Update on the Geographic Distribution of the Intermediate Host Snails of Schistosoma mansoni on St. Lucia: A Step Toward Confirming the Interruption of Transmission of Human Schistosomiasis.

To provide information to guide considerations of declaring interruption of transmission of human schistosomiasis due to Schistosoma mansoni on St. Lucia, we undertook an island-wide survey in June-July 2022 to determine the presence of Biomphalaria snails, the intermediate hosts of S. mansoni, and their infection status. Snail surveys were carried out at 58 habitats to determine presence of Biomphalaria snails followed by examination of the collected snails for evidence of infection with S. mansoni. Furthermore, water samples were collected at the snail habitats and screened for presence of S. mansoni DNA using an eDNA approach. We found B. glabrata present in one habitat (Cul de Sac) where it was abundant. Specimens provisionally identified as Biomphalaria kuhniana were recovered from 10 habitats. None of the Biomphalaria specimens recovered were positive for S. mansoni. None of the eDNA water samples screened were positive for S. mansoni. Experimental exposures of both field-derived and laboratory-reared St. Lucian B. glabrata and B. kuhniana to Puerto Rican and Kenyan-derived S. mansoni strains revealed B. glabrata to be susceptible to both and B. kuhniana proved refractory from histological and snail shedding results. We conclude, given the current rarity of B. glabrata on the island and lack of evidence for the presence of S. mansoni, that transmission is unlikely to be ongoing. Coupled with negative results from recent human serological surveys, and implementation of improved sanitation and provision of safe water supplies, St. Lucia should be considered a candidate for declaration of interruption of human schistosomiasis transmission.

Schistosoma mansoni Vector Snails in Antigua and Montserrat, with Snail-Related Considerations Pertinent to a Declaration of Elimination of Human Schistosomiasis.

Investigations leading to a WHO-validated declaration of elimination of schistosomiasis transmission are contemplated for several countries, including Caribbean island nations. With assistance from the Pan American Health Organization, we undertook freshwater snail surveys in two such nations, Antigua and Barbuda, and Montserrat in September and October 2017. Historically, the transmission of Schistosoma mansoni supported by the Neotropical vector snail Biomphalaria glabrata occurred in both countries. Transmission on the islands is thought to have been interrupted by the treatment of infected people, improved sanitation, introduction of competitor snails, and on Montserrat with the eruption of the Soufrière volcano which decimated known B. glabrata habitats. Guided by the available literature and local expertise, we found Biomphalaria snails in seven of 15 and one of 14 localities on Antigua and Montserrat, respectively, most of which were identified anatomically and molecularly as Biomphalaria kuhniana. Two localities on Antigua harbored B. glabrata, but no schistosome infections in snails were found. For snail-related aspects of validation of elimination, there are needs to undertake basic local training in medical malacology, be guided by historical literature and recent human schistosomiasis surveys, improve and validate sampling protocols for aquatic habitats, enlist local expertise to efficiently find potential transmission sites, use both anatomical and molecular identifications of schistosomes or putative vector snail species found, if possible determine the susceptibility of recovered Biomphalaria spp. to S. mansoni, publish survey results, and provide museum vouchers of collected snails and parasites as part of the historical record.

Status of Schistosomiasis Elimination in the Caribbean Region.

Schistosomiasis elimination status in the Caribbean is reviewed with information on historical disease background, attempts to control it and current situation for each locality in the region where transmission has been eliminated (Sint Maarten, Saint Kitts, Vieques), eliminated but not yet verified (Puerto Rico, Dominican Republic, Antigua, Montserrat, Guadeloupe, Martinique) and still ongoing (Saint Lucia, Suriname). Integrated control initiatives based on selective and mass treatment and snail control using environmental, chemical and biological methods along with public service improvements (housing, safe water, sanitation) and changes in demography (urbanization) and economy (change from sugarcane and banana production to tourism) have resulted in reduction in the burden of schistosomiasis over the past century. Introduction of Biomphalaria-competitor snails into the region as a cost-effective, low maintenance control method appears to have had the most sustainable impact on transmission reduction. A regional inventory of B. glabrata, other Biomphalaria species and Biomphalaria-competitor snails as well as investigation of possible animal reservoir hosts in persisting endemic areas would be helpful for control. Elimination of schistosomiasis appears achievable in the Caribbean. However, a regional surveillance and monitoring program is needed to verify elimination in the various localities and identify and monitor areas still endemic or at risk.