A study of the dynamics of digital dermatitis in 742 lactating dairy cows

Digital dermatitis (DD) is a contagious disease of cattle affecting the skin of the claw. The disease presents with a range of severities and can be associated with lameness. Information about the disease dynamics of DD is scarce. Parity and lactation stage have been identified as risk factors for DD and studies have also indicated that not all cows are equal regarding their risk of recurrent disease and prospects for cure from DD. The aim of this study was to investigate host heterogeneity to DD and to identify disease patterns of DD and factors associated with the development and resolution of lesions. In three commercial dairy herds, 742 lactating cows were observed for DD lesions weekly for 11 or 12 weeks. The effects of parity, lactation stage and duration of preceding episode on the hazard of transitions between healthy and lesion states were analysed using a multilevel multistate discrete-time model. One or more DD lesions were observed in 460 cows and lesions were observed in 2426 out of 10,585 observations. In total, 1755 uncensored episodes with DD lesions were observed. Early lactation was associated with a reduced risk of developing lesions compared with mid and late lactation. Lesions that developed in late lactation had a greater likelihood of resolution compared with lesions that developed during early lactation. There was a reduced risk of lesions developing in parity 3 compared with parity 1 cows, but an increased risk of lesions developing in parity 2 compared with parity 1 cows. In the present study, the mean duration of uncensored DD episodes was 1.7 weeks indicating that the duration of the majority of DD lesions might be shorter than the 42 days reported previously. The transitions between disease states suggest that DD is a dynamic disease, and that the early stage lesions are more transient than expected from previous studies. We conclude that studies with shorter observation intervals than one week are needed to fully understand and describe the individual and group dynamics of DD.