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Abstract

Animal parasitic nematodes cause a wide range of diseases in humans and animals. Most Strongyloides and Onchocerca species are highly host specific, being restricted to one or a few species, but a few of them have a broader host range (eg. S. stercoralis). If a parasitic nematode species can infect multiple hosts, the infection efficiency and dynamics, reproductive success and the gene expression patterns may vary depending on the host. Also within the species, different genotypes that are specialized for a particular host may exist. In this thesis I: 1) compared the infection dynamics and the transcriptomes of S. ratti in its natural host rat and in the sub-optimal host gerbil; 2) analyzed the transcriptomes of different developmental stages of S. papillosus; 3) investigated the population structure of S. stercoralis from humans and dogs in rural Cambodia and 4) compared the genetic variation in the whole mitochondrial genomes of the O. ochengi population in northern Cameroon. To 1) I show that the infection success of S. ratti in gerbils is lower than in rats, but those worms that are successful, survive and reproduce much longer compared with S. ratti in rats. They produce a very high percentage of male progeny. The gene expression patterns are very similar in both hosts. Hence, gerbils are good laboratory hosts for the long-term maintenance of S. ratti. To 2) I report a comparative analysis of the transcriptomes of six developmental stages of S. papillosus. Different members of Astacin and CAP gene families, which are strongly expanded in Strongyloides spp., are specifically up-regulated particularly in parasitic adults and infective larvae suggesting biologic importance of these gene families for the parasitic life cycle of this organism. To 3) Using the nuclear 18S rDNA locus and mitochondrial markers, I demonstrate the existence of multiple genotypes among S. stercoralis isolated from humans and dogs in rural Cambodia. Whilst some make up the vast majority of Strongyloides spp. in dogs and are specific for this host, others are predominant in humans but also occur in dogs. This illustrates the possibility of zoonotic transmission of this parasite. I never found hybrids between the different 18S genotypes. This indicates either that intermixing only occurs within the same 18S genotypes, and as a result S. stercoralis represents a species complex of closely related species, or that these worms reproduce only asexually, as it had been proposed for Strongyloides spp. by some authors (but disproven for S. ratti and S. papillosus). I provide preliminary evidence that at least the laboratory isolate of S. stercoralis is capable of sexual reproduction. To 4) I present the complete mitochondrial genome sequences of eleven different individuals of Onchocerca ochengi, a filarial nematode of cattle. I confirm the existence of two O. ochengi mitochondrial clades at the whole genome level and resolve the mitochondrial phylogeny of the two O. ochengi clades and the very closely related human pathogen O. volvulus.