Bovine digital dermatitis: natural lesion development and experimental induction

Abstract

Bovine Digital Dermatitis (DD) is a leading cause of lameness in dairy cattle in the US with more than 70% of herds affected. Despite 40 years of research, the definitive etiologic agent(s) associated with the disease process is unknown. While clinical lesions have been well described, little is known about the macroscopic, microscopic, and bacterial changes associated with the early stages of lesion development from normal skin to clinical lesions. The goal of this dissertation was to describe the temporal changes associated with lesion development in Holstein dairy cattle, particularly early stage lesions, and develop a model for lesion induction. By following a cohort of Holstein dairy cows for a three year period, several important epidemiologic findings were recognized. In the absence of control measures, DD lesions developed at a rate of 4 lesions per 100 cow feet-months, with the average time for a lesion to develop being 133 days. From the recognition of the macroscopic changes that preceded clinical DD lesions, a novel scoring system was developed. While 20% of the feet observations had clinical DD lesions, an additional 55% of observations had lesions that were indicative of early DD lesion development. Biopsies from these different stages of lesion development were submitted for metagenomic analysis using next generation sequencing. The bacterial microbiota of these biopsies was found to progress through a systematic series of changes that correlate with the macroscopic lesion scoring system with the microbiota of each stage being statistically different from other stages. As one of the major goals for these studies was to gain a better understanding of the etiology of disease, an experimental model was needed to test candidate pathogens. Four preliminary studies were conducted to optimize conditions and methodologies for induction of DD lesions that led to a