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In the course of the work presented here, the prevalence of thermophilic Campylobacter species found in dogs and cats from two different regions of Germany was investigated. Identification, characterisation and typisation of the Campylobacter strains isolated was performed using phenotyping and genotyping methods. By comparing the animal isolates with Campylobacter isolates from humans who were suffering from campylobacteriosis, it was possible to assess the significance of dogs and cats as a source of infection for humans.From November 1997 to January 1999, samples of faeces from a total of 261 dogs and 46 cats from Berlin and North Rhine-Westplialia (NRW) were examined for thermophilic Campylobacter species. The results of biochemical analysis of cultures produced a clear identification of bacteria of the C. jejuni species; this was confirmed by specific PCR. Bacteria of the species C. upsaliensis and C. helveticus could not be clearly distinguished from one another by their biochemical properties; they could, however, be distinguished from other Campylobacter species. But also in this case, PCR analysis produced clear results, so that nearly all the isolates could be classified as one or the other species.Bacteria of the species C. jejuni, C. upsaliensis and C. helveticus were found in altogether 41.0% of the dogs and 45.7% of he cats investigated. C. jejuni was thereby isolated with almost the same frequency from dogs as from cats. Infections with C. upsaliensis, on the other hand, were found more frequently in dogs, and those with C. helveticus significantly more frequently in cats. The age of the animals appeared to exercise a decisive influence on the infection rates, as was shown in the case of the dogs by predominantly significant differences in the results. At least in the case of the dogs, the prevalence of C.jejuni was also surmised to be influenced by the provenance of the animals. The administration of antibiotics also had an influence on the level of the infection rates. No significant differences, on the other hand, could be identified in relation to the presence of enteritis, the sex of the animals, the manner in which they were kept, how they were fed, or the time of the year at which the samples were taken.Serotyping according to PENNER and HENNESSY (1980) proved a good method for performing an initial grouping of the isolated Campylobacter strains on the basis of phenotypic properties. This was true both for C. jejuni as well as C. upsaliensis and C. helveticus. While a connection between the age of the dogs and cats and the serotype did at least seem possible in the case of C. upsaliensis, the provenance of the animals had no influence on the serotype. Nor could any link be identified between serotype and health status.Further differentiation of the Campylobacter isolates at genetic level was performed by the production of macrorestriction profiles and subsequent cluster analysis. Presentation of the relationships between Campylobacter isolates of one species was done in the form of dendrograms. On the one hand, these showed the genomic heterogeneity within a species. On the other, the grouping of isolates with identical serotype in clusters with a high level of genetic similarity suggested a clonal population structure of bacteria of the Campylobacter species investigated.The question of the extent to which thermophilic Campylobacter species are responsible for the occurence of gastrointestinal illness in dogs and cats could ot be unequivocally clarified on the basis of the results obtained.A comparison of the canine and feline C.jejuni isolates with those of human origin which had been isolated during the same period showed the occurence of identical serotypes and macrorestriction profiles in both animals and humans. It could therefore be demonstrated that genetically identical C. jejuni isolates were responsible for infections both in dogs and cats and also humans.In view of the high prevalence of the human pathogenic species C. jejuni and C. upsaliensis which was found, he large number of dogs and cats living in close social contact with humans constitutes an important reservoir for these pathogens and hence a source of infection for humans which should not be underestimated.