Anderson N
CSIRO, Division of Animal Health, Victoria, Australia.
Vet Parasitol. 1988 Feb;27(1-2):13-21. doi: 10.1016/0304-4017(88)90057-x.
Adaptations such as the capacity of free-living stages to survive environmental stress, inhibition and density-dependent effects on population size and fecundity show O. ostertagi to be a consummate parasite of cattle in temperate environments. Knowledge of these adaptations, within an animal management context, provides the key to understanding the occurrence of disease and a basis for control of parasite numbers. Substantial infections do arise from low egg contamination rates of pastures. Discontinuities in rates of infection are caused by poorly predicted seasonal events such as the effects on eggs of oxygen-deficient environments, fluctuating temperatures and the absence of sufficient moisture for migration of infective larvae from faeces to herbage. Time delays of several months between pasture contamination and availability of infection are therefore common. Ingestion of large numbers of larvae, over a short period early in the grazing season, gives rise to Type I disease or subclinical infections which decrease liveweight gains. In some ecotypes, environmentally induced inhibition leads to the accumulation of large populations of early 4th stage parasites within the host. These populations, under poorly defined conditions, can mature synchronously to produce severe diarrhoea, debility and even death in a proportion of mature cattle--the Type II disease.
诸如自由生活阶段在环境压力下存活的能力、对种群数量和繁殖力的抑制以及密度依赖效应等适应性表明，奥斯特他线虫是温带环境中牛的完美寄生虫。在动物管理背景下了解这些适应性，是理解疾病发生的关键，也是控制寄生虫数量的基础。大量感染确实源于牧场低虫卵污染率。感染率的不连续性是由预测不佳的季节性事件引起的，如缺氧环境对虫卵的影响、温度波动以及缺乏足够的水分以供感染性幼虫从粪便迁移到牧草上。因此，牧场污染和感染可用性之间通常存在数月的时间延迟。在放牧季节早期的短时间内摄入大量幼虫会引发I型疾病或亚临床感染，从而降低体重增加。在某些生态型中，环境诱导的抑制会导致宿主体内大量早期第四阶段寄生虫的积累。在定义不明确的条件下，这些虫群会同步成熟，导致一部分成年牛出现严重腹泻、虚弱甚至死亡——即II型疾病。
