The severity of footrot lesions induced by aprV2‐positive strains of Dichelobacter nodosus varies between strains

Abstract

Objectives 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. Methods 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. Results 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. Conclusion 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.

Prevalence of Dichelobacter nodosus and Ovine Footrot in German Sheep Flocks

“…This can be explained, because footrot is a multifactorial disease. The virulence of D. nodosus has an impact on the capacity of causing severe lesions, while recent studies suggest that the presence of the aprV2 gene in D. nodosus isolates may not be the only characteristic determining the clinical outcome of lesions [23]. In addition, environmental conditions and the inherent susceptibility of the sheep play a role in the expression of clinical footrot [7].…”

The virulence of Dichelobacter nodosus, measured by the elastase test, is an important predictor for virulent footrot diagnosis in New South Wales sheep flocks

“…Although benign footrot normally presents as interdigital dermatitis, there is the potential for a small proportion of animals to show underrun lesions. 19,20 In conclusion, virulent footrot diagnosis was best predicted by combining the incidence of severe foot lesions, the virulence of D nodosus, measured by the elastase test and environmental conditions. It is important to realise that similar modelling of footrot investigations performed under different conditions (different sheep breeds, ages and environment) might highlight other factors important in the diagnosis of virulent footrot, so an extension of this monitoring will better establish the relationship between bacterial virulence and virulent footrot diagnosis across more variable factors.…”

“…[4][5][6][7] Many attempts have been made to develop a laboratory test that supports virulent footrot diagnosis by measuring the expression of D. nodosus virulence genes such as the acidic protease gene that encodes a thermostable protease involved in hoof tissue degradation. 5,8,11,12,19,20 Activity of the acidic protease gene can be measured by both the gelatin gel test and the elastase test and the presence of the virulent genotype of the aprV2gene, detected with a quantitative PCR assay. 10 In an in vivo pen study, both the elastase and gelatin gel tests correlated with clinical disease under controlled environmental conditions, but the elastase test was more specific, and the gelatin gel test was more sensitive.…”

“…Many attempts have been made to develop a laboratory test that supports virulent footrot diagnosis by measuring the expression of D. nodosus virulence genes such as the acidic protease gene that encodes a thermostable protease involved in hoof tissue degradation 5,8,11,12,19,20 . Activity of the acidic protease gene can be measured by both the gelatin gel test and the elastase test and the presence of the virulent genotype of the apr V2gene, detected with a quantitative PCR assay 10 .…”

“…Some flocks with intermediate forms of footrot may also have been diagnosed as benign if the veterinarian believed that they were unlikely to respond to footrot eradication programs. Although benign footrot normally presents as interdigital dermatitis, there is the potential for a small proportion of animals to show underrun lesions 19,20 …”