Dichelobacter nodosus and Footrot in Swedish Sheep Increased Knowledge and Improved Laboratory Diagnostics

Ovine footrot is a contagious bacterial disease primarily caused by Dichelobacter nodosus. Footrot affects the feet of sheep and is characterised by two major clinical presentations. The milder form consists of inflammation confined to the interdigital space (interdigital dermatitis or benign footrot) and the more severe form includes underrunning of the hoof horn (underrunning or virulent footrot). Disease severity is dependent on several factors including the virulence of the D. nodosus strain, environmental conditions, farming practices, host susceptibility, and co-infecting bacteria. In Sweden, ovine footrot was first diagnosed in 2004, but fast and sensitive diagnostics for D. nodosus were not available. Likewise, knowledge was missing about the D. nodosus strains and co-infection with other lameness-associated bacteria. Hence the overall aim of this thesis project was to increase the knowledge about ovine footrot in Swedish sheep and to improve laboratory diagnostics for it. In this thesis project, sensitive and specific real-time PCR methods to detect and discriminate between virulent and benign strains of D. nodosus were developed and used to characterise D. nodosus from Swedish sheep. The results showed that most of the Swedish D. nodosus are benign and that the virulent type is uncommon. D. nodosus isolates from seven other countries included in the study showed that the D. nodosus genome is highly conserved and that it exists as a globally distributed bimodal population. Furthermore, D. nodosus is mainly associated with the early stages of footrot whereas Fusobacterium necrophorum is associated with the later ones. This confirms the suggested role of F. necrophorum as an opportunistic pathogen rather than the primary pathogen. Although previously proposed, there was no evidence of Treponema spp. in disease development. Finally, a sample pooling method was developed to meet the demands for cost-efficiency in control programs. The method allows samples to be analysed in groups of five with no loss of sensitivity compared to individual samples. It has been implemented in the Swedish Footrot Control Program as a result of this thesis project.