Abstract
BackgroundGlobal changes promote the spread of infectious diseases worldwide. In this context, tropical urogenital schistosomiasis is now permanently established in Corsica since its first emergence in 2013. The local persistence of the tropical pathogens (schistosomes) responsible for urogenital schistosomiasis at such latitudes might be explained by (i) the presence of its intermediate host, the snail Bulinus truncatus, (ii) the recurrent local reseeding of schistosomes by their vertebrate hosts (either human or animal) every summer, and/or (iii) the maintenance and survival of schistosomes within their snail hosts over winter.MethodsIn this study we conducted an ecological experiment to assess the ability of temperate and tropical schistosome strains to survive in classical winter temperatures in Corsican rivers when infecting temperate (local) snail strains. We also quantified the ability of the schistosomes to complete their life-cycle post-overwintering when returned to classical summer water temperatures.ResultsOur results show that Mediterranean molluscs are locally adapted to winter conditions compared to tropical molluscs. Moreover, temperate and tropical schistosome strains equally survived the cold and produced viable offspring when returned to optimal temperatures. These results indicate that schistosomes can overwinter under temperate climates when infecting locally adapted snails and might partly explain the establishment and maintenance of schistosomes in Corsica from year to year.ConclusionsThe observed broader thermal range of schistosomes compared to that of their snail hosts was unexpected and clearly indicates that the spread and establishment of schistosomiasis in temperate countries relies primarily on the presence of the locally adapted snail host lineages, currently known to be present in France, Italy, Portugal, Spain and Greece.
Highlights
Global changes promote the spread of infectious diseases worldwide
Nuclear (ITS2) and mitochondrial gene sequencing revealed that the Cameroon S.h is a pure S. haematobium strain [18] and Corsica strain S.h-b is a hybrid strain resulting from the interbreeding between S. bovis and S. haematobium most likely originating from Senegal [17]
Our study demonstrates that (i) B. truncatus can survive during a long-term cold period; (ii) B. truncatus originating from temperate regions (i.e. Spain, Corsica) better survive to cold stress than the tropical strain (i.e. Cameroon); (iii) infected snails display a higher survival rate at cold temperature than at the theoretical optimal temperature, and (iv) the schistosomes inside their snail hosts survive cold stress for several weeks and produce viable cercariae once returned to optimal temperatures
Summary
Global changes promote the spread of infectious diseases worldwide In this context, tropical uro‐ genital schistosomiasis is permanently established in Corsica since its first emergence in 2013. In April 2014, cases of urogenital schistosomiasis were simultaneously diagnosed in French and German hospitals with several local people infected [9,10,11]. These European patients had never visited schistosomiasis endemic areas before (i.e. Arabian Peninsula or Africa [12]) and all patients were infected in August 2013 in Corsica, a French Mediterranean Island very popular for its touristic attractiveness [9,10,11]. According to an epidemiological model built on an exhaustive dataset from the summer 2013 outbreak at the country scale, up to 338 (95% confidence interval 166–510) people could have been infected in Corsica, several of whom are probably asymptomatic with low level infections that will not be diagnosed
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