A longitudinal study of the role of Dichelobacter nodosus and Fusobacterium necrophorum load in initiation and severity of footrot in sheep

Key unadjusted results were that D. nodosus was detected more frequently on feet with ID, whereas F. necrophorum was detected more frequently on feet with SFR. In the multinomial model, ID was associated with increasing log10 load of D. nodosus the week of observation (OR = 1.28 (95% CI = 1.08-1.53)) and the week prior to development of ID (OR = 1.20 (95% CI = 1.01-1.42). There was no association between log10 load2 of F. necrophorum and presence of ID (OR = 0.99 (95% CI = 0.96-1.02))). SFR was associated with increasing log10 load of D. nodosus the week before disease onset (OR = 1.42 (95% CI = 1.02-1.96)) but not once SFR had occurred. SFR was positively associated with log10 load2 of F. necrophorum once disease was present (OR = 1.06 (95% CI = 1.01-1.11)). In summary, there was an increased risk of increasing D. nodosus load the week prior to development of ID and SFR and during an episode of ID. In contrast, F. necrophorum load was not associated with ID before or during an episode, and was only associated with SFR once present. These results contribute to our understanding of the epidemiology of footrot and highlight that D. nodosus load plays the primary role in disease initiation and progression, with F. necrophorum load playing a secondary role. Further studies in more flocks and climates would be useful to confirm these findings. This study identifies that D. nodosus load is highest during ID. This supports previous epidemiological findings, which demonstrate that controlling ID is the most effective management strategy to prevent new cases of ID and SFR.