Electron-microscopic observations on the female worms of six Onchocerca species from cattle and red deer.

The midbody regions of female worms of six Onchocerca species (O. flexuosa, O. tarsicola, O. lienalis, O. gutturosa, O. armillata, O. gibsoni) were studied by transmission electron microscopy. The cuticular layering was rather similar in all species with the ridges built up by the cortical layers and the inner cuticular striations by the median or basal layers. Differences in the epicuticular morphology were considerable. O. flexuosa and O. lienalis had a thin epicuticle without protuberances, the epicuticle of O. armillata carried small knobs, and O. tarsicola, O. gutturosa, and O. gibsoni had a thick trilaminar epicuticle with long protuberances. Extreme hypertrophy of hypodermis and reductions of somatic musculature were observed in O. flexuosa and O. gibsoni. Less extended thickenings of the hypodermis were observed in the other species. No degenerative alterations were found in the muscle cells of O. gutturosa and O. lienalis. The intestinal lumen of most of the species was in a central position, but in O. tarsicola and O. gibsoni the lumen was reduced to small clefts between the intestinal cells. In these species, numerous electron-dense, concentric granules were observed in the cytoplasm of the intestinal cells. The proportions of the various organs differed considerably from species to species, e.g., the uteri contained the embryos filed one behind the other in O. tarsicola, whereas 50 or more embryos were found beside one another in cross-sections of the uterus of O. gibsoni. The comparative study showed that O. gibsoni and O. volvulus have many derived morphological characteristics in common and that in the other species more primitive stages of development of these morphological marks can be observed.