The stomal morphology of parasitic females of Strongyloides spp. by scanning electron microscopy.

For the discrimination of the species of the genus Strongyloides, the shape of the stoma in the apical view of parasitic females is the most important feature. Microscopical interpretations divided the shape of the stoma into four groups (simple, angular, complex, and simple but with esophageal teeth); however, the stomas classified in the latter two groups, in particular, remained unclear. In the present study, parasitic females of eight representative Strongyloides spp. with the angular stoma (S. procyonis and S. martis), complex stoma (S. robustus, S. callosciureus, S. planiceps, S. papillosus and S. myopotami), and stoma with esophageal teeth (S. ransomi) were studied by scanning electron microscopy. All species examined in this study having complex stoma or with esophageal teeth possessed at least four linguiform or verruciform projections submarginally in the stomal cavity. Namely, the number, size, shape, and the site of attachment of intra-stomal projections, along with the depth of oral cavity, determine a range of complex images of the stoma such as X-shaped or ornate stomas under light microscopy. Similarly, with the standard examination, which usually uses a lateral view of the head, erroneous interpretations of the number of lobes on the circumoral elevation of parasitic females might occur due to such intra-stomal projections in some Strongyloides spp.