Morphological demonstration of essential functional changes after in vitro and in vivo transition of infective Onchocerca volvulus to the post-infective stage.

Transmission electron microscopic investigation of the morphogenesis of Onchocerca volvulus through the third moult to the post-infective stage revealed essential alterations in various larval organs. Complete rebuilding of surface structures, the reduction of secretory granules in the glandular oesophagus, the unfolding of the intestine, an increase in the number of nerve fibres in the nerve ring and novel sensory papillae were significant findings. Transition from third- to fourth-stage larvae (L4) started as early as 48 h after transfer to vertebrate conditions in vivo in surrogate hosts and in vitro. After a resting period of about 60 h to enable a reduction in gland size and the loosening of the old cuticle and formation of the new one, the larvae started to cast the infective-stage cuticle. Young L4 exhibited a thin, monolayered cuticle and did not rebuild a glandular oesophagus. The body cavity widened, the intestine unfolded and the increased number of microvilli indicated the resumption of metabolic activity.