Trematode Parasite Infections in Freshwater Leeches from the Central Region of Lithuania: First Record of Posthodiplostomum (Dubois, 1936) in a Leech Host

Leeches play a critical role in the transmission of digenean trematodes, yet their parasitic infections remain understudied in the Baltic region. This study investigates the diversity, prevalence, and molecular identification of trematode infections in freshwater leeches from central Lithuania. A total of five leech species (<i>Alboglossiphonia heteroclita</i>, <i>Glossiphonia complanata</i>, <i>Glossiphonia verrucata</i>, <i>Helobdella stagnalis</i>, and <i>Erpobdella octoculata</i>) were examined using compression and dissection techniques to detect trematode cysts, which were predominantly found in the soft tissues rather than the intestinal tract. Molecular sequencing of 18S rRNA, COI, and ITS markers, combined with phylogenetic analyses, confirmed the presence of three trematode genera: <i>Cotylurus</i>, <i>Australapatemon</i>, and <i>Posthodiplostomum</i>. The overall infection rate among leeches was 40.8%, with the highest prevalence observed in <i>G. complanata</i> (53.3%). <i>Cotylurus</i> spp. were the most frequently detected parasites, with genetic analyses revealing close affinities to <i>Cotylurus syrius</i> and <i>Cotylurus</i> spp. from Poland. <i>Australapatemon</i> species were also identified, though species-level classification remained inconclusive. Notably, this study provides the first molecular evidence of <i>Posthodiplostomum cuticola</i> utilizing leeches as intermediate hosts, extending the known range of hosts for this trematode. Phylogenetic analyses demonstrated the broad geographic distribution of these parasites, with close genetic matches to isolates from Poland, Russia, Canada, and Japan. The findings highlight the ecological significance of leeches in parasite transmission networks and contribute to the understanding of trematode diversity and host interactions in the Baltic region. Further molecular and ecological studies are needed to clarify species diversity and the role of leeches in the life cycles of aquatic parasites.