M V Fernández, M Laidemitt
Echinostomes are a globally distributed group of digeneans with complex life cycles. In South America, many species are known only from definitive hosts, and in Argentina, most larval stages and adult records remain unlinked. Formal morphological descriptions and molecular data are essential for identifying larval stages; however, molecular reference sequences of larval digeneans remain scarce. We present an integrated morphological and molecular characterisation of echinostome cercariae from Biomphalaria aff. straminea collected between January 2017 and February 2018 in a rice field from Corrientes Province, Argentina, based on partial DNA sequences of the nuclear 28S rRNA (28S) gene and the mitochondrial marker nad1 gene. Eighty-three snails (2.12%) of the 3,913 sampled were infected with three echinostomatid morphotypes: Cercaria Echinostoma sp. 20 (1.63%); Cercaria Echinostomatidae gen. sp. 21 (0.25%); and Cercaria Drepanocephalus sp. (0.23%). Available Echinostomatidae sequences in GenBank were aligned with newly generated sequences from echinostomatid cercariae. Phylogenetic analyses showed that Echinostoma sp. 20 represents a distinct lineage that does not group with other Echinostoma sequences in GenBank; Echinostomatidae gen. sp. 21 does not cluster with Echinostoma and likely belongs to a genus related to Patagifer, although its position differs between the 28S and nad1 trees and it shows morphological differences; and Drepanocephalus sp. clusters with D. spathans from Mexico with low genetic divergence (<4.6%), although additional sequence data are needed to clarify species boundaries. This study provides the first molecular identification of Drepanocephalus and Echinostoma species in freshwater snails from Argentina, expanding the molecular dataset for Drepanocephalus in South America.