River Blindness (Onchocerciasis)
River Blindness (Onchocerciasis)
Disease History, Characteristics, and Transmission
Introduction
Onchocerciasis (on-kough-sir-KY-A-sis) is caused by a type of parasitic worm called a helminth and occurs mainly in Africa. The worms are spread by the bite of infected black flies, which live mainly near fast-running rivers and streams. Hence, the alternative name—river blindness—for the condition.
Once they have invaded the body, the worms reproduce and millions of microscopic offspring migrate to the eye. When they die, the toxic effects cause severe and chronic inflammation of the cornea and related areas of the eye that lead to loss of vision. The threat of river blindness led to mass migration of people in West Africa away from areas infested with the black fly. This had severe economic consequences, since they settled in less productive upland areas. Fortunately, the anti-parasitic drug ivermectin can be used to treat river blindness. Mass treatment programs have decreased the burden of river blindness in recent years.
Disease History, Characteristics, and Transmission
River blindness, known clinically as oncocerciasis, is caused by a tiny parasitic worm called Onchocerca volvulus. The vector of the disease is a black fly belonging to the Simulium genus. These flies breed near fast moving rivers and streams in the savannas and rainforests in several African countries. Therefore, people living in such areas are prone to infection. The incubation time of the parasite varies between nine and 24 months.
River blindness does not always cause any symptoms. But the parasitic worms, known as microfiliae, may accumulate in characteristic nodules under the skin. Dermatitis, with severe itching, is common and the skin may become wrinkled and thickened. The resulting disfigurement is sometimes called “leopard” or “lizard” skin. More seriously, the parasites may migrate to the eyes. The microfiliae have been found in all parts of the eye except the lens and, when they die, they cause toxic effects, such as inflammation and bleeding, which can ultimately lead to blindness.
Transmission of river blindness occurs when someone is bitten by an infected black fly. This introduces the parasitic worms in a larval form under the skin where they mature. Then the mature female worm releases millions of microfiliae, which migrate towards the eyes. The microfiliae may also move to the surface of the skin, where they may be ingested by other black flies, which may go on and bite someone else, thereby spreading the infection.
Unlike malaria, which can be transmitted by just a single mosquito bite, it usually takes several black fly bites to transmit river blindness. The intensity of infection in an individual depends upon the number of microfiliae they are carrying, which, in turn, depends upon how many bites they have sustained. Blindness usually occurs in people with intense infection.
Scope and Distribution
River blindness is the second leading cause of preventable blindness worldwide. (Trachoma is the leading cause of blindness.) According to the World Health Organization (WHO), around 120 million people worldwide are at risk of river blindness. There are nearly 18 million actual cases of the disease, with about 270,000 cases of blindness and 6.5 million cases of severe itching and dermatitis resulting. The vast majority of these cases occur in Africa, with the remainder occurring in Yemen and in Central and South America.
In Africa, 30 countries in equatorial West, Central, and East Africa are affected by river blindness—areas where there are the fast-running rivers and streams frequented by Simulium black flies. In Central and South America, the affected countries are Mexico, Guatemala, Ecuador, Colombia, and Venezuela. Because infection with river blindness normally requires several bites, it is the populations of these countries, rather than visitors, who are most affected. However, there have been cases among adventure travelers, missionaries, and Peace Corps volunteers.
Treatment and Prevention
The oral anti-parasitic drug ivermectin is effective against the microfilariae, but is less effective against the adult worms. An annual dose, for two years, should clear the infection and relieve dermatitis as well as prevent blindness. Previously, the drug diethylcarbamazine was used, but this drug has severe side effects and the WHO no longer recommends it.
There are no vaccines against river blindness. Insecticides can help control black flies in areas where river blindness is a problem. The flies bite during the day and those at risk should wear long sleeved shirts and pants to avoid being bitten.
Impacts and Issues
Blindness has a profound impact on someone's earning capacity and quality of life. River blindness has therefore proved a severe obstacle to socioeconomic development in many African countries. According to the WHO, in the 1970s, around 50% of all men under the age of 40 in some West African communities had been blinded by onchocerciasis. This caused migration away from fertile river valleys infested by black fly to less productive upland country. The resulting economic losses were around $30 million.
Clearly the world had to take action to halt the economic and social toll of river blindness in the world's poorest countries. Over the last 30 years, there have been various coordinated efforts to control the disease. In 1974, the Oncocerciasis Control Program (OCP) began in West Africa. This program was based on vector control—treating the breeding sites of the black fly with larvicides, that is, insecticides that kill fly larvae. The OCP expanded over the next several years to cover many river systems in seven countries, and it eventually doubled in size to cover 11 countries. The program ended in 2002, by which time river blindness had been virtually eliminated as a public health problem in 11 West African countries.
In 1989, a second strategy was added, involving the distribution of drugs to treat river blindness. Then, in the mid-1990s, the African Program for Oncocerciasis (APOC) began, with the aim of covering a further 19 countries, which comprised the rest of Africa affected by river blindness. In these countries, vector spraying was not a viable option because of environmental conditions. Therefore APOC is based upon the distribution of ivermectin, donated by Merck & Co, the company that discovered the drug. To date, the APOC has protected more than 600,000 people from blindness and reclaimed more than 61 million acres (25 million hectares) of previously infested land for resettlement and agricultural cultivation.
APOC is based in Burkina Faso, in West Africa, and aims to eliminate oncocheriasis throughout the African continent. It works by placing communities themselves at the heart of ivermectin distribution, supported by a number of partners, such as international agencies and national governments. It is financed by voluntary contributions via The World Bank and run by the WHO. APOC aims to prevent one million cases of blindness each year and hopes to achieve its goals by the year 2010. The program may also be a model for other health care interventions in developing countries.
Meanwhile, oncocheriasis is also covered by the WHO initiative, VISION 2020: The Right to Sight, which aims to eliminate preventable blindness worldwide by the year 2020. It is a partnership between the WHO and the International Agency for the Prevention of Blindness (IAPB), a coalition of eye-care professional groups and nongovernmental organizations involved in eye care. The initiative grew from the positive experience of APOC, recognizing that river blindness is just one of five preventable conditions causing 75% of cases of blindness. The other four are cataracts, refractive errors and low vision, trachoma, and a group of conditions that cause childhood blindness. Poor communities are disproportionately affected by these conditions, and there is a cost-effective solution for each of them. The strategy of VISION 2020 is to bring these solutions to as many people as possible.
WORDS TO KNOW
HELMINTH: A representative of various phyla of worm-like animals.
MICROFILIAE: Live offspring produced by adult nematodes within the host's body.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
See AlsoDeveloping Nations and Drug Delivery; Economic Development and Infectious Disease; Parasitic Diseases; Trachoma.
BIBLIOGRAPHY
Books
Wilson, Walter R., and Merle A. Sande. Current Diagnosis & Treatment in Infectious Diseases. New York: McGraw Hill, 2001.
Web Sites
Centers for Disease Control and Prevention.
“Oncocerciasis (River Blindness).” September 27, 2004. <http://www.cdc.gov/ncidod/dpd/parasites/onchocerciasis/factsht_onchocerciasis.htm> (accessed April 20, 2007). The World Bank. “Defeating Onchocerciasis
(Riverblindness) in Africa.” <http://www.worldbank.org/afr/gper/defeating.htm> (accessed April 22, 2007). World Health Organization. “Magnitude and Causes of
Visual Impairment.” November 2004. <http://www.who.int/mediacentre/factsheets/fs282/en/> (accessed April 22, 2007).
Susan Aldridge