Molecular identification of Haplorchis taichui and other trematodes infecting thiarid freshwater snails from Southern Philippines.

Fish-borne zoonotic trematodiases (FZT) are neglected parasitic infections of global public health significance, persisting in communities where there is a cultural practice of consuming raw or undercooked freshwater fish. In the Philippines, FZT surveillance efforts remain largely focused on human stool-based diagnostics, leaving intermediate host transmission stages poorly characterized. In this study, we provide molecular characterization of the zoonotic trematode cercariae infecting thiarid snails from five FZT-endemic barangays in New Corella, Davao del Norte, Southern Philippines. Of 387 thiarid snails collected in freshwater near human settlements, 26 (6.72%) were infected with three cercarial morphotypes. DNA barcoding and phylogenetic analyses using partial 28S and cox1 sequences identified parapleurolophocercous cercariae from Melanoides tuberculata as Haplorchis taichui, the most prevalent fish-borne zoonotic fluke species reported in the Philippines. The other two morphotypes represented lineages with distinct, non-fishborne transmission routes. Megalurous cercariae confirmed as Philophthalmus gralli, an ocular trematode of birds capable of aberrant human infection, were recovered from Stenomelania sp. representing a putative new first intermediate host record for this species. Virgulate xiphidiocercariae recovered from M. tuberculata were assigned to family Lecithodendriidae, though limited reference sequence coverage precluded genus- or species-level resolution. These findings characterize co-occurring trematode diversity in thiarid snails from Southern Philippines and corroborate active transmission of H. taichui in areas with documented human heterophyid infections. Future work integrating molecular surveillance of both snail and fish intermediate hosts into FZT monitoring may provide a more complete assessment of FZT transmission and human infection risk in these endemic areas.