River blindness is caused by a parasitic filarial worm, Onchocerca volvulus. Symptoms include intense itching and eye lesions, making it a leading cause of visual impairment and permanent blindness in the tropics.
The worm is spread between people by a species of blackfly. The authorities in Colombia managed to break this cycle by giving communities in affected areas the anti-parasitic drug ivermectin every six months for 12 years.
"This is a historic milestone," says Mark Taylor at the Liverpool School of Tropical Medicine in the UK. But he says that elimination may be harder in African countries, where co-infection with another parasite can result in severe side effects following drug treatment.