Experimental transmission of bovine digital dermatitis to sheep: development of an infection model.

Authors: Wilson-Welder, Jennifer; Nally, Jarlath; Alt, David; Palmer, Mitchell; COTANEY, JOHN - Iowa State University; PLUMMER, PAUL - Iowa State University

Submitted to: Veterinary Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2017
Publication Date: 11/16/2017
Citation: Wilson-Welder, J.H., Nally, J.E., Alt, D.P., Palmer, M.V., Cotaney, J., Plummer, P. 2017. Experimental transmission of bovine digital dermatitis to sheep: development of an infection model. Veterinary Pathology. 55(2):245-257. https://doi.org/10.1177/0300985817736572.
DOI: https://doi.org/10.1177/0300985817736572

Interpretive Summary: Digital Dermatitis is a leading cause of lameness in cattle and has been identified by both dairy and beef cattle producers as a disease of economic importance. The exact cause is unknown, but numerous studies have implicated multiple spirochete bacteria from the genus Treponema and other anaerobic bacteria as agents for causing the lesion. In order to better understand the role of these bacteria in lesion pathogenesis, a reproducible, manageable animal model needs to be developed. Sheep have a similar immune system to cattle, are naturally susceptible to similar digital dermatitis disease, and can be easily handled for frequent hoof inspections. We used material from a bovine digital dermatitis lesion to inoculate naive sheep. These experimentally inoculated sheep developed lesions in 4 weeks that were similar to bovine disease. Visual and molecular tests confirmed treponemes were present in the sheep lesions. We were also able to infect a second group of naive sheep from the first group of experimentally infected sheep, thus perpetuating the lesions. Development of a reproducible model in sheep, will allow for investigations into the host-pathogen interactions of the treponemes, their role in pathogenesis of digital dermatitis and will also allow work to develop protective vaccines.

Technical Abstract: Digital dermatitis is an infectious cause of lameness primarily affecting cattle, but also described in sheep, goats, and North American wild elk. Digital dermatitis is a polybacterial infection, comprised predominantly by several Treponema species, and other anaerobic bacteria. While the exact etiology has not been demonstrated, a number of bacterial, host, and environmental factors are thought to contribute to disease development. In order to study host-bacterial interactions, a reproducible laboratory model of infection is required. Crossbred sheep were obtained from a flock free of hoof disease. Skin on the caudal surface of the pastern between the heel bulb and dewclaw was compromised to create a small abrasion before wrapping to emulate the moist, anaerobic environment. After 3 days, abraded and macerated areas were inoculated with lesion material from active bovine digital dermatitis, and remained wrapped. Experimentally inoculated feet developed lesions by two weeks post-inoculation. Swabs of lesions showed the presence of spirochetes. At four weeks post-inoculation, histologic changes in the dermis and epidermis were consistent with those described for bovine digital dermatitis, including erosion of the keratin layer, ballooning keratinocytes and the presence of neutrophilic infiltrate. Silver staining of lesion biopsies confirmed that spirochetes had penetrated host tissue. The model was then perpetuated by passaging lesion material from experimentally infected sheep into naive sheep. The continued development and refinement of this model of bovine digital dermatitis will allow for novel insights into pathogenic mechanisms of infection, as well as the development of improved diagnostics and therapeutics for affected ruminants.