- Diseases, Disorders & Conditions
Onchocerciasis or River Blindness: Eye Disease and a Parasite
What Is Onchocerciasis?
Onchocerciasis is a serious parasitic disease that causes skin and eye inflammation. The skin inflammation results in severe, debilitating itching. The eye inflammation often leads to blindness.
The disease is triggered by the bite of a black fly, which is a blood-sucking insect. The fly isn't the direct cause of the disease, however. Its bite inserts a parasitic nematode—a type of roundworm—into its victim's body. This leads to the often devastating symptoms of the disease. The technical term for the illness is derived from Onchocerca volvulus, the nematode's scientific name.
The black fly breeds in fast-flowing rivers. Onchocerciasis is also known as river blindness because it affects people who live near rivers or streams. Most victims of the disease live in Africa south of the Sahara, but some people in Central and South America and in Yemen have been affected, too.
Community Volunteers Help to Treat River Blindness
According to the World Health Organization, an estimated 270,000 people in Africa have lost their sight due to river blindness and more than 500,000 have a visual impairment due to the infection. Millions of people have been infected by the parasite.
Infection by Onchocerca: A Parasitic Nematode
When a black fly of the genus Simulium bites a human, larvae of the nematode escape from the fly's saliva and enter the person's bloodstream. The larvae then leave the person's blood and enter the skin, settling in subcutaneous tissue (the deepest layer of the skin). Here they complete their development to adulthood inside nodules. The adult nematodes in the nodules may live for as long as fifteen years.
The adult worms are long and slender. Mature females are 33 to 50 cm in length (1.1 to 1.6 feet) but only 0.27 to 0.40 millimeters (0.011 to 0.016 inches) in width. Males are shorter and narrower than females.
The nematodes are believed to either ingest blood or to absorb blood nutrients through their skin. Nodules contain lots of blood vessels to nourish the roundworms. It's thought that the worms stimulate the formation of these blood vessels.
Male and female nematodes mate inside the nodules, producing eggs that hatch into tiny microfilariae. One female can release 1000 or more microfilariae per day. Each may live for as long as two years. The female roundworm is capable of producing eggs for about nine to eleven years.
The microfilariae leave the nodules and travel through the subcutaneous tissue. They may eventually reach the eye. In a severe infection they may also enter the blood, urine or sputum.
Release of a Bacterium Named Wolbachia
When microfilariae die inside humans, a bacterium called Wolbachia is released from their bodies. This bacterium normally lives inside the cells of the worms. It's thought that it may play a role in the disease process. The release of Wolbachia from the nematode may trigger an inflammatory response in the body, which could contribute to the unpleasant and potentially debilitating symptoms of onchocerciasis.
Completion of Onchocerca's Life Cycle
When a black fly bites an infected person and sucks up some of the person's blood, it withdraws microfilariae from the person's body. These enter the black fly's gut along with the blood. The microfilariae then travel through the wall of the gut and settle in the thoracic muscles. Here they change into different larval forms. The larvae eventually migrate to the fly's head and mouth parts and may infect a new person when the fly bites another human.
Symptoms of Onchocerciasis
One of the first symptoms of onchocerciasis is a skin rash with intense itching. The itching can make sleeping extremely difficult. Lacerations may appear on the skin as the person frantically scratches. There may also be visible nodules where the adult roundworms congregate.
Later there may be other skin changes, which may be disfiguring. The skin often becomes swollen and thickened and may develop hanging folds. Sometimes white patches form where pigment has been lost. In other cases patches with too much pigment appear.
The most serious effects of the infection are the life-altering itching and the eye disease. The inflammation damages the normally transparent layer over the surface of the eye, or the cornea, as well as injuring deeper parts of the eye. The changes eventually prevent the person from seeing. The blindness is usually the last symptom to appear and develops in adults and older people rather than in children. The Sightsavers website states if a person is infected at birth and is untreated, they have a high chance of being blind by the time they reach forty years of age.
Social Effects of the Disease
It's usually impossible for people in infected areas to avoid contact with a river or stream. River blindness usually develops in rural areas where the people are dependent on the land and water for their survival. They catch fish from the river and also use it to wash or to collect water for their village. They are repeatedly being exposed to black flies, which are most common around the water.
