Exploring a novel method for inoculation to experimentally induce digital dermatitis in dairy calves

Digital dermatitis (DD) is a polymicrobial, multifactorial hoof disorder with an undefined etiology, and existing animal models fail to induce a fast, standardized infection in bovine skin. This study explored bacterial inoculation methods for bacterial skin penetration to optimize a DD infection model. A pilot study tested various microneedles in calf feet, identifying tattoo needles as the most effective for skin penetration based on the ink depth and amount. Two animal trials followed. In trial 1, eight calves were inoculated once with DD lesion macerates (n = 5, 10 feet) or a sham-inoculum (n = 3, 6 feet). Light wraps were applied, and calves were monitored for 14 days, but DD lesions failed to develop. In trial 2, five calves underwent a 3-day pretreatment with foot wrapping and daily water application before being inoculated with a mixture of pure isolates (n = 2, 8 feet) or lesion macerates (n = 3, 12 feet). Monitoring continued for three weeks, with swabbing every three days. On day 18, skin biopsies were taken. Once again, DD lesions failed to develop. DD-associated Treponema spp. were detected in swabs from days 2 to 9 in both groups, while non-Treponema anaerobes persisted in the pure isolates group until day 18. In biopsies, only DD-Treponema spp. were detected (12/20), and the presence of spirochetes was confirmed by silver staining. Although tattoo-based inoculation successfully delivered inoculum, bacteria failed to replicate and induce lesions, likely due to unknown missing factors necessary for DD pathogenesis in this model.