One of the world’s leading infectious causes of blindness is river blindness (onchocerciasis). Some 18 million people are infected with the disease-causing worm, which leaves 300,000 permanently blind and 500,000 suffering from visual impairment. Roughly 99 percent of all these cases occur in thirty countries in sub-Saharan Africa, with the remainder in Yemen and six Latin American countries.
The larva of the microscopic worm Onchocerca volvulus causes onchocerciasis, and the disease is spread by the multiple bites from infected blackflies that breed in fast-flowing rivers. Single oral doses of ivermectin (Mectizan, Stromectol), given at six- to twelve-month intervals, prevent the release of and kill the microfilariae (immature worms responsible for the disease), most likely by causing its paralysis. The drug also relieves the painful symptoms of onchocerciasis that affect the skin, and it prevents the disease from progressing to blindness. Ivermectin cannot kill the adult worm, however, neither can it reverse blindness once it has occurred. After the initial course of therapy, this generally well-tolerated drug is given at yearly intervals for ten to twenty years—the lifespan of the adult worm.
Since 1987, Merck, working with international aid organizations, has donated Mectizan. In 2008 alone, some 80 million people were treated, and thanks to this program, an estimated 600,000 cases of blindness have been prevented. There is hope that onchocerciasis will be eradicated in the coming years. However, in 2007, early signs of drug resistance were observed in the parasitic worm, and this could lead to a resurgence of the disease in communities previously thought to be brought under control.
Although the most dramatic effects have been seen in treating onchocerciasis, ivermectin is among the most broad-spectrum antiparasitic drugs for use in humans and animals. In humans, it is effective in treating threadworm and roundworm, as well as scabies and head lice. It is also given to dogs and cats to prevent heartworm and ear ticks.
SEE ALSO Praziquantel (1972), Artemisinin (1972).
The sculpture Gift of Sight by R. T. Wallen depicts a boy helping an older man afflicted with river blindness. It is located in the World Bank’s great atrium in Washington, D.C.