化和发育得更慢。在较高温度下，发育速度会加快至最大值，之后发育会受到不利影响，幼虫会死亡。如果不考虑湿度，就很难评估温度的影响。必须有湿度来防止发育中的幼虫干燥和死亡。湿度会影响幼虫的移动和活动能力。随着粪堆变干，幼虫可能会干燥死亡。如果环境干燥，幼虫可能无法移动到周围的牧草上，从而被迫移动或迁移到粪堆下方的土壤中。过多的水分（降雨）可能会破坏粪堆，将幼虫移动到周围的牧草上或随径流冲走。当湿度适宜时，牧场幼虫的移动可能会受到温度影响。最佳温度范围内的温度有利于幼虫活动，进而促进其移动。湿度提供了幼虫可以移动的介质（在干燥表面上可能几乎没有移动）。降雨对幼虫扩散的影响很重要，一滴水可能将幼虫从粪堆运输到90厘米远的地方。然而，最佳的幼虫回收距离仅约为粪堆边缘5厘米（2 - 3英寸），向外数量逐渐减少，直至约25厘米处。几位作者报告幼虫会迁移到土壤中，深度可达15厘米，距离粪堆中心可达40厘米。土壤类型可能对幼虫的迁移能力有重大影响。幼虫倾向于相对靠近粪堆，这可能对传播有重大影响，因为直到草料非常有限时牛才会靠近粪堆吃草。因此，密集轮牧可能会通过迫使动物啃食高度污染区域来增强传播。与此同时，密集轮牧会导致粪堆受到更多破坏，从而加速干燥。许多研究人员利用示踪小牛在规定时间内啃食牧场，以表明存在的蠕虫种类和数量。一些研究人员使用牧场幼虫计数，报告每千克干牧草中的幼虫数量以及存在的属。
