6.2 Mansonella perstans, distribution The infection is widely distributed in Africa but is more localised in Central and South America. M. perstans does not occur in Asia. 6.3 Mansonella perstans, clinical features The adult worms live in body cavities (peritoneum, pleura, pericardium) and in perirenal fat. Most patients infected with Mansonella perstans are asymptomatic. A number of different symptoms and allergic reactions are sometimes ascribed to this parasite, but the disease spectrum has not yet been fully established. 6.4 Mansonella perstans, diagnosis The adult worms are 4 to 8 cm long and very thin. They are only rarely observed, for example during a laparotomy. The microfilariae are small (100-200 m m by 5 m m) and have no sheath. The tail is short and contains nuclei. They are aperiodic. The diagnosis is established by detecting the typical small microfilariae in the peripheral blood. The tail is rounded with nuclei at the extremity. The head spot sometimes has a V-shaped appearance. Serology is of little use. Mansonella perstans frequently occurs together with other filariae. * Mansonella perstans should be distinguished from Microfilaria semiclarum (a parasite of animals which sometimes causes accidental infections in humans). Sometimes confusion is possible if the blood smear is randomly infected during or after preparation with a mould such as Helicospora. This organism, however, is considerably smaller and thinner than a microfilaria. 6.5 Mansonella perstans, treatment If asymptomatic, no treatment is necessary. If disease is suspected, therapy with mebendazole (Vermox®), best in combination with levamisole (Ergamisol®), is indicated. Ivermectin, albendazole and DEC are inactive.