Farm Animal Health, Sydney School of Veterinary Science, Faculty of Science, The University of Sydney, 425 Werombi Road, Camden, New South Wales, 2570, Australia.
Aust Vet J. 2021 Jul;99(7):279-287. doi: 10.1111/avj.13065. Epub 2021 Mar 28.
The primary objective of this study was to evaluate the clinical virulence of aprV2-positive lesser virulent field isolates of footrot bacteria Dichelobacter nodosus in comparison with an aprV2-positive clinically virulent reference strain. Correlations between the clinical expression of the disease and the presence of aprV2 (detected using PCR tests) have been inconsistent. A second objective was to evaluate the elimination of D. nodosus following treatment of sheep as some strains of D. nodosus have been reported to be difficult to eliminate.
The virulence of three aprV2-positive field isolates of D. nodosus which had lesser virulent phenotypes, and an aprV2-positive virulent reference strain was evaluated in a sheep trial using a pasture-based experimental infection model. In the second phase of the study, treatments including footbathing and a long-acting antibiotic were administered and their efficacy in elimination of these strains was evaluated.
Severe underrun (score 4) lesions developed in sheep infected with the aprV2-positive virulent reference strain but not in sheep infected with the field isolates; they had mild lesions (score 2 or 3). The three field isolates and the virulent reference strain of D. nodosus were eliminated by intensive foot bathing and antibiotic therapy in combination with housing the animals in dry conditions post-treatment.
The results suggest that the presence of aprV2 gene in isolates of D. nodosus may not be a reliable indicator of virulence and that further investigation of the factors that determine clinical virulence is required. While the treatment regime was successful, based on a range of considerations, the use of such an intensive treatment involving antibiotics should be limited to small groups of high-value animals, such as rams.
本研究的主要目的是评估 aprV2 阳性、毒力较弱的腐蹄病细菌双球菌（Dichelobacter nodosus）田间分离株的临床毒力，与 aprV2 阳性、临床毒力强的参考株进行比较。疾病的临床表现与 aprV2 的存在（通过 PCR 检测）之间的相关性一直不一致。第二个目的是评估在对绵羊进行治疗后消除 D. nodosus 的情况，因为据报道一些 D. nodosus 菌株难以消除。
在基于牧场的实验性感染模型中，用绵羊试验评估了三种 aprV2 阳性、毒力较弱的田间分离株和 aprV2 阳性、毒力强的参考株的毒力。在研究的第二阶段，对羊进行了包括泡脚和长效抗生素在内的治疗，并评估了这些治疗方法对消除这些菌株的效果。
感染 aprV2 阳性、毒力强的参考株的绵羊会出现严重的跗关节下（评分 4）病变，但感染田间分离株的绵羊不会；它们只有轻度病变（评分 2 或 3）。通过密集泡脚和抗生素治疗，结合对动物进行干燥条件下的治疗后管理，可以消除这三种田间分离株和 D. nodosus 的毒力参考株。
结果表明，D. nodosus 分离株 aprV2 基因的存在可能不是毒力的可靠指标，需要进一步研究决定临床毒力的因素。虽然该治疗方案是成功的，但基于一系列考虑，应将这种涉及抗生素的密集治疗方案仅限于少数高价值动物，如公羊。
