Medicine for Africa - Medical Information Service
Onchocerciasis, also called River Blindness, is a chronic disease in which a parasitic nematode (roundworm), called Onchocerca volvulus, is transmitted to humans via the bite of black flies (simulium damnosum).
The microfilariae (minute larval form of thread-like roundworms) of O. volvulus were first associated with a popular dermatitis called craw-craw, in Ghana, in 1874 by Dr. John O’Neill, an Irish surgeon. In 1917, the Guatemalan physician Dr. Rudolfo Robles Valverde published his findings on a new disease associated with subcutaneous nodules, ocular lesions, skin disease and microfilariae. And finally, in Sierra Leone in 1923, Donald Blacklock discovered that the black fly of the genus Simulium damnosum, was the transmission vector of the disease.
The black flies are active during the day and breed near fast flowing, well-oxygenated rivers, thus the prevalence of the disease is highest in areas near those rivers, which is the reason why the disease is also called river blindness.
An estimated 18 million people are infected with the O. volvulus microfilariae. The infection can be found in 38 countries worldwide, including Central and South America (Brazil, Colombia, Ecuador, Guatemala, Mexico, Venezuela), and the Arabian Peninsula (Saudi Arabia, Yemen), but occurs most often in Africa with 95-96% of all cases reported. The worst affected countries in Africa include, among others, Angola, Burundi, Cameroon, Chad, the Central African Republic (CAR), the Congo, the Democratic Republic of Congo (DRC), Ethiopia, Equatorial-Guinea, Gabon, Kenya, Malawi, Mozambique, Nigeria, Rwanda, Uganda, Sudan and Tanzania. Of those people infected with the parasite, nearly 300,000 are blind, while an additional 500,000 have severe visual impairment.
The transmission from fly to man occurs during a blood meal of an infected fly, when it injects the O. volvulus larva (microfilaria) into the human host. These larvae develop over the next one to two years into adult worms within subcutaneous nodules. A female worm can grow up to 30 to 80 cm in length, and can live coiled up in those nodules for 10 to 15 years, while producing between 1,000 and 2,000 new microfilariae each day, after mating. Male worms are much smaller (3 to 5 cm in length) and migrate between nodules in order to inseminate female worms. When the new larvae are taken up by a feeding fly, the cycle of infection repeats itself, over and over again.
The adult worms rarely ever cause symptoms other than subcutaneous nodules, usually around bony prominences. The major symptoms occur one to three years after first infection, and express themselves as reactive immune responses by the body to dead or dying microfilariae. The production of antibodies, and especially eosinophils, as well as cellular responses can lead to severe itching, skin rashes, and skin depigmentation, especially on the lower limbs, leading to the so-called ‘leopard skin’. Loss of skin elasticity results in thick, rough or loose, hanging folds (‘hanging groins’), while lymph nodes become inflamed and swollen, particularly in the genital area.
However, the most serious and devastating effects of onchocerciasis happen when the microfilariae enter the eye, leading first to more or less serious visual impairments and finally causing blindness.
At first, the eye becomes inflamed and may appear red; exposure to bright light can cause pain. Without appropriate treatment, the cornea will become more and more opaque and may finally scar, causing blindness. The iris, pupil and retina can also be affected, leading to atrophy, inflammatory glaucoma and cataract; and the optic nerve can become inflamed and degenerate, another cause of blindness.
In Africa, depigmented skin orders occur more frequently in forests, while the development of blindness is more commonly reported in the savanna and woodland areas.
While a transplacental infection from mother to child can occur with microfilariae, the active transmission of infective larvae from the black fly is required for the disease to erupt again and progress.
A definitive diagnosis of onchocerciasis, or river blindness, requires the demonstration of microfilariae in the skin. This is achieved by taking skin biopsies (skin snips) and examining them under the microscope. A positive skin biopsy is 100% specific for the disease. However, in the early or latent stage of the disease, the presence of microfilariae may be very low, thus producing false-negative results. A positive diagnosis may also be made by demonstrating adult worms from subcutaneously excised nodules.
In advanced disease, slit-lamp examination may reveal free floating microfilariae in the anterior chamber of the eye.
Blood tests may show an increase in eosinophils, but this is not a specific, nor a sensitive test. At times, especially in endemic areas, microfilariae can also be found in urine and cerebrospinal fluid (CSF). While positive findings are obviously diagnostic, negative results do not exclude the possibility of an infection.
Serological tests such as an enzyme-linked immunoabsorbent assay (ELISA), detect antibodies against the antigens of O. volvulus. Recently, Rapid Antibody Cards have been developed that detect a recombinant antigen of O. volvulus – these tests can usually be performed fast and easily by any medically trained personnel, without the need of special and expensive equipment. However, serological tests cannot reliably distinguish between a past and current infection.
Polymerase chain reaction (PCR) assays can amplify the DNA sequences of O. volvulus in skin biopsies. While this test can be very sensitive and specific for diagnosing onchocerciasis, technical problems and considerable costs make the test unsuitable for practical use, especially in less developed countries.
In addition, there are several forms of diagnostic diethylcarbamazine (DEC) tests available:
Treatment for onchocercosis or river blindness consists of one single dose of ivermectin, in order to kill all currently present microfilariae!
However, since this drug does not kill the adult worms, it is necessary to repeat the treatment every six months until the adult worms will have died of old age (usually around 12 years).
Ivermectin is given in an annual or semi-annual dose of 150 to 200 micrograms/kg body weight. This dosage will reduce the amount of microfilariae to near zero within just one month, and maintain a low overall level of microfilariae for up to one year.
Regular treatment will resolve most eye and skin related problems. However, once chorioretinitis or blindness have developed, treatment will not be able to reverse these stages, nor will treatment be effective in curing skin atrophy or skin depigmentation. The average life expectancy of blind patients is about 10 years.
Adult worms that live in subcutaneous nodules (see above) can also be removed surgically by removing the entire nodule; however, it may often be difficult to find all of the nodules, in order to guarantee complete success.
In 1987, the pharmaceutical company that produces ivermectin, has generously decided to donate its drug to less developed countries in order to fight the disease. In association with the World Health Organization (WHO), non-governmental organizations, and the countries ministries of health, more than 530 million treatments have been delivered worldwide over the last twenty years, making this one of the most effective Public-Private Partnership ventures in the history of health care. The company has remained steadfastly committed to donating the drug for as long as needed to eradicate this disease.
Unfortunately, after a treatment has been shown to be very effective over a long period of time – more than twenty years – the development of drug resistance is bound to occur. Thus, in a recent study, a Canadian research team has found that the prevalence (the total number of cases of disease in the population at a given point in time) of microfilariae in the skin of affected individuals has doubled between 2000 and 2005 in two communities that had been treated for the disease. While ivermectin cleared 100% of the microfilariae in 99% of patients treated, after just three months, 40% of the communities showed a significant repopulation of larvae.
An alternative drug that has been given at times is an antibiotic, called doxycycline. In dosages of 100 mg/day given over six to eight weeks, it has demonstrated efficacy not only in reducing the number of larvae, but also sterilizing adult worms and decreasing their overall viability. However, side effects can be severe, and the efficiency in using this drug in mass treatment plans is questionable.
Ivermectin given once yearly can prevent the parasite’s transmission, and thus is equally effective in preventing as in treating the disease.
Of course, avoiding fly-infested areas, wearing protective clothing and using insect repellents generously, can theoretically help to reduce the risk of an infection; however, in reality, the risk of being bitten by an infected fly can never be totally eliminated. The wide ranging use of spraying insecticides may have some effect, which, however, should be evaluated versus its short and long term negative environmental effects.
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