Abstract:
The parasitic nematode Strongyloides stercoralis is the main causative agent of human strongyloidiasis, a neglected tropical disease. S. stercoralis can undergo three different life cycles. Non-human primates and dogs have been described as possible sources for zoonotic infections of humans. Different isolates of S. stercoralis show considerable genetic diversity and different life cycle preferences. Two types of S. stercoralis have been identified as ‘dog only’ and ‘human and dog shared’. However, the studies on which these finding are based are concentrated in East Asia, Southeast Asia and Australia. Strongyloides fuelleborni can also infect humans. In Asia, S. fuelleborni infections were considered to be rare and restricted to zoonotic cases. Given the biological and medical importance and the difficulties of establishing transgenic lines of these obligatory parasites in hosts, genetic tools such as RNAi developed as a straightforward and effective method is highly desirable. In this thesis, I present two projects, one that compares Strongyloides wild isolates of different hosts from three countries and the other that explores method development for Strongyloides using the free-living model nematode Pristionchus pacificus as a stepping stone.
(1) The aim was to enhance the current understanding of the Strongyloides spp. responsible for human infections, as well as their potential zoonotic threat by expanding the geographic range for which molecular taxonomic information is available. I approached this by collecting and or analysing human, dog and monkey-derived wild isolates of Strongyloides spp. from Iran, Bangladesh and Sri Lanka. Worms collected were subjected to molecular taxonomic and genomic analysis. We found S. stercoralis in humans in Iran and Bangladesh and the results further support the existence of multiple populations with different host specificities within S. stercoralis. While the samples from Bangladesh did not separate from the Southeast Asian samples, in Iran we found indication for different subpopulations that, however, appear to interbreed at least occasionally. For the first time we observed a genomically ‘dog only’ type worm in a human and found first evidence of occasional interbreeding between the two S. stercoralis types, thus exchange of genetic properties, for example drug resistance, between the two types is conceivable. In Sri Lanka, we did not identify any human S. stercoralis cases, suggesting the success of recent STH control measures. Contradicting the literature, we noticed a rather high incidence of human S. fuelleborni infections in Bangladesh suggesting S. fuelleborni may play a more prominent role in human strongyloidiasis in Asia than previously thought. We also found for the first time, S. fuelleborni in dog samples in Sri Lanka, suggesting the possibility of dogs as an additional host for S. fuelleborni. Based on the mitochondrial sequences, S. fuelleborni from Sri Lanka clustered together with the Asian samples and did not mix with the African samples.
(2) The aim was to explore if a novel RNAi method described for C. elegans using poly(UG)- tailed RNA can be used in other nematodes, in particular Pristionchus pacificus and Strongyloides spp. where long dsRNA-mediated RNAi does not work reliably or not at all. I established this method as a tool in P. pacificus. Naturally existing poly(UG)-tailed RNAs throughout the genome were identified in P. pacificus using custom 3'-end RNA sequencing. In S. stercoralis and in S. ratti the detection of poly(UG)-tailed RNAs was not convincing. Although a negative result, we think that this finding indicates that the corresponding biological mechanism is used much less in S. stercoralis and in S. ratti, compared with C. elegans and P. pacificus, if it is not absent altogether.
