Molecular characterization of the parasitic nematode genus Crassicauda; larvae parasitic on the firefly squid and adults on beaked whales off the coast of Japan

The Spirurina type X larvae, which infect firefly squid (Watasenia scintillans), are known to cause cutaneous creeping eruption and intestinal obstruction in humans. Although it has been reported that the adult of this larva is Crassicauda giliakiana, which was recovered from a Baird's beaked whale (Berardius bairdii), it is not well known internationally. In this study, to reconfirm the identification of this species, we determined the mitochondrial cox1 gene and the partial sequence of 18S–28S ribosomal DNA from larvae recovered from firefly squid. As the results, we confirmed that the larvae were C. giliakiana, and partial ribosomal DNA sequences were also performed for phylogenetic analysis. Furthermore, to determine the distribution of the genus Crassicauda in Japan as a definitive host, DNA was extracted from archival specimens of adult worms recovered from the kidneys of family Ziphiidae that had stranded on the coast in Japan, and phylogenetic analysis using ITS2 region was conducted. As a result, C. giliakiana were detected from not only B. bairdii but also Mesoplodon stejnegeri, and Ziphius cavirostris, and C. anthonyi was also detected in Z. cavirostris. Furthermore, the kidney parasitic nematoda Crassicauda sp., which is not registered in the database, was found in both M. densirostris and M. gingkodens. This study provides new insights into the distribution and the lifecycle of genus Crassicauda in Japan based on the phylogenetic relationship between larvae and adults.