Severe onchocerciasis is disabling for the affected person, but it has also damaged communities. Sometimes villages with good farmland have been abandoned due to the prevalence of river blindness. Young, healthy people have been especially keen to leave. This has meant fewer workers to care for crops and famine or poverty for the remaining community members. Children have been unable to go to school because they have to look after blind relatives. Sometimes the whole community has moved to an upland area, which is further away from the black flies but has less productive soil than the valley. This has cause increased hardship for the community.
Treating Preventable Blindness
The treatment for Onchocerciasis is a medication called Ivermectin, or Mectizan®, which kills the nematode microfilariae that are responsible for the disease symptoms. It doesn't kill the adults, but it does prevent them from producing new microfilariae.
Mectizan has been donated by its manufacturer, Merck, since 1987. Merck has pledged to provide the drug for as long as necessary and in whatever quantity is needed. The medication stops the itching and prevents further eye damage. Since it kills the microfilariae that enter black flies when they suck up human blood, it also stops the transmission of the disease. The medication is given to all members of a community, even those that aren't infected. An antibiotic that kills Wolbachia is sometimes administered in addition to Mectizan. Although Mectizan is free, there is still a cost involved in transporting the drug to all the areas that need it.
One or two doses of Mectizan are required each year for at least fifteen years (the estimated maximum lifespan of the worms). It's uncertain if treatment can then be stopped or if there is a danger of reinfection. Preliminary evidence suggests that in at least some areas it's safe to stop treatment after fifteen to seventeen years.
A Potentially Devastating Disease
One problem is that Mectizan cannot be used to treat people infected with the Loa loa parasite, another type of roundworm, since in these people the drug may be deadly. Other control methods have to be used in areas affected by Loa loa.
Black flies have been killed by insecticides in some areas as a control mechanism for onchocerciasis, but this method also has problems. The insecticide is expensive and needs to be applied repeatedly. In addition, there are concerns about the safety of pesticides for human health and for the environment.
River blindness is most common in rural villages which are located by rivers or streams that are infested with black flies. Health experts say that temporary visitors are not likely to develop river blindness, since the appearance of symptoms and severity of the disease depend on the number of black fly bites received. The risk increases slightly for visitors who stay in an infected area for longer than normal, such as aid workers, field scientists or military personnel. Visitors are advised to use protection methods such as insect repellents and bed nets.
Eliminating River Blindness
Eliminating river blindness from the planet is a wonderful goal but an ambitious task. In some parts of the world the task is progressing well, while in others more work is needed. Mectizan is an effective treatment (at least at the moment), and Merck's generosity has been essential in the treatment plan so far.
Another helpful component in the elimination program has been the determination of certain people in Africa to solve the problem. These people include not only doctors, health workers and health organizations, but also community representatives. These local people have helped to educate their communities about the medication. In some cases, they are in charge of distributing the Mectizan and maintaining written records related to the distribution.
The Carter Center is an organization whose aim is to encourage peace and reduce disease. It was founded by former U.S. President Jimmy Carter and his wife Rosalynn. According to the center, transmission of river blindness ended in 2013 in Columbia, in 2014 in Ecuador, in 2015 in Mexico, and in 2016 in Guatemala. The effort to prevent transmission is continuing in Brazil and Venezuela. Most of the world's cases of river blindness occur in Africa, however.
In some previously affected parts of Africa there are no longer new incidences of onchocerciasis and the focus has shifted to helping people who have already become blind. In other areas the disease is still being transmitted.
A common symbol of river blindness is a child leading a blind adult around with a stick. The child holds one end of the stick and the adult holds the other. Hopefully this sad symbol will gradually fade as river blindness continues to be eliminated from the world.
River blindness facts from the Centers for Disease Control and Prevention (CDC)
Onchocerciasis information from the World Health Organization (WHO)
Mectizan Donation Program plus information about onchocerciasis
Control and Elimination of River Blindness from The Guardian newspaper
© 2012 Linda Crampton