Infection reservoirs and transmission of digital dermatitis in the dairy herd

Digital dermatitis (DD) is a multifactorial disease, both in terms of the pathogens that can inhabit lesions, and in terms of the environmental and management conditions which are presumed to cause damage to the stratum corneum and changes to the foot-skin microbiome, thus allowing pathogens to cause lesion development. Gaps in our understanding of the complex pathophysiology of DD alongside practical problems with implementing control measures are hindering our efforts to eliminate disease.

A wide variety of bacteria have been detected from DD lesions, including Fusobacterium spp., Bacteroides spp., Guggenheimella bovis, Campylobacter spp. and Peptococcus spp; and more recently Porphyromonas levii, Mycoplasma spp. and Prevotella spp. (Berry et al, 2010). However, it is currently understood that there are specific Treponema phylogroups that are important in causing DD and they are found in combination (Evans et al, 2016). More detailed genotypic and phenotypic characterisation carried out on UK isolates demonstrated three distinct taxonomic groups, which were designated as Treponema medium/vincentii-like, Treponema phagedenis-like, and Treponema putidum/denticola-like (Evans et al, 2008). More recently the latter phylogroup has been re-classified as Treponema pedis (Evans et al, 2009); further taxonomic scrutiny of the other two groups is still needed to distinguish them from the human treponemes they were found to be similar to. Only treponemes have been found deep in the epidermis (Figure 1), with tissue invasion considered a major virulence trait (Blowey et al, 1994; Dopfer et al, 1997; Moter et al, 1998). The importance of treponemes in DD is supported by successful development of infection models (Read and Walker, 1996; Gomez et al, 2012; Krull et al, 2016), however skin needs to be macerated to allow infection to become established; and the use of tissue homogenate prepared from fresh lesion material is more effective than inoculation with pure Treponema spp. cultures.