River blindness

Chibuzo Odigwe explains the history and current treatment strategies for a disease that has been a large scale problem in Africa since recorded history began

From the beginnings of recorded history, onchocerciasis has been a problem in Africa, as folklore and orally preserved tradition indicate. But it has also been described on the Arabian Peninsula and the Americas. An estimated 18-40 million people are currently affected worldwide, with about 270 000 people having lost their sight as a result. About 99% of the people living with onchocerciasis are in Africa with about half of this population in Nigeria alone.

The black fly

Onchocerciasis is caused by Onchocerca volvulus, a nematode of the family Filariiadae, whose vector is the blackfly, Simulium damnosum. Because the black fly prefers to lay its eggs in fast flowing rivers, onchocerciasis is endemic in these areas--giving rise to its more familiar name of river blindness.

Leopard skin and blindness

Onchocerca volvulus can live in the human body for as long as 14 years, during which time they produce microfilariae, which produce the characteristic features of the illness.

Onchocerciasis mainly affects the skin and eyes. It induces blindness by causing corneal opacification, which occurs as a result of an eosinophil mediated inflammatory reaction to dead microfilarial larvae.

The early skin related signs include mild pruritus, which may later be followed rash, erosion, and lichenification. Adult worms also lodge in nodules under the skin and releasing large numbers of microfilariae into surrounding tissues. When these worms die they elicit an inflammatory reaction which produces a skin rash. This is itchy and gives a characteristic skin appearance that is sometimes described as "leopard skin" and occurs most commonly on the shins. If the condition is allowed to progress, lymphadenitis and elephantiasis of the genitals may occur. And in most extreme cases it results in squamous cell cancer of the skin.

Apart from the classic skin and ocular manifestations, a controversial association between onchocerciasis and epilepsy was proposed. The seizure disorders were thought to be due to an inflammatory reaction to dead microfilarial larvae in the cerebral cortex.

Uninhabitable land

The preponderance of river blindness in Nigeria has created some socioeconomic problems. Not only does it rob communities of labour by affecting young men and women, it deprives poor communities of good land, forcing them to relocate till the rains stop. It also renders well watered areas uninhabitable, forcing farmers to relocate to dry land, which soon gets exhausted from overuse.

Diagnosis

The classical manifestations of onchocerciasis form the basis of its diagnosis. But there are tests too. Skin snips may show microfilariae, and by using slit lamp biomicroscopy you may see these in the cornea or anterior chamber of the eye. You can also biopsy and histologically examine skin nodules, which are the most obvious sign. O volvulus is identified histologically by special staining. The adult is a threadlike, coiled, white worm, which measures about 50-100 cm (females) or 2.5-5 cm (males). Dead microfilariae are straight and surrounded by granulation tissue. Evidence also shows that early detection of onchocerciasis is possible via assay of parasite specific IgG and IgG isotype responses, preferably IgG4.

Treatment regimens

An earlier method of treating onchocerciasis consisted of nodulectomy. This was done in the belief that once the adult worm was eradicated the infection was potentially cured. However, this method is no longer in use. Current standard treatment entails administration of diethycarbamazine, administration of ivermectin, or surgery in some cases where removal of onchocermata and hanging groin is necessary

By and large, the current method of choice is ivermectin treatment. Much of the success of the onchocerciasis control programmes is due to its efficacy and tolerability. It acts by interrupting the development of microfilariae in the uterus of the adult worm.

Intervention and control strategies

Onchocerciasis control

This was created in 1974, with the objectives of vector control and ivermectin distribution in eleven countries. This programme has been hailed as one of the most successful partnerships in the entire history of development assistance. It has helped to stop the transmission of onchocerciasis in the 11 country area , avoiding about 60 000 new cases of blindness, preventing some 16 million children from infection, and freeing up of about 25 million hectares of arable land in oncho-endemic areas.

The African programme

Launched in 1995, its goals include the annual treatment of over 50 million people, and it relies mainly on the establishment of community directed ivermectin treatment. This approach also helps to empower the community and so is the favoured approach in developing countries such as Nigeria. It hopes to eliminate onchocerciasis as a public health problem from the whole of Africa by the end of the programme in 2007.

The way forward

Although a lot has been done in stemming this scourge, it would be wrong to assume that all that needs to be done has been done. More enlightenment campaigns and education for patients are needed. Again communities should be encouraged to take active part in the control programmes. Governments should also fund more extensive surveys of areas suspected of being oncho-endemic but have as yet not been fully surveyed.

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