Unravelling the consortium of the cercarial dermatitis in lake from a basin of Argentinian Central Andes: histological and phylogenetic insights of Chilina snail and their fluke partner.

In the context of a research program dealing with the symbiotic associations between aquatic snails and trematodes, we studied a host-parasite consortium in Lake El Nihuil (Mendoza, Argentina), a water body from Atuel river basin (Central Andes) in which recent outbreaks of cercarial dermatitis ('swimmer's itch') occurred. According to the morphological traits and molecular phylogenetic analyses (mitochondrial COI and 16S rRNA genes), the gastropod host was assigned to the endemic genus Chilina. The snails hosted and released a schistosomatid brevifurcate apharingeate cercaria, whose phylogenetic position was inferred using 28S rRNA subunit gene and 18S-ITS1-5.8S rRNA ribosomal region. The parasite was placed in a clade together with sequences from 'Furcocercariae Lineage II', avian schistosomes closely related to the recently named genus Nasusbilharzia, whose final host, the endemic black-necked swan, occurs in Lake El Nihuil. The infected specimens of Chilina sp. showed abundant parasite larvae placed between haemocoelic spaces and connective tissues, modifying the histological architecture of the digestive gland-gonad complex. Parasites produced a decrease in the number of glandular acini and atrophy of the reproductive tissue. This work suggests a displacement of the swimmer's itch outbreaks towards lower latitudes, highlighting the need for the implementation of multidisciplinary studies to monitor these emerging diseases in association with specific symbiotic consortiums.