Interdigital dermatitis

Abstract: A condition of digital dermatitis causing lameness in a dairy herd is described. The lesion was a small circumscribed area of epidermal inflammation in the skin immediately above the coronet between the bulbs of the heel. Topical treatment, consisting of excoriation and application of a gentian violet and tetracycline aerosol spray, was effective. Although the high incidence and rapid onset of the condition suggested an infectious cause, no organism was isolated consistently.

Abstract: Dichelobacter nodosus is the essential causative agent of footrot in sheep. The major D. nodosus-encoded virulence factors that have been implicated in the disease are type IV fimbriae and extracellular proteases. To examine the role of the fimbriae in virulence, allelic exchange was used to insertionally inactivate the fimA gene, which encodes the fimbrial subunit protein, from the virulent type G D. nodosus strain VCS1703A. Detailed analysis of two independently derived fimA mutants revealed that they no longer produced the fimbrial subunit protein or intact fimbriae and did not exhibit twitching motility. In addition, these mutants were no longer capable of undergoing natural transformation and did not secrete wild-type levels of extracellular proteases. These effects were not due to polar effects on the downstream fimB gene because insertionally inactivated fimB mutants were not defective in any of these phenotypic tests. Virulence testing of the mutants in a sheep pen trial conducted under controlled environmental conditions showed that the fimA mutants were avirulent, providing evidence that the fimA gene is an essential D. nodosus virulence gene. These studies represent the first time that molecular genetics has been used to determine the role of virulence genes in this slow growing anaerobic bacterium. Footrot is a highly contagious disease of the feet of sheep and is characterized by the separation of the keratinous hoof from the underlying epidermal tissue, resulting in severe lameness and loss of body condition (11, 39). The consequences of the disease are very significant for the wool and sheep meat industries, and footrot is among the most significant ovine bacterial diseases, causing economic losses in most producer countries. The disease is dependent on a mixed bacterial infection, but the essential causative agent is Dichelobacter nodosus, a slow-growing, anaerobic, gram-negative rod (2, 37). D. nodosus exhibits a spectrum of virulence ranging from virulent strains, which lead to severe underrunning of the horn of the hoof, to benign strains, which cause a self-limiting interdigital dermatitis (37). Little is known about the pathogenesis of ovine footrot, although the polar type IV fimbriae (12) and extracellular proteases (23) of D. nodosus have been traditionally considered virulence factors (2). In addition, the vap and vrl genomic islands have been shown to be preferentially associated with virulent isolates (2). Analysis of the role that these proposed virulence factors play in the disease process has been hampered by the lack of a genetic system in D. nodosus. However, we recently reported the successful transformation of several D. nodosus strains (22). In these experiments, a tetracycline resistance gene, tet(M), which was present on a suicide plasmid, was inserted into the chromosome by double-reciprocal crossover events. These studies have provided the essential tools required to enable the use of reverse genetics to examine the pathogenic role of the putative virulence factors of D. nodosus. The fimbriae of D. nodosus are classified as type IV because of their highly conserved amino-terminal region, polar location, association with twitching motility, and the presence of an N-methylphenylalanine residue as the N-terminal amino acid (41). D. nodosus fimbriae are highly immunogenic, with agglutination reactions involving the fimbrial antigens providing the basis of the classification of D. nodosus into nine major serogroups designated A to I (4). Vaccination of sheep with whole cells of D. nodosus or with purified fimbriae protects against the disease, although this protection is serogroup specific (8, 12, 13). Multivalent recombinant fimbrial vaccines have been prepared by overexpression of each of the nine fimbrial subunit genes in Pseudomonas aeruginosa (9). However, antigenic competition has limited the application of these vaccines (20, 21, 33) so that the most effective strategies are based on univalent and divalent formulations.