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INTRODUCTION
Parasitic infections are non-randomly distributed in space (Ostfeld et al. 2005). For instance, physical habitat structure can influence the distribution and abundance of organisms, and thus affect interspecific interactions such as host-parasite associations (Sousa and Grosholz, 1990). Several illustrations can be found in the literature. Disease was influenced by altitude in amphibian-fungal disease (Pounds et al. 2006), patch size and connectivity in tick-borne disease (Allan et al. 2003), deforestation in mosquito-borne malaria (Molyneaux, 2002) and basin-shape in Daphnia-fungal disease (Hall et al. 2010). Lotic systems (rivers) are very particular ecosystems in that they have a dendritic shape and continuous unidirectional water flow. In any river, the physical properties change drastically from its source to its mouth in ways that could affect infection patterns. Several theories exist in river ecology to explain such changes. The River Continuum Concept (RCC) developed by Vannote et al. (1980) predicts the way in which biotic changes occur longitudinally instream in accordance to downstream changes in hydrologic and geomorphic properties. On the other hand, discrepancies have been explained by considering alternative factors such as heterogeneity of habitats, stochastic disturbance and hierarchical scaling (e.g., Perry and Schaeffer, 1987; Townsend, 1989; Benda and Dunne, 1997; Gomi et al. 2002). Only a handful of studies have addressed spatial patterns of parasite distributions in rivers (e.g. Kennedy, 1990; Barker et al. 1996; Weichman and Janovy Jr, 2000; Barger and Esch, 2001; Kennedy, 2001; Barger, 2006; Loot et al. 2007), but mostly focused on the similarities and heterogeneities in richness and abundance at the component community and infracommunity levels. In addition to those, a few have focused on the environmental determinants of myxozoan infections such as sediment type, temperature and flow rate in salmonid whirling disease (e.g. Krueger et al. 2006; Hallett and Bartholomew, 2008). So far, very little is known about disease distribution in rivers and the processes and environmental factors structuring infection foci in these habitats. Such information will undoubtedly be valuable for disease risk assessment in freshwater ecosystems.
In a river, a common process originating from the unidirectional river flow, stream drift, causes the displacement and downstream dispersion of typically benthic invertebrates. This process could promote a gradient in infection levels, leading to decreasing numbers...