Woolaston R R, Baker R L
CSIRO Division of Animal Production, Armidale, NSW Australia.
Int J Parasitol. 1996 Aug-Sep;26(8-9):845-55. doi: 10.1016/s0020-7519(96)80054-3.
Resistance to nematode parasites can be improved by selection, but efforts to include appropriate traits in commercial livestock breeding programs are only a recent development. Procedures for including resistance in breeding programs are similar to those involving other traits. The steps are described with special reference to sheep, and areas are highlighted where particular considerations exist. Three approaches are described and contrasted: breeding for resistance (reduced parasite numbers, as determined by faecal worm egg count); resilience (production during parasitism); or number of treatments required during parasitism. It is necessary, but difficult, to assess the economic benefits of improving resistance relative to other traits. Disease costs vary widely depending on the prevalence of the disease and on the availability, effectiveness and sustainability of alternative control measures. Costs of treatment and control are relatively simple to estimate for a given situation, but production losses are more difficult. Methods of dealing with this problem are discussed. Breeding for disease resistance usually requires that either selection candidates, or their relatives, are exposed to the pathogen so that resistance levels can be compared. Parasitic diseases generally create no special ethical problems in a breeding program unless natural challenge levels are insufficient to enable discrimination between hosts in their susceptibility. In the longer term, it is desirable that selection criteria for all major diseases be developed that will be informative in healthy animals. Molecular genetic markers offer promise, but simple genetic markers have so far been as elusive as physiological traits to predict resistance in undiseased animals. In the longer term, useful genetic markers will be found and techniques for combining these with phenotypic information need to be developed. Commercial breeding programs for sheep which include resistance to gastrointestinal roundworms are now operating in Australia and New Zealand, and issues related to breeding in the tropics are discussed.
通过选择可以提高对线虫寄生虫的抗性，但将适当性状纳入商业家畜育种计划的努力只是最近才发展起来的。将抗性纳入育种计划的程序与涉及其他性状的程序相似。以下步骤将特别参照绵羊进行描述，并突出存在特殊考虑因素的领域。文中描述并对比了三种方法：抗生性育种（通过粪便虫卵计数确定寄生虫数量减少）；恢复力（寄生期间的生产性能）；或寄生期间所需的治疗次数。评估提高抗性相对于其他性状的经济效益是必要的，但也很困难。疾病成本因疾病流行程度以及替代控制措施的可用性、有效性和可持续性而有很大差异。对于给定情况，治疗和控制成本相对容易估算，但生产损失则更难估算。文中讨论了处理这个问题的方法。抗病育种通常要求选择候选个体或其亲属接触病原体，以便比较抗性水平。除非自然感染水平不足以区分宿主的易感性，否则寄生虫病在育种计划中一般不会产生特殊的伦理问题。从长远来看，希望制定所有主要疾病的选择标准，以便在健康动物中提供有用信息。分子遗传标记有前景，但到目前为止，简单的遗传标记和生理性状一样难以用于预测未患病动物的抗性。从长远来看，将会找到有用的遗传标记，并且需要开发将这些标记与表型信息相结合的技术。澳大利亚和新西兰目前正在实施包括对胃肠道线虫抗性的绵羊商业育种计划，并讨论了与热带地区育种相关的问题。
