Within-Flock Population Dynamics of Dichelobacter nodosus

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.

Genomic heterogeneity of Dichelobacter nodosus within and between UK sheep flocks and between age groups within a flock

“…Similar to the findings reported by Smith et al [20], there was no correlation with lesion prevalence or severity (normal, interdigital dermatitis, footrot), additionally, the apparent lack of correlation between the diversity of the D. nodosus population and the purchasing policies/ biosecurity policies of the flocks may indicate that the bacterial population on the foot changes over time, influenced primarily by the farm environment rather than by the establishment of host populations which are stable over sustained periods. However, larger, multiflock, longitudinal studies would be required to robustly address these questions with sufficient statistical power to fully elucidate the transmission and colonisation dynamics at the ewe and flock level over time.…”

Genomic heterogeneity of Dichelobacter nodosus within and between UK sheep flocks and between age groups within a flock

“…Similar to the findings reported by Smith et al (2017), there was no correlation with lesion prevalence or severity (normal, interdigital dermatitis, footrot), additionally, the apparent lack of correlation between the diversity of the D. nodosus population and the purchasing policies/biosecurity policies of the flocks may indicate that the bacterial population on the foot changes over time, influenced primarily by the farm environment rather than by the establishment of host populations which are stable over sustained periods. However, larger, multi-flock, longitudinal studies would be required to robustly address these questions with sufficient statistical power to fully elucidate the transmission and colonisation dynamics at the ewe and flock level over time.…”

Serogroups of Dichelobacter nodosus, the cause of footrot in sheep, are randomly distributed across England

“…The strength of the current study is the number of flocks investigated nationally which provides the robust estimates of the prevalence of serogroups. The percentage of flocks and feet with multiple serogroups and the maximum number of serogroups per flock are higher than reported in previous UK studies [12][13][14]39 this is probably partly due to the increased sensitivity of PCR directly from DNA without culture 15 . Despite this, the number of serogroups detected in flocks is likely to be underestimated.…”

“…There is evidence for this in the current study where most flocks were open, 83.5% of the 164 farmers purchased sheep, often from distant locations 30 and only 50% quarantined sheep for at least 3 weeks 30 , and less than 25% treated sheep with footrot or interdigital dermatitis on arrival 4 . It is also possible that within flock seroconversion of a serogroup occurred, this has been demonstrated www.nature.com/scientificreports/ in the laboratory 43 and possibly in a UK flock 39 . Although the continuing dominance of serogroups H and B indicate national stability, the current study was limited to a single timepoint on each farm and so we could not determine stability of serogroups within flocks over time.…”