Geographical surveys

A detailed geographical outline of the fascioliasis endemic area was established during the 1990s [34]. Past surveys on humans, cattle and lymnaeids demonstrated that this endemic area is completely isolated. Cattle were used as a marker because of the shorter lifespan of fasciolids in bovines [34, 45]. A total of 57 lymnaeid-inhabited freshwater sites, and around 14,000 specimens collected in different years and different seasons of the year, allowed for the delimitation of the geographical boundaries of the hyperendemic area (Fig. 2a) [34]. The boundaries proved to cover from the southern surroundings of the locality of Ancoraimes at the coast of Lake Titicaca, in the north, to a little southward from the locality of Viacha on the route from El Alto to Oruro, in the south. Longitudinally, the endemic area proved to extend from the valleys of La Paz and the River Cala Jahuira, in the east, to the Bolivian coast of Lake Titicaca in the west (Figs. 1, ​,2a)2a) [34, 45]. This distribution concerned four provinces of the Department of La Paz (Fig. 1c, d); Los Andes, Ingavi, Omasuyos and Murillo (Fig. 2).

In the present study we describe the results of the 2018–2019 lymnaeid snail surveys made for two geographical purposes: (i) assessing presence/absence of lymnaeids in freshwater habitats located inside the whole endemic area to verify the stability of the patchy distribution established in the 1990s [34]; and (ii) analysis of freshwater habitats located in a wide perimeter outside the endemic boundaries established in the 1990s [34], to assess potential present and further spread of the disease occurred during the last 25 years. For both purposes, the same lymnaeid-inhabited water bodies studied in the past and other freshwater habitats in which lymnaeids were not found in the past, whether inside or outside the old endemic boundaries, were surveyed again.