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Originally described in Ghana, Mansonella streptocerca is distributed through the western portion of Africa. It is especially common in Ghana and Zaire.
The life cycle of M. streptocerca is similar to that of other filarial nematodes in that it goes through six life stages. The third-stage infective larva are transmitted by the midge Culicoides grahami (picture below taken from Peters and Gilles) and once inside a human host the larvae move to the upper thorax and shoulders where they mature in the dermis.
Adult streptocercae are thin sinuous worms, the males averaging 17x0.05mm and the females 27x0.075 mm. Once the adults have mated they produce microfilaria which average 180-240 x 2.5-5 micrometers in size and they also inhabit the dermis. The microfilaria are unsheathed and have a characteristic pattern of nuclei posterior to the clear cephalic space with four oval nuclei followed by seven to ten smaller, more rounded nuclei and an oval rounded terminal nucleus in a characteristic curved posterior portion of the worm producing the 'walking stick' or 'shepherds crook' tail. These features distinguish them from M. ozzardi and M. perstans (picture below taken from Peters and Gilles).
In many cases there are no clinical or pathological features of streptocercian infection. IgE mediated immune responses to filarial antigens can occur against dead worms and microfilariae. They are similar but usually less severe than those caused by onchocerciasis and the eye is never affected. The incubation and maturation time of the third-stage infective larvae is three to four months. Persons with symptoms often have an itching dermatitis over the thorax and shoulders. The skin is thickened and there are hypopigmented macules. The axillary lymph glands can also be enlarged.