The role of arthropod second intermediate hosts as avenues for and constraints on the transmission of frog lung flukes (Digenea: Haematoloechidae)

Avenues for and constraints on the transmission of frog lung flukes to frog definitive hosts were studied through examination of the role and fate of parasite metacercariae stages. This study was based on four species of North American frog lung flukes, Haematoloechus coloradensis (Cort, 1915), H. complexus (Seely, 1906), H. longiplexus Stafford, 1902 and H. parviplexus Stafford, 1902. Semi-terrestrial leopard frogs are commonly infected with Haematoloechus coloradensis and H. complexus but exhibit ecological host specificity and are rarely infected with H. parviplexus and H. longiplexus. The latter two lung flukes infect aquatic bullfrogs. Field and experimental infection data indicated that leopard frogs became infected with H. coloradensis and H. complexus by feeding on small non-odonate arthropods that served as second intermediate hosts for these parasites. Comparative experimental examinations of metacercarial survival patterns and distribution in second intermediate odonate hosts revealed that most metacercariae of H. longiplexus were lost during dragonfly metamorphosis, but most metacercariae of H. coloradensis, H. complexus, and H. parviplexus survived dragonfly metamorphosis. These observations suggest that the ecological host specificity of H. longiplexus in semi-terrestrial leopard frogs is due to few metacercariae of H. longiplexus reaching these frogs in a terrestrial environment. Experimental infections of leopard frogs and bullfrogs with H. parviplexus indicate that leopard frogs are resistant to infection with this species. This result indicates that the synonymy of worms from leopard frogs and bullfrogs is not warranted and species from leopard frogs and bullfrogs are distinct. A phylogenetic study of 12 species of North American and European frog lung flukes was conducted using the internal transcribed spacer region. The phylogenetic analysis revealed that these different lineages shared similar patterns of arthropod host specificity distinct from patterns found in the other lineages. These results suggest that second intermediate host specificity may be a trait that has been conserved through evolutionary time. The phylogenetic data presented in this study reveal the importance of second intermediate host specificity among the evolutionary lineages of frog lung flukes because second intermediate hosts serve as avenues for and constraints on the movement of these parasites to their respective definitive amphibian hosts