Human onchocerciasis is found in both the Old and New World but about 96% of all cases are in Africa and mostly in Western Africa. Of the 36 countries where the disease is endemic, 30 are in sub-Sahara Africa (plus Yemen) and six are in the Americas. Indeed, important foci exist also in Mexico, Guatemala, Venezuela and Ecuador. A total of 18 million people are infected with the disease and have dermal microfilariae, of whom 99% are in Africa. (WHO's Fact Sheets #95)
Human onchocerciasis is caused by the filarial parasite Onchocerca volvulus whose life cycle occurs in two different hosts: black flies, and human. The infective larvae (a: stage L3) are normally transmitted by the bite of Simulium flies (see picture below). Once in the human body, the larvae undergo molting to stage L4 (e), to then reach the adult stage in about one year (f). Adult female are able to produce millions of microfilariae (h) that they shed in the blood of their human host. When female blackflies take a bloodmeal they ingest those microfilariae that are going to undergo in the fly host a first transition to L2 life stage (j). L2 larvae then molt to L3 (a), the infective stage for human.
Simulium flies breed in fast flowing rivers i.e well oxygenated water, because their larvae have an obligatory aquatic stage during which they require high oxygen tension (see picture below). Hence, Onchocerciasis is associated with fast flowing rivers including rapids. That's why the blindness Onchocerciasis can lead to is often referred to as 'river blindness'.
The infective larvae of Onchocerca (stage L3) enter the body through the wound made by the bite of its host fly. The larvae then move to the subcutaneous tissues where they become encapsulated within nodules and mature into adults in approximately one year (see cross section of worms in a subcutaneous nodule below).
After mating the female sheds microfilariae 300 mm in length and 0.8 mm in diameter. The microfilariae are sheathless with sharply pointed, curved tails.
The microfilariae can be found free in the fluid within the nodules and in the dermal layers of the skin spreading away from the nodules containing the adults. Microfilariae also can be found in the blood and eye during heavy infections. They infect their fly vectors while the flies are feeding on the human host and mature into third stage infective larvae in the flies' flight muscles (about 10 days total).
One early sign of infection with Onchocerca is the raised nodules that can be seen under the skin. These are most often seen in areas over bony prominence (see picture below).
It is suggested that this phenomenon occurs because the larvae are immobilized in these locations (while the host is sleeping) long enough for them to be trapped by the body's cellular defense mechanisms.
Reactions to dead microfilariae around these nodules can lead to several unpleasant conditions. In the skin there is destruction of the elastic tissues and the formation of redundant folds. There is also often a loss of pigmentation and the histological appearance of advanced cases often resembles the skin of very old normal subjects (see pictures below taken from Peters and Gilles 1991).
The microfilariae can also enter the eye by passing along the sheaths of the ciliary vessels and nerves from under the bulbar conjunctiva directly into the cornea, via the nutrient vessels into the optic nerve, and via the posterior perforating ciliary vessels into the choroid. Dead microfilariae in the eye lead to an inflammatory immune response and the eventual formation of secondary cataracts and ocular lesions. Because of this, heavy infections often lead to progressive blindness.
The microfilariae can also cause inflammation of regional lymph glands which remove foreign material from the distal skin. This inflammation along with the loss of tissue elasticity can lead to protruding lymph glands enfolded in pockets of skin. This condition is especially prominent in the areas around the scrotum (often called the 'hanging groin' effect) and in severe cases is classified as minor elephantiasis (see picture below).