Onchocerciasis treatment distribution. Third World health worker handing out ivermectin (Mectizan) tablets for the treatment of onchocerciasis (insriver blindnessins). The health worker is distributing the tablets to a head of household (at left) as part of a house-to-house distribution system. Onchocerciasis is caused by the infective tiny worms (microfilariae) of Onchocerca volvulus. The worms are transmitted from person to person by biting flies which breed in rivers. Blindness occurs as an allergic reaction when these worms die in or near tissues of the eye. Ivermectin is highly effective at killing the worms when given in doses 6-12 months apart. Photographed in Mali.