Sequential emergence and quantitative dynamics of key bacterial species preceding digital dermatitis lesion onset in dairy cattle

Abstract

Digital dermatitis (DD) is a skin infection of cattle’s feet with multiple bacteria suspected to be involved, yet its precise etiopathogenesis remains unclear. This longitudinal study explored the temporal changes of seven DD-associated bacteria in feet developing lesions or remaining healthy, while simultaneously investigating their persistence in potential reservoirs as sources of infection. Weekly swabs were collected from feet skin and saliva of 53 Holstein cows without DD lesions sequentially enrolled at calving in a commercial dairy herd. At the end of the study, samples from all cases and a subset of matched controls were analyzed (1:2 ratio) at five-time points (weeks −3, −2, −1, 0 - when early signs of DD were observed - and +1) and subjected to qPCR targeting Treponema phagedenis, T. medium, T. pedis, Porphyromonas levii, Bacteroides pyogenes, Fusobacterium necrophorum, and F. mortiferum. Linear mixed-effect models assessed the bacterial number changes within cows (cases) and between cows (cases vs controls). Throughout the study, 8 cows developed signs of DD. P. levii, F. necrophorum, and B. pyogenes numbers increased two weeks before the first visible lesion. T. phagedenis and T. pedis numbers increased one week before, suggesting a sequential colonization and potential synergism in triggering DD. Only P. levii and F. necrophorum were persistently present in saliva and skin, while Treponema spp. persisted solely in lesions. Our results inform specific bacterial dynamics associated with DD pathogenesis and might advise future attempts to effectively treat and control DD.