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.