Host and environmental reservoirs of infection for bovine digital dermatitis treponemes

The effect of slurry on skin permeability to methylene blue dye in dairy cows with and without a history of digital dermatitis

This study aimed to determine whether there was a difference in skin permeability to methylene blue dye or skin morphology between dairy cows that differed in their susceptibility to digital dermatitis (DD) and to assess the effect of contact with slurry on skin permeability. Twenty nine dairy cows were monitored for DD during the winter housing period and classed as DD+ (previous DD infection, n = 17), or DD- (no recorded infection, n = 12). The animals were culled and a skin sample was taken from above the heel of each hind foot and frozen. Samples were later defrosted and one sample from each cow was tested for permeability, whereas the other was treated with slurry for 24 h before permeability testing. To test permeability, methylene blue dye was applied to the skin surface in a Franz diffusion cell. After 48 h, the amount of dye that had passed through the skin was estimated. The stratum corneum thickness and the density of hair follicles were determined from additional heel skin samples. Skin permeability to methylene blue dye was significantly greater for samples that had been treated with slurry but did not differ between DD+ and DD- animals. No difference was found in the stratum corneum thickness or density of hair follicles between DD+ and DD- animals. These findings imply that individual differences in general skin permeability are not a major factor in determining DD susceptibility and suggest that contact with slurry could contribute to DD infection by increasing the permeability of the skin, which may facilitate pathogen entry. Further work is required to clarify the role played by slurry in the pathogenesis of DD.

Targeting the treponemal microbiome of digital dermatitis infections by high-resolution phylogenetic analyses and comparison with fluorescent in situ hybridization

Modern pyrosequencing technology allows for a more comprehensive approach than traditional Sanger sequencing for elucidating the etiology of bovine digital dermatitis. We sought to describe the composition and diversity of treponemes in digital dermatitis lesions by using deep sequencing of the V3 and V4 hypervariable regions of the 16S rRNA gene coupled with species-level taxonomic identification. Treponema-specific 16S rRNA gene PCRs and pyrosequencing were performed on biopsy specimens originating from 10 different Catalan dairy herds (n = 36) with digital dermatitis, and this analysis yielded 75,297 sequences. We identified 20 different taxa, including a potentially novel phylotype that displayed 95% sequence identity to members of the Treponema denticola/Treponema pedis-like cluster. Species frequencies and abundances that were determined by pyrosequencing analysis were highly correlated with the results of fluorescent in situ hybridization using phylotype-specific oligonucleotide probes. In a limited number of animals from a single geographic region, we detected most of the Treponema phylotypes that were described in previous investigations of digital dermatitis. Additionally, we identified a number of phylotypes that mapped to oral treponemes of humans and dogs that had not been reported for digital dermatitis lesions. The results presented here support previous observations of a polytreponemal etiology of infections, with Treponema phagedenis-like, Treponema medium/Treponema vincentii-like, and T. denticola/T. pedis-like phylotypes being highly associated with disease. Using this new approach, it has become feasible to study large herds and their surrounding environments, which might provide a basis for a better understanding of the pathogenesis of this disease.