The wide geographic distribution of Schistosoma mansoni, a digenetic trematode and parasite of humans, is determined by the occurrence of its intermediate hosts, freshwater snails of the genus Biomphalaria (Preston 1910). We present phylogenetic analyses of 23 species of Biomphalaria, 16 Neotropical and seven African, including the most important schistosome hosts, using partial mitochondrial ribosomal 16S and complete nuclear ribosomal ITS1 and ITS2 nucleotide sequences. A dramatically better resolution was obtained by combining the data sets as opposed to analyzing each separately, indicating that there is additive congruent signal in each data set. Neotropical species are basal, and all African species are derived, suggesting an American origin for the genus. We confirm that a proto-Biomphalaria glabrata gave rise to all African species through a trans-Atlantic colonization of Africa. In addition, genetic distances among African species are smaller compared with those among Neotropical species, indicating a more recent origin. There are two species-rich clades, one African with B. glabrata as its base, and the other Neotropical. Within the African clade, a wide-ranging tropical savannah species, B. pfeifferi, and a Nilotic species complex, have both colonized Rift Valley lakes and produced endemic lacustrine forms. Within the Neotropical clade, two newly acquired natural hosts for S. mansoni (B. straminea and B. tenagophila) are not the closest relatives of each other, suggesting two separate acquisition events. Basal to these two species-rich clades are several Neotropical lineages with large genetic distances between them, indicating multiple lineages within the genus. Interesting patterns occur regarding schistosome susceptibility: (1) the most susceptible hosts belong to a single clade, comprising B. glabrata and the African species, (2) several susceptible Neotropical species are sister groups to apparently refractory species, and (3) some basal lineages are susceptible. These patterns suggest the existence of both inherent susceptibility and resistance, but also underscore the ability of S. mansoni to adapt to and acquire previously unsusceptible species as hosts. Biomphalaria schrammi appears to be distantly related to other Biomphalaria as well as to Helisoma, and may represent a separate or intermediate lineage.

Isolates of Cuban Pseudosuccinea columella susceptible and non-susceptible to Fasciola hepatica miracidial infection were compared for cellular reaction to miracidial development, shell morphometrics, mantle pigmentation pattern, and egg laying behaviour. At 24 h post-exposure, non-susceptible snails showed an early cellular host reaction to encapsulate and phagocytise the miracidium, whereas viable transforming miracidia were observed in exposed susceptible snails, indicating that non-susceptibility resulted from resistance rather than physiological unsuitability. Susceptible isolates (more than 80% susceptible to infection) showed significantly higher values of the morphometric indices shell width/ length and aperture width/length than resistant isolates (100% resistant). Mantle pigmentation pattern was analysed using Visilog software ; the cumulative distances between mantle spots were plotted and compared between isolates, showing a clear distinction between groups. Egg-laying behaviour also showed significant differences with susceptible snails preferring to lay their eggs on the container's walls whereas resistant snails preferred to lay their eggs on the bottom of the container.

Population genetic studies are efficient for inferring the invasion history based on a comparison of native and invasive populations, especially when conducted at species scale. An expected outcome in invasive populations is variability loss, and this is especially true in self-fertilizing species. We here focus on the self-fertilizing Pseudosuccinea columella, an invasive hermaphroditic freshwater snail that has greatly expanded its geographic distribution and that acts as intermediate host of Fasciola hepatica, the causative agent of human and veterinary fasciolosis. We evaluated the distribution of genetic diversity at the largest geographic scale analysed to date in this species by surveying 80 populations collected during 16 years from 14 countries, using eight nuclear microsatellites and two mitochondrial genes. As expected, populations from North America, the putative origin area, were strongly structured by selfing and history and harboured much more genetic variability than invasive populations. We found high selfing rates (when it was possible to infer it), none-to-low genetic variability and strong population structure in most invasive populations. Strikingly, we found a unique genotype/haplotype in populations from eight invaded regions sampled all over the world. Moreover, snail populations resistant to infection by the parasite are genetically distinct from susceptible populations. Our results are compatible with repeated introductions in South America and flash worldwide invasion by this unique genotype/haplotype. Our study illustrates the population genetic consequences of biological invasion in a highly selfing species at very large geographic scale. We discuss how such a large-scale flash invasion may affect the spread of fasciolosis.

An uncommon snail-trematode interaction pattern was found for the Pseudosuccinea columella - Fasciola hepaticamodel under laboratory conditions. Week-old juveniles from two isolates showed very high rates of infection (90.0% and 93.3%) after exposure to five miracidia. They also showed differences in their life history traits when compared to a control group. However, they did not exhibit the increase in size and reduction/cessation in host fecundity which is usually observed in most of the other related snail-trematode systems. In contrast, infected juvenile P. columella showed increased egg laying after the onset of cercarial emission and there was no effect on growth. A third isolate of P. columella was found to be refractory to miracidial infection. This constitutes the first report of non-susceptibility to F. hepatica in a lymnaeid species which is usually susceptible. These non-susceptible snails exhibited lower fecundity and survival compared to non-exposed susceptible snails under identical laboratory conditions. The differences observed in terms of life span and reproduction could result from the cost of resistance for individuals from this isolate.

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In Cuba fasciolosis is regarded as one of the most important parasitic diseases with both economical, showing high prevalence in cattle (Mauri 1972, Verdecia et al. 2001) and medical significance (Kourí 1948, Brito et al. 1987, Millan et al. 2000. This disease is caused by the liver fluke Fasciola hepatica, which needs a lymnaeid snail as intermediate host to complete its life cycle. Only two lymnaeids are known to be present in Cuba: Fossaria cubensis and Pseudosuccinea columella. Both species can host larvae of the trematode, but only the former has been found naturally infected in the island. Currently, only the curative aspect of the disease is taken into account by a chemotherapy treatment of the final host (Ritchter et al. 1999, Millan et al. 2000. No integrated program aiming at stopping or at limiting the transmission of the parasite, associating the control of intermediate snail hosts to other control strategies is currently carried out at the national level.The development of an effective strategy of integrated control, with the precondition of a good knowledge on the epidemiology of this parasitosis requires including, among other things, the study of population dynamics of intermediate hosts and its relation to environmental factors. This study had as objective to determine the variations of the abundance of F. cubensis and of P. columella as well as their relationship with various factors of the environment such as certain physico-chemical parameters of the water, the snail diversity and the abundance of other snail species. MATERIALS AND METHODSThe field work was carried out in the municipality San Juan y Martinez, located south of Pinar del Rio province. Recently, high cattle infections by F. hepatica were referred in this locality as well as two human cases (which appear in the files of the local center of hygiene and epidemiology). Five sampling sites were selected in various water collections: (A) a culvert of residual waters from houses located close to the dispensary of the city; (B) a site on the river which crosses the city; (C) a drainage canal from a vegetable plantation located in the city; (D) a site on a small river outside the city and close to the small village of El Cañón; and (E) a small pond that originated and is fed by the water of an irrigation system. The sites A, C, and D are in or close to the city, whereas the sites B and E are more rural and with no or a little contact with the local population. Sampling took place once a month during one year (from April 2000 to May 2001)The species abundance was estimated by collecting snails using a sieve and forceps, always by the same collector during 15 min; the abundance was then measured as the number of snails collected for each 15-min sampling. The snails were carried to the laboratory in plastic boxes and then identified and counted. The following environmental variables were measured: water temperature, pH, total hardness, nitrites, and nitrates. The snail diversity at each sampling was estimated by the index of Shannon and Wea...

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