Abstract
Footrot causes 70–90% of lameness in sheep in Great Britain. With approximately 5% of 18 million adult sheep lame at any one time, it costs the UK sheep industry £24–84 million per year. The Gram-negative anaerobe Dichelobacter nodosus is the causative agent, with disease severity influenced by bacterial load, virulence, and climate. The aim of the current study was to characterize strains of D. nodosus isolated by culture of swabs from healthy and diseased feet of 99 ewes kept as a closed flock over a 10-month period and investigate persistence and transmission of strains within feet, sheep, and the flock. Overall 268 isolates were characterized into strains by serogroup, proline–glycine repeat (pgr) status, and multi-locus variable number tandem repeat analysis (MLVA). The culture collection contained 87 unique MLVA profiles and two major MLVA complexes that persisted over time. A subset of 189 isolates tested for the virulence marker aprV2 were all positive. The two MLVA complexes (76 and 114) comprised 62 and 22 MLVA types and 237 and 28 isolates, respectively. Serogroups B, and I, and pgrB were associated with MLVA complex 76, whereas serogroups D and H were associated with MLVA complex 114. We conclude that within-flock D. nodosus evolution appeared to be driven by clonal diversification. There was no association (P > 0.05) between serogroup, pgr, or MLVA type and disease state of feet. Strains of D. nodosus clustered within sheep and were transmitted between ewes over time. D. nodosus was isolated at more than one time point from 21 feet, including 5 feet where the same strain was isolated on two occasions at an interval of 1–33 weeks. Collectively, our results indicate that D. nodosus strains persisted in the flock, spread between sheep, and possibly persisted on feet over time.
Highlights
In England, approximately 5% sheep are lame at any one time
There were 113 isolates that were duplicates and were removed from the analysis. This resulted in a final dataset of 268 isolates from 157 interdigital skin and 35 footrot lesion swabs; this included one occasion where the same strain was isolated from a footrot lesion and the interdigital skin of the same foot at the same time
A total of 96 strains of D. nodosus were isolated from 192 D. nodosus-positive swabs in a 10-month study of a single flock of 99 ewes
Summary
In England, approximately 5% sheep are lame at any one time. Footrot, caused by Dichelobacter nodosus [1], causes 70–90% of lameness and costs the UK sheep industry £24–84 million per year [2, 3]. There are two distinct clinical presentations of footrot, an inflammation of the interdigital skin [interdigital dermatitis (ID)] and separation of the hoof horn from the underlying tissue (severe footrot). A number of approaches have been used to characterize D. nodosus strains, isolates, and populations. These have included visual assessment by microscopy and descriptions of colony morphology and growth patterns [4,5,6]. Serotyping, virulence determination, analysis of single and multiple loci, and whole-genome sequencing have been used [7,8,9,10,11,12,13,14,15]
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