Authors

Discussion

Although the two fasciolid trematode species Fasciola hepatica and F. gigantica are causing fascioliasis in humans and livestock, only F. hepatica is involved in the New World. The absence of F. gigantica in the Americas is due to the absence of lymnaeid vectors belonging to the Radix group. So, fascioliasis in the New World is transmitted by many species of the Galba/Fossaria group and one species of the genus Pseudosuccinea, P. columella, the fascioliasis transmission capacity of the Patagonian Pectinidens diaphana still pending verification. The Galba/Fossaria group is very rich in North America and the numerous species of this group described in the Nearctic region indeed suggests that the palaeobiogeographic origin of this group should be looked for in this region. However, in Latin America, from Mexico up to Argentina and Chile, including the Caribbean islands, the number of Galba/Fossaria species is relatively small and only a few have been described to be present in human fascioliasis endemic areas.

     Two of these species are of Old World origin, imported by the Spanish 'conquistadores' around 500 years ago: Galba truncatula and Lymnaea schirazensis. The first is the most efficient fascioliasis vector known and has proved to be the responsible for human hyperendemic areas in Andean countries such as Bolivia and Peru, but also Venezuela, where it appears as the only or the main vector. Worth mentioning is its capacity to give rise to high transmission rates at very high altitudes. Lymnaea schirazensis was always confused with G. truncatula due to their morphological similarity, has proved to be unable to transmit Fasciola, and has been so far found in the Caribbean, Mexico, Venezuela, Colombia, Ecuador and Peru.

The remaining Galba/Fossaria species are all lymnaeids endemic to the Americas. Lymnaea cubensis assures the transmission in Caribbean islands and Venezuela, but also in Mexico and southern USA. Lymnaea viator (= L. viatrix) appears to be a species restricted to the latitudes of the Southern Cone countries of Argentina and Chile, whereas Lymnaea neotropica has recently proved to be widely spread throughout South America thanks to man-made passive transport attached to livestock hooves. The latter has shown to be the main vector in lowland plains such as in Uruguay.

     The molecular characterization by marker sequencing of the nuclear ribosomal DNA (ITS-2, ITS-1) and mitochondrial DNA (16S, cox1) allows not only for the differentiation of these species but also for a quick and easy classification of specimens. The analysis of genetic distances, SNPs, and the reconstruction of trees by different phylogenetic methods indicates that the morphological similarity of these species may be interpreted as an evolutionary convergence towards adaptation to a similar amphibious way of life. Indeed, genetic distances are too high between several of these species as to include them inside the same genus. Moreover, the phylogenetic trees suggest that at least three, and perhaps more, different genera may be involved. Old malacologists already proposed many genera among this group of species.