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Footrot is a contagious hoof disease of ruminants. It is endemic in New Zealand and throughout sheep and goat farming regions of the world. The disease results from a mixed bacterial infection, but the essential agent is Dichelobacter nodosus, a Gram-negative, anaerobic bacterium that possesses type-IV fimbriae on its surface. Genetic variation in the fimbriae of D. nodosus was investigated in this study. Using the polymerase chain reaction (PCR), the variable region of the gene encoding the fimbrial subunit (fimA) was amplified from bacterial DNA extracted from footrot lesions. Different fimA amplimers were differentiated by single-strand conformation polymorphism (SSCP) analysis. In conjunction with DNA sequencing, 15 unique sequences of D. nodosus fimA were obtained from 14 footrot samples taken from 6 farming regions throughout New Zealand. When these sequences were compared to fimA of known serogroups, it revealed that there were at least 15 D. nodosus strains, representing 8 serogroups, present on New Zealand farms. The predominant serogroup was B which contained 6 strains, followed by serogroups F, H and G. No strains from serogroups D and I were detected in this investigation. Twelve out of the 15 New Zealand D. nodosus strains had fimbriae different to those previously reported and the presence of multiple strains on a single hoof was common (86% of samples). The fimA sequences from the 12 D. nodosus strains incorporated into the footrot vaccine currently available in New Zealand were determined. A primer set targeting the relatively conserved fimA regions and based on the published sequence of serogroup M Nepalese isolates (designated M-Nep), failed to amplify fimA from the vaccine serotype M strain (designated as M-SPAHL). When the downstream primer was substituted with a primer that was specific for other serogroups of D. nodosus, the fimA gene was successfully amplified. Cloning followed by DNA sequencing, revealed that M-SPAHL fimA was different to M-Nep fimA. The predicted amino acid sequence of M-SPAHL fimA did not show homology to any known serogroups or serotypes. The most similar sequence was from serotype F1, and not M-Nep. The sequence difference between M-SPAHL and M-Nep was larger than that expected within a serogroup. The consequences of serological relatedness and sequence dissimilarity are discussed. Only eight of the 15 New Zealand field strains had fimbriae identical to those of the vaccine strains, while the remaining seven strains possessed different fimbriae. In addition, the vaccine contained two more D. nodosus strains, representing two sera groups, that were not found on the New Zealand farms investigated in this study. This may, to some extent, explain why the current footrot vaccine is at times less efficient in New Zealand. Another 17 footrot samples were screened for new or additional D. nodosus strains. Two PCR amplimers (designated X and Y) derived from footrot samples generated SSCP patterns different to those of previously identified strains. DNA sequencing revealed that these two fragments possessed novel sequences. The upstream of X (nt 1-183) was identical to serotype M1 while its downstream (nt 223-414) was identical to serotype F1; the upstream of Y (nt 1-116) was identical to serotype E1 whereas its downstream (nt 148-423) was identical to serotype F1. A 14-mer sequence consisting of two partially overlapping Chi-like sequences, 5'-GCTGGTGCTGGTGA-3', was also found in these fragments. Two primer sets with the downstream primer specific for serotype Fl and the upstream primer specific for serotype M1 or E1, produced PCR products of the expected sizes from the footrot samples from which fragments X and Y were isolated, respectively. These primer sets did not appear to amplify artificially mixed genomic DNA from serotypes M1 and F1 or E1 and F1. However, when the reactions were re-amplified, PCR recombination artefacts were observed, suggesting that PCR recombination does occur, but at a low frequency. It therefore seems more likely that fragments X and Y reflect genuine fimA sequences of D. nodosus which have resulted from in vivo DNA recombination, than from a PCR recombination artifact. The genetic capability for recombination at the fimbrial subunit locus may therefore endow D. nodosus with the ability to alter its antigenic appearance. D. nodosus strains present in footrot lesions can be genotyped using a PCR-SSCP/sequencing technique. However, this typing technique requires cloning and screening of D. nodosus fimA sequences, which is both laborious and costly. A rapid molecular typing system for D. nodosus was therefore developed in this study. A close examination of available D. nodosus fimA sequences revealed regions that appear to be specific for serogroups and serotypes. These regions were used to design a panel of sequence-specific oligonucleotide probes (SSOPs), and a rapid and accurate D. nodosus typing system using PCR and reverse dot-blot hybridisation (PCR/oligotyping) was subsequently developed. The variable region of D. nodosus fimA, amplified and labelled with digoxigenin (DIG) in a single multiplex PCR amplification, was hybridised to a panel of group- and type-specific, poly-dT tailed oligonucleotides that were immobilised on a nylon membrane strip. A mixture of positive control poly-dT tailed oligonucleotides was also included on the membrane. After hybridisation the membrane was washed to a defined specificity, and DIG-labelled fragments that had hybridised were detected. The specificity of the oligonucleotides was verified by the lack of cross-reactivity with D. nodosus fimA sequences that had a single base difference. DNA from 14 footrot samples previously genotyped by PCR-SSCP/sequencing, was assayed using the PCR/oligotyping technique. All types of D. nodosus which had been detected previously with a PCR-SSCP/sequencing method were detected by this procedure. However, for three of the 14 footrot samples, PCR/oligotyping detected additional types of D. nodosus. Further PCR amplification using type-specific primers, confirmed that these types were present in the original footrot samples. These results indicate that PCR/oligotyping is a specific, accurate, and useful tool for typing footrot samples. In combination with a rapid DNA extraction protocol, D. nodosus present in a footrot sample can be accurately genotyped in less than two days. Individual animals from the same farm, or the same paddock, were often infected by different strains of D. nodosus. This suggests a host role in mediating footrot infection, or that the interaction between the pathogen and the host is important. In order to better understand the interaction between the bacterium and the host, two polymorphic ovine class II MHC genes DQA1 and DQA2, which have been previously shown to be important in footrot infection, were also investigated in this study. PCR-SSCP/sequencing analysis of the DQA1 locus revealed ten unique ovine DQA1 sequences, with five of them being newly identified. This increases the number of known ovine DQA1 alleles from 8 to 13 (including a null allele), implying a high level of polymorphism at the ovine DQA1 locus. D. nodosus present on 20 footrot infected sheep from the same flock were genotyped, together with the ovine DQA1 and DQA2 genotypes of their hosts. Preliminary results showed that sheep with the same DQA1 and DQA2 genotypes tended to be infected by similar types of D. nodosus. Different types of D. nodosus were generally found on sheep with different genotypes at either the DQA1 or the DQA2 locus. This suggests the diversity in D. nodosus infection may be associated with the heterogeneity in the host MHC. However, as only a small number of animals from the same sire were analysed, further investigation is needed to gain a better understanding of the interaction between D. nodosus and the host MHC.