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Human onchocerciasis is found in both the Old and New World but about 95% of all cases are in Africa. Important foci exist in Mexico, Guatemala, Venezuela, and most of Western Africa.
(taken from Peters and Gilles 1991)
Human onchocerciasis is caused by the filarial parasite Onchocerca volvulus. The infective larvae are normally transmitted by the bite of Simulium flies (see picture below taken from Peters and Gilles 1991)
Simulium flies can only breed in well oxygenated water because their larvae have an obligatory aquatic stage during which they require high oxygen tension (see picture below taken from Peters and Gilles 1991).
Accordingly onchocerciasis and the blindness it can lead to are associated with fast flowing rivers with rapids and onchocerciasis is often referred to as 'river blindness'.
The infective larvae of Onchocerca 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 nodules and mature into adults in approximately one year(see cross section of worms in a subcutaneous nodule below, picture taken from Peters and Gilles 1991).
After mating the female ovivipariously gives birth to microfilariae 300 mm in length and 0.8 mm in diameter. The microfilariae are sheathless with sharply pointed recurved tails.
The microfilariae can be found free in the fluid within the nodules and in the dermal layers of the skin spreading centrifugally from the area where an adult lies. 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 stage three infective larvae in the flies' flight muscles (in about 10 days).
One of the earliest signs of infection with Onchocerca is the raised nodules that can be seen under the skin around areas over bony prominence (See picture below taken from Peters and Gilles 1991).
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 advances cases often resemble 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 taken from Peters and Gilles 1991).
