Introducing the land snail Bradybaena pellucida increased infection risk of the avian parasite Postharmostomum commutatum in the Kanto region of Japan.

The trematode Postharmostomum commutatum is a parasite of the chicken Gallus gallus domesticus. Its heavy infection can cause inflammation and hemorrhage in the cecum of host birds. We found a severe infection of metacercariae of P. commutatum, which was identified based on DNA barcodes with morphology, in the introduced land snail Bradybaena pellucida and its related species in the Kanto region of Japan. Our field survey revealed that metacercariae were detected in 14 of 69 sampling locations in this region. B. pellucida was thought to be the major second intermediate host of metacercariae of the trematode because this snail was most frequently found in the study area and the prevalence and infection intensity were higher than those of the other snail species. The observed increase in metacercariae in introduced populations of B. pellucida can enhance the infection risk of chickens and wild host birds, probably owing to the spillback effect. Our seasonal field study showed that the prevalence and infection intensity of metacercaria seemed to be high in populations of B. pellucida during the summer and early autumn. Therefore, chickens should not be bred outdoors during these seasons to prevent severe infection. Our molecular analysis, based on cytochrome c oxidase subunit I sequences, showed a significantly negative value for Tajima's D in P. commutatum, suggesting an increase in its population size. Thus, P. commutatum distributed in the Kanto region may have increased its population size with the introduction of the host snail.

Brachylaima phaedusae n. sp. (Trematoda: Brachylaimidae) from door snails in Japan.

The metacercarial infections of door snails (Gastropoda: Clausiliidae) with unknown species of the genus Brachylaima (Trematoda: Brachylaimidae) have recently been reported in eastern Honshu and Kyushu, Japan. A large scale snail survey was carried out to clarify their taxonomic status. From the period of 2015 to 2020, a total of 1239 land snails (768 door snails and 471 others) were collected from 32 localities in Honshu, Shikoku, and Kyushu. The resulting trematode isolates were identified as Brachylaima sp. by mitochondrial DNA barcoding. The sporocysts were found only a few from Megalophaedusa sublunellata (Clausiliidae), Tauphaedusa subaculus (Clausiliidae), and Aegista trochula (Camaenidae), while the metacercariae were frequently detected from 14 species of Clausiliidae and 2 species of other families. Although Brachylaima sp. showed a broad range of intermediate hosts, door snails seem to be very important to drive the life cycle. The gravid adults of Brachylaima sp. was experimentally raised from metacercariae using immunosuppressed mice. Morphological, phylogenetical, and ecological considerations prompted us to propose Brachylaima phaedusae n. sp. for this unknown species. The definitive hosts of the new species are completely unknown. The wide geographic distribution and high genetic diversity of the new species suggest a possibility that the definitive host is ground-foraging birds, which prefer door snails.

Natural history study on Riccardoella tokyoensis: life history with ontogeny and host distribution records.

Riccardoella tokyoensis is a snail parasite recently recorded in Japan. To understand the basic ecology of this mite, we surveyed the life history of a population, including seasonal dynamics, host range, and geographical distribution in East Japan. Seasonal dynamics of the mite were studied in Rinshi-no-mori Park, Tokyo, Japan, from June 2016 to May 2018; thereafter, the morphology of each stage was described. In this park, the host population was infected with the mite in all seasons; however, the prevalence and infection intensities were highest during the warm seasons. Larvae and nymphs were found in the warm seasons. Proto- and tritonymphs were uncommon. To study the host range and geographical distribution, we sampled 1135 terrestrial mollusks from 16 families at 21 locations in Kanto and east Tokai areas from June 2016 to May 2020. Mites were detected at 13 locations, and only clausiliid snails were infected with the mite. Through a laboratory experiment, our observations confirmed that the mite was transmitted to the host lung via the pneumostome. None of the artificially infected snails died, although we maintained the snails for 2 months. No inflammation was found in naturally infected host tissue, suggesting that their virulence against the host snails was low. Riccardoella tokyoensis was not found in the litter layer in the field surveys, and no mite was observed on the external surface of the hosts, suggesting that the major habitat of this mite was the host lung.

MOLECULAR AND MORPHOLOGICAL DISCRIMINATION OF DICROCOELIID LARVAE (TREMATODA: DIGENEA) FROM TERRESTRIAL MOLLUSKS IN JAPAN.

Trematodes of the family Dicrocoeliidae commonly use terrestrial mollusks as the first intermediate host. Despite the abundant studies on the adult worms in birds and mammals, few reports exist on their larval stage in snail intermediate hosts. A present survey of mollusks in Japan led us to the discovery of dicrocoeliid sporocysts with cercariae in 16 out of 303 individuals, encompassing eight snail species and one slug species. A DNA barcoding based on sequencing of mitochondrial cytochrome c oxidase subunit 1 showed that the larvae consisted of five species. Phylogenetic trees of nuclear 18S and 28S ribosomal DNAs confirmed the five species to be members of the Dicrocoeliidae. These were temporarily termed dicrocoeliid spp. 1 to 5, since conclusive identification was impossible without adult worms. These unknown species were phylogenetically related to each other, except sp. 5. The phylogenetic trees demonstrated close genetic relationships between sp. 3 and the genus Lutztrema, and between sp. 5 and the genus Lyperosomum. The phylogenetic analysis also suggested that a division into the subfamilies Dicrocoeliinae and Leipertrematinae is a wrong classification due to the paraphyly of the Dicrocoeliinae. Morphological characterization of the cercariae and their DNA barcodes provide a primary platform for differentiating dicrocoeliids from various mollusks in Japan. The DNA barcodes, in particular, will enable tracing the parasite life cycles, in case of finding metacercariae and adults from presently unknown hosts.