Streptocercosis is caused by Mansonella streptocerca (formerly Dipetalonema streptocerca). This nematode is confined to Central and West Africa. The parasite is transmitted by Culicoides midges. It could be a zoonosis, since morphological identical parasites are found in chimpanzees. The adult worm measures 2 cm by 0.06 mm. Microfilariae are 180 to 240 m m by 2.5 to 5 m m in size. The tail is characteristically coiled (hook) and contains nuclei. Adult worms live in the skin. Live worms cause no lesions, but a local inflammatory reaction occurs when they die, with papules and possibly subsequent fibrosis. There are no eye lesions. Differentiation from onchocerciasis is necessary.

Many infected people are asymptomatic. The most frequent symptom is chronic pruritus. The skin is thickened and there are papules. Hypopigmented patches can occur, which must be distinguished from leprosy, endemic treponematosis and onchocerciasis. Lymph nodes can be enlarged.

The microfilariae are found in the skin. Detection is as for onchocerciasis (skin snip, scarification with collection of dermal fluid). In the event of doubt or suspicion of leprosy, a biopsy is useful. DEC causes a Mazzotti reaction as in onchocerciasis.

DEC is micro- and macrofilaricidal for Mansonella streptocerca. Ivermectin is highly active against this parasite.