Bisset S A, Vlassoff A, West C J, Morrison L
AgResearch, Wallaceville Animal Research Centre, Upper Hutt, New Zealand.
Vet Parasitol. 1997 Jul 1;70(4):255-69. doi: 10.1016/s0304-4017(96)01148-x.
Field trials were undertaken to compare nematode population dynamics, lamb productivity and levels of breech soiling in experimental flocks of Romney lambs selectively bred for increased resistance or susceptibility to nematode infection. In each year of the 2 year study, spring-born ewe lambs derived from Wallaceville Animal Research Centre's divergent nematode-resistant and nematode-susceptible breeding lines were grazed as separate flocks on matched farmlets from weaning (at 3 months old) until they were approximately 10-11 months old. Allocation of farmlets was reversed between Years 1 and 2 of the study to account for any possible paddock-related effects. Within each year both flocks were subjected to identical management conditions, including anthelmintic treatment (which was administered only when the overall mean faecal worm egg count measured across both genotypes reached 1500 eggs g-1). In both years, by mid-autumn (April) nematode larval infestation levels on pasture were approximately 5-6-fold greater on the farmlet grazed by susceptible (S) genotype lambs than on that grazed by their resistant (R) counterparts (Year 1: 2506 cf. 544 larvae kg-1 herbage; Year 2: 431 cf. 74 larvae kg-1 herbage). This led to 51-fold and 56-fold differences in faecal egg count between R and S lambs by late autumn (May) and winter (July) in Years 1 and 2, respectively. Although mean growth rates were similar in the R and S lambs over summer (while pasture infestation levels on the farmlets were still in the process of diverging), significantly higher growth rates occurred in the R than in the S lambs over autumn-winter in both years of the study (P < 0.01). In contrast, no significant differences in growth rate occurred in either year between male lambs derived from the nematode-resistant and nematode-susceptible breeding lines which were grazed together on another area of the Wallaceville farm from weaning until late autumn. Despite the substantially lower pasture infestation levels encountered by the R ewe lambs, they nevertheless temporarily suffered more breech soiling (dags) than their S counterparts (P < 0.01) in both years. Yearling fleece-weights of the R and S genotypes did not differ significantly in either year. Although the results of our study confirmed that there are potentially significant epidemiological benefits to be derived from breeding sheep for resistance to nematode infection, these benefits did not appear to be associated with large advantages in animal performance. Further work is needed to establish how these results should be interpreted with respect to anthelmintic drench requirements of genetically resistant animals.
开展了田间试验，以比较选择性培育出对线虫感染具有增强抗性或易感性的罗姆尼羔羊试验羊群中的线虫种群动态、羔羊生产力和臀部污染水平。在为期两年的研究中的每年，从华莱士维尔动物研究中心的抗线虫和易感线虫的不同品系中选取春季出生的母羊羔，从断奶（3个月大）开始，在匹配的小农场中作为单独的羊群放牧，直到它们大约10 - 11个月大。在研究的第1年和第2年，小农场的分配进行了颠倒，以考虑任何可能的牧场相关影响。在每年内，两个羊群都接受相同的管理条件，包括驱虫处理（仅当两种基因型的总体平均粪便蠕虫卵计数达到1500个卵/克时才进行）。在这两年中，到中秋（4月）时，易感（S）基因型羔羊放牧的小农场的牧场线虫幼虫侵染水平比抗性（R）基因型羔羊放牧的小农场高约5 - 6倍（第1年：2506对544幼虫/千克牧草；第2年：431对74幼虫/千克牧草）。这导致在第1年和第2年的深秋（5月）和冬季（7月），R和S羔羊的粪便卵计数分别相差51倍和56倍。尽管在夏季R和S羔羊的平均生长率相似（此时小农场的牧场侵染水平仍在分化过程中），但在研究的两年中，R羔羊在秋冬季节的生长率均显著高于S羔羊（P < 0.01）。相比之下，从断奶到深秋在华莱士维尔农场的另一个区域一起放牧的抗线虫和易感线虫品系的公羊羔，在任何一年的生长率都没有显著差异。尽管R母羊羔遇到的牧场侵染水平显著较低，但在这两年中，它们暂时比S母羊羔遭受更多的臀部污染（粪便结块）（P < 0.01）。R和S基因型的周岁羊毛重量在任何一年都没有显著差异。尽管我们的研究结果证实，培育抗线虫感染的绵羊可能会带来显著的流行病学益处，但这些益处似乎与动物性能的大幅优势无关。需要进一步开展工作，以确定如何根据抗线虫动物的驱虫需求来解读这些结果。
