The first stage larva (so-called microfilaria)
Around 1000 live microfilariae are released every day by the gravid female worm during its reproductice cycles. They leave the female through the vulva located very close to the worm’s anterior end. They are still enclosed in the nodule and have to find their way out into the skin, where they migrate in the lymphatic clefts of the cutis, close under the epidermis.
They seem to be in constant move, at least when isolated from a skin-snip. Their movements in a drop of saline do not have any direction. This is because there is no medium that would provide any grip for a directed movement. We therefore developed a method to cast the living microfilariae in a semi-fluid medium (Agar-agar or gelantine) to make their movements visible:
It is yet unknown whether the microfilariae are in constant move in the skin. For example to remove any cells of the host’s immune system that would attach to them. If so, one would have to postulate any mechanism of orientation that prevents the microfilariae to leave those sites where the vector flies are biting. In the case of O. ochengi, where the nodules are located in the ventral skin of cattle, mostly in the inguinal reagion of the belly, the vector blackflies almost exclusively bite at the same body region. The microfilariae therefore are located in the same body region like to nodules.
It is different, however, for the microfilariae of O. volvulus in man, where – in Africa! – the nodules concentrate around the lower body (iliac crest, knee), whilst the vector Simulium damnosum s.l. almost exclusively bites on the legs below the knees (Renz & Wenk, 1983).
Schema der Ultrastruktur einer Mikrofilarie. Aph Amphide, AnP Analporus, Bcc Buccalbucht, BiK Binnenkörper, ExP Ekretionsporus, ExZ Exkretionszelle, GZ Geschlechtszelle, Hk Haken, MsZ Muskelzellen, NR Nervenring, Phm Phasmide, RZ Rektalzellen (n. McLaren 1972).
In cattle in Cameroon, we find microfilariae of 5 different Onchocerca-species in the skin: