Height measurement. Third World health worker measuring the height of a young boy. As a safety precaution, children must be at least 90 cms in height before they are eligible to receive ivermectin tablets for the treatment of onchocerciasis (insriver blindnessins). 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 Uganda.