Larval Cucullanid Nematode Infection in the Liver of Ayu

Many white, elliptical to spherical cyst-like bodies (0.43–1.57 mm in diameter) were found in the liver of ayu Plecoglossus altivelis in Shimane Prefecture, Japan. ​Each cyst contained one or two (rarely three) nematodes, 1.11–1.20 mm in body length. ​In parasitological surveys of ayu conducted from 2008 to 2013, infection was confirmed in ayu from three out of six river systems in the prefecture, with prevalence and intensity of infection ranging widely from 0 to 100% and from 1 to 837 cysts/ fish, respectively. ​We believe that infection occurred in rivers, inferred from the fact that uninfected artificially produced ayu became infected after release to rivers. ​Morphologically, the nematodes were identified as third stage larvae. ​They stayed motionless inside the spherical bodies up to 24 h after capture of the host but started to move actively when exposed to physiological saline, where they survived for up to 81 h at room temperature of 22–25°C,