River blindness is a chronic parasitic (helminthic) infection. It is endemic in dozens of countries in Latin America, Africa, Asia, and the Middle East. At any given time, approximately 37 million people are infected with the worm called Onchocerca volvulus .
This organism is a filarial (threadlike) helminthic worm transmitted by small biting vectors called black flies. These voracious flies often attack in large numbers, and it is not uncommon in endemic areas to be bitten several hundred times a day. The disease gets its name from the habitat where these flies are most often found, rural settlements along rivers bordered with overhanging vegetation.
The Onchocerca larvae are deposited into a bite wound and develop into adults in the immediate subcutaneous tissues, where disfiguring nodules form within 1 to 2 years after initial contact. Microfilariae given off by the adult female migrate via the bloodstream to many locations but especially to the eyes. While the worms are in the blood, they can be transmitted to other feeding black flies. Some cases of onchocerciasis result in a severe itchy rash that can last for years. It was previously thought that the condition was caused by degeneration of the worms and the inflammation and granulomatous lesion formation that result from the release of their antigens.
It is in fact the case that the worms eventually invade the entire eye, producing much inflammation and permanent damage to the retina and optic nerve. In 1999, researchers first discovered large colonies of bacteria called Wolbachia living inside the Onchocerca worms. There is convincing evidence that the damage caused to human tissues is induced by the bacteria rather than by the worms. Of course, the worms serve as the delivery system to the human as it does not appear that the bacteria can infect humans on their own. These bacteria enjoy a mutualistic relationship with their hosts; they are essential for normal Onchocerca development.
In regions of high prevalence, it is not unusual for an ophthalmologist to see microfilariae wiggling in the anterior chamber during a routine eye checkup. Microfilariae die in several months, but adults can exist for up to 15 years in skin nodules.
River blindness has been a serious problem in many areas of Africa. In some villages, nearly half of the residents are affected by the disease. A campaign to eradicate onchocerciasis is currently underway, supported by the Carter Center, an organization run by former U.S. President Jimmy Carter. The approach is to treat people with ivermectin, a potent antifilarial drug, and to use insecticides to control the black flies. Eliminating the protozoan will still eliminate the disease . The drug company that manufactures ivermectin has promised to provide the drug for free for as long as the need for it exists.