Onchocerca volvulus is a filarial worm that causes onchocerciasis (a.k.a. river blindness). The adult worms form a subcutaneous nodule called an onchocercoma, from which the female releases larvae into the surrounding skin. Diagnosis is most commonly made through microscopic examination of skin snips, looking for the microfilariae, but can also be made through removal and examination of the onchocercoma. Shown here are hematoxylin and eosin-stained sections of an onchocercoma showing the adult worms in cross-section. Microfilariae can also be seen within the uterus of the female worm and in the surrounding tissue.

You can view a whole slide image from a case of onchocerciasis HERE and another case HERE.