Oxytetracycline reduces inflammation and treponeme burden whereas vitamin D₃ promotes β-defensin expression in bovine infectious digital dermatitis

Digital dermatitis (DD), a common ulcerative disease of the bovine foot causing lameness and reducing productivity and animal welfare, is associated with infection by spirochete Treponema bacteria. Topical tetracycline, the most common treatment, has inconsistent cure rates; therefore, new therapeutic options are needed. We compared effects of topical oxytetracycline and vitamin D₃ on innate immunity in DD-affected skin. Cows with active DD lesions were treated topically with oxytetracycline or vitamin D₃ and skin biopsies were collected from lesions. Tissue samples were examined histologically, transcriptional expression of pro-inflammatory cytokines, Toll-like receptors (TLRs), and host defense peptides assessed, and the presence of specific treponeme species determined. Effects of treatments at a mechanistic level were studied in a human keratinocyte model of treponeme infection. Oxytetracycline promoted hyperplastic scab formation in ulcerated DD lesions and decreased transcriptional expression of Cxcl-8 (neutrophil chemoattractant). Oxytetracycline also reduced numbers of Treponema phagedenis and T. pedis and enhanced Tlr2 mRNA expression. Vitamin D₃ did not modify expression of cytokines or Tlrs, or bacterial loads, but enhanced transcription of tracheal antimicrobial peptide (Tap), a key bovine β-defensin. Combing oxytetracycline and vitamin D₃ provides complementary clinical benefits in controlling DD through a combination of antimicrobial, immunomodulatory, and pro-healing activities.