A pasture-based experimental infection model for footrot in sheep

Footrot is a contagious disease of sheep and other small ruminants. The causative agent, Dichelobacter nodosus, is transmitted between hooves via contaminated pasture and soil. However, the bacterium cannot invade the epidermis unless it is compromised in some manner and water maceration, which follows prolonged exposure to a moist substrate, is the most common predisposing factor. Experimental infection models have been used extensively to study the aetiology, pathogenesis, treatment and prevention of footrot. In Australia, an indoor, pen-based, infection model involving standing sheep on wet mats and bandaging inoculum onto the foot has been used commonly but does not accurately represent the pasture environment and has an impact on animal welfare. Instead, we predisposed sheep on irrigated pasture and compared three methods for infecting them with D. nodosus: application of inoculum to the interdigital skin with a swab, dripping inoculum onto the interdigital skin with a pipette, and spraying inoculum onto the pasture on which the sheep grazed. We were able to establish D. nodosus infections using all three methods but selected the swab method for further use because the dose can be controlled. It was used in two field trials and infection was established in 70 %–100 % of sheep. The infection model developed in this study improves animal welfare by reducing the level of intervention. However, as the model is pasture-based, seasonal weather patterns need to be considered to ensure predisposition and disease expression. Further research will be required to understand the environmental conditions required for transmission of D. nodosus and the clinical expression of disease.