Ultrastructure of Rhabdias tokyoensis, a pulmonary nematode of the Japanese newt (Cynops pyrrhogaster)

The parasitic nematode, Rhabdias tokyoensis, Wilkie, 1930, which is found primarily in the saccular lungs of the Japanese newt, Cynops pyrrhogaster, Boie, 1826, is described from the clinical and ultrastructural perspectives. Newts are commonly infected with 1-3 nematodes per lung and often appear healthy, but may present in a wasted state and rarely, if the parasites have penetrated the lungs, will appear grossly bloated with air. This distention alters the newt diving behavior due to impaired buoyancy control. The nematodes induce ulcerative lesions on the inner, pulmonary surface and, if they escape from the lungs, may attach to abdominal organs such as the liver. Rhabdias tokyoensis has minor annulations in the head region, with an oral cavity devoid of crown or lips, and with narrow, sub-cuticular, bacillary band-like longitudinal ridges, as observed by scanning electron microscopy (SEM). Cuticular wrinkling due to dehydration is evident by SEM. Transmission electron microscopy revealed that the cuticle was distinctive, with a particularly thick inner cortex, two thin laminations comprising the basal, fibrous layer, and a convoluted, outer cortex which reflected the surface wrinkling observed by scanning electron microscopy. Cells lining the intestinal tract had apical surfaces lined by microvilli and both large (2.5 μm) and small (0.3 μm) electron dense secretory granules. Somatic muscles encircling the periphery of the nematode body were typical for this genus. The life cycle, prevalence in the natural environment, and possible intermediate hosts for R. tokyoensis remain unknown. Infection of Japanese newts by R. tokyoensis, which otherwise are in good health, may not significantly impair this primarily aquatic amphibian because it relies much on integumentary respiration and the nematode-damaged, primitive saccular lungs do not undergo significant hemorrhage.