River blindness, also known as onchocerciasis, is a disease caused by a parasitic worm. It is the second most common infectious cause of blindness in the world.
River blindness is a disease responsible for a high incidence of partial or total blindness. The World Health Organization (WHO) estimates that between 18 and 37 million people are infected with the parasite that causes river blindness and that about 750,000 people have visual impairment or are blind as a result of the disease.
River blindness is found in tropical climates and is most common in sub-Saharan Africa. Onchocerca volvulus, the parasite that causes river blindness, is also found in parts of Yemen and Central and South America; however, the number of reported cases in these areas is quite low. In total, WHO estimates that over 90 million people are at risk for becoming infected with O. volvulus.
In the areas where river blindness is found, people who live near fast-running streams and rivers are most susceptible to being infected. The Simulium blackfly found in rural agricultural areas transmits the parasite that causes river blindness. Typically, it takes many bites for a person to become infected. For this reason, beyond locals who live in these areas, researchers, missionaries, and members of the Peace Corps who stay in blackfly-infested areas for long periods are the most likely to be infected with O. volvulus.
River blindness is found in 28 African countries, Yemen, and small, localized areas of six Central and South American countries. Ninety-nine percent of all infections occur in Africa. O. volvulus infects individuals independent of ethnicity, race, age, or sex. However, symptoms of those infected do vary depending on region. Although there is no clear understanding to why this is the case, one suspicion holds that it is due to genetic variations of the host.
River blindness is a disease caused by infection with O. volvulus, a thread-shaped roundworm (a nematode), which is transmitted by the bite of a blackfly. The larvae of O. volvulus develop into the infective stage, called L4, inside the blackfly. It takes between 10 and 12 days for larvae to become infective for humans. O. volvulus is introduced into humans by the bite of an infected blackfly. Typically, it takes many bites for the person to become infected.
Larvae take between three months and one year to develop into adults. Female adult O. volvulus live coiled within subcutaneous nodules several inches in diameter and are 13–20 inches (33–50 cm) in length. Males live near females and are smaller, between 7 and 17 inches (19–42 cm). O. volvulus can live up to fourteen years inside the human body, and their larvae can live up to two years. Adult females produce up to 1,000 larvae each day. These larvae can begin to be detected in the skin between 10 and 20 months after the first infection.
Symptoms of river blindness vary depending on the region and person infected. Whereas some people experience no symptoms and no pain, others experience skin rashes, nodules under the skin, itchiness, and problems with vision. Itching and inflammation of the skin, caused by the immune system's reaction to dead larvae, can cause long-term damage. People with skin damage from O. volvulus are often said to have “leopard skin.” Others may find that their skin becomes extremely thin, causing them to suffer from a condition called “hanging groin.” Although not all nodules are painful, they are unattractive. Additionally, dead larvae that make their way to the eye can cause vision problems. At first, these problems are reversible; however, with time, the cornea becomes cloudy, and blindness can result. Inflammation of the optic nerve may occur also, resulting in blindness.
The standard diagnosis for river blindness is a skin snip biopsy. This is done through removing 1–2 mg of skin by scraping with a scalpel. The skin sample is then kept at room temperature in a saline solution for about 24 hours. After 24 hours, a physician can use a microscope to see if any microfilariae have developed. For best results, six samples will be taken from various areas of the body such as the scapula, over the iliac crest, and from the lower extremities. This diagnosis is quite effective for later and high intensity infections. For infections that are under one year old or low in intensity, the results are limited, with only 20–50% accuracy.
Once a diagnosis is made, it is important that the infected individual be treated quickly in order to prevent any further health problems, including blindness. The most common and recommended treatment is ivermectin, a medicine that must be given every six months for the entire life span of the adult worms infecting the patient(about 14 years). Ivermectin kills larvae but does not kill adult worms. Another treatment, which does kill adult worms, is doxycycline. Doxycycline kills the bacteria, known as Wolbachia, on which adult O. volvulus worms live. Some physicians choose to treat infected patients with both drugs simultaneously. It is important that physicians check for Loa loa, another parasite found in the regions where river blindness is common, before beginning any type of drug treatment. Loa loa can cause severe side effects to the medications used in the treatment of river blindness.
Although as of 2012, river blindness had not been blamed for any deaths, it can be debilitating for communities in infected areas. WHO estimated that blindness, as a result of river blindness, can reduce life expectancy by 4–10 years. Onchocerciasis is the second most common infectious cause of blindness, and it has been estimated that in West Africa, up to 10% of villagers may be blind as a result of river blindness.
Physical symptoms are not the only public health threat river blindness poses. Blackflies live near fast-flowing water and therefore often very fertile areas. These areas are sometimes abandoned due to fear of river blindness. People migrate away from these areas to higher ground, which they then clear for cultivation. Clearing vegetation in Africa frequently results in soil erosion and the formation of gullies that channel water during heavy rains. The moving water allows blackflies to breed, spreading the disease to the new area. The reappearance of river blindness results in still further human migration, until large areas of badly eroded land are left unpopulated and unproductive, and entire villages are abandoned. These changes cause severe socioeconomic problems for local villagers.
Organizations such as WHO, World Bank, United Nations Development Programme, and the Food and Agriculture Organization of the United Nations have worked together to control the reproduction of blackflies and the spread of river blindness. The first program, known as the Onchocerciasis Control Programme in West Africa, took place in seven West African countries beginning in 1974. The goal of the program was to control the spread of blackflies through the use of insecticide. The program continued until 2002 and reduced the risk of infection for millions of people. Beginning in 1988, Merck donated the drug ivermectin to help people infected with river blindness. This program continued, and in 2007, Merck made a pledge to make unlimited donations of ivermectin through 2020.
In 1995, African Program for Onchocerciasis Control was developed to help limit the spread of river blindness in the remaining areas of Africa where it is endemic. The program is run by affected communities who work together to deliver medication and monitor the spread of the disease. The program was set to run through 2015. A similar program, the Onchocerciasis Elimination Program for the Americas, was also taking place in affected areas of Central and South America.
Although still a serious problem in areas where blackflies breed, the river blindness has been brought largely under control through the distribution of medicine and the control of the blackfly population. Those infected with river blindness who receive early and consistent treatment and medication can expect to experience few symptoms and keep their vision.
There are many ways to lower the risk of being infected by river blindness. People can wear clothing that covers their entire bodies and use insect repellant to reduce the chance of being bitten by blackflies. Staying fewer than three months in areas with blackflies also greatly reduces the likelihood of becoming infected. On a larger scale, the major method of prevention of river blindness in humans is the control of the blackfly intermediate hosts and vectors. In Kenya, the larval stages of S. neavei have been killed by releasing dichlorodiphenyltrichloroethane (DDT) into streams where the blackflies breed and evolve. However, the waterways must be treated frequently to prevent the re-establishment of blackfly populations, and there have been serious questions raised over the safety of DDT. In West Africa, S. damnosum is more difficult to control, since the adults can fly considerable distances and can easily re-infect cleared sites from up to 60 miles (97 km) away.
See also Parasites ; Tropical disease .
